linux/kernel/rcu/tasks.h
Linus Torvalds 3fe2f7446f Changes in this cycle were:
- Cleanups for SCHED_DEADLINE
  - Tracing updates/fixes
  - CPU Accounting fixes
  - First wave of changes to optimize the overhead of the scheduler build,
    from the fast-headers tree - including placeholder *_api.h headers for
    later header split-ups.
  - Preempt-dynamic using static_branch() for ARM64
  - Isolation housekeeping mask rework; preperatory for further changes
  - NUMA-balancing: deal with CPU-less nodes
  - NUMA-balancing: tune systems that have multiple LLC cache domains per node (eg. AMD)
  - Updates to RSEQ UAPI in preparation for glibc usage
  - Lots of RSEQ/selftests, for same
  - Add Suren as PSI co-maintainer
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'sched-core-2022-03-22' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler updates from Ingo Molnar:

 - Cleanups for SCHED_DEADLINE

 - Tracing updates/fixes

 - CPU Accounting fixes

 - First wave of changes to optimize the overhead of the scheduler
   build, from the fast-headers tree - including placeholder *_api.h
   headers for later header split-ups.

 - Preempt-dynamic using static_branch() for ARM64

 - Isolation housekeeping mask rework; preperatory for further changes

 - NUMA-balancing: deal with CPU-less nodes

 - NUMA-balancing: tune systems that have multiple LLC cache domains per
   node (eg. AMD)

 - Updates to RSEQ UAPI in preparation for glibc usage

 - Lots of RSEQ/selftests, for same

 - Add Suren as PSI co-maintainer

* tag 'sched-core-2022-03-22' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (81 commits)
  sched/headers: ARM needs asm/paravirt_api_clock.h too
  sched/numa: Fix boot crash on arm64 systems
  headers/prep: Fix header to build standalone: <linux/psi.h>
  sched/headers: Only include <linux/entry-common.h> when CONFIG_GENERIC_ENTRY=y
  cgroup: Fix suspicious rcu_dereference_check() usage warning
  sched/preempt: Tell about PREEMPT_DYNAMIC on kernel headers
  sched/topology: Remove redundant variable and fix incorrect type in build_sched_domains
  sched/deadline,rt: Remove unused parameter from pick_next_[rt|dl]_entity()
  sched/deadline,rt: Remove unused functions for !CONFIG_SMP
  sched/deadline: Use __node_2_[pdl|dle]() and rb_first_cached() consistently
  sched/deadline: Merge dl_task_can_attach() and dl_cpu_busy()
  sched/deadline: Move bandwidth mgmt and reclaim functions into sched class source file
  sched/deadline: Remove unused def_dl_bandwidth
  sched/tracing: Report TASK_RTLOCK_WAIT tasks as TASK_UNINTERRUPTIBLE
  sched/tracing: Don't re-read p->state when emitting sched_switch event
  sched/rt: Plug rt_mutex_setprio() vs push_rt_task() race
  sched/cpuacct: Remove redundant RCU read lock
  sched/cpuacct: Optimize away RCU read lock
  sched/cpuacct: Fix charge percpu cpuusage
  sched/headers: Reorganize, clean up and optimize kernel/sched/sched.h dependencies
  ...
2022-03-22 14:39:12 -07:00

1726 lines
58 KiB
C

/* SPDX-License-Identifier: GPL-2.0+ */
/*
* Task-based RCU implementations.
*
* Copyright (C) 2020 Paul E. McKenney
*/
#ifdef CONFIG_TASKS_RCU_GENERIC
#include "rcu_segcblist.h"
////////////////////////////////////////////////////////////////////////
//
// Generic data structures.
struct rcu_tasks;
typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
typedef void (*pregp_func_t)(void);
typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
typedef void (*postscan_func_t)(struct list_head *hop);
typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
/**
* struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
* @cblist: Callback list.
* @lock: Lock protecting per-CPU callback list.
* @rtp_jiffies: Jiffies counter value for statistics.
* @rtp_n_lock_retries: Rough lock-contention statistic.
* @rtp_work: Work queue for invoking callbacks.
* @rtp_irq_work: IRQ work queue for deferred wakeups.
* @barrier_q_head: RCU callback for barrier operation.
* @cpu: CPU number corresponding to this entry.
* @rtpp: Pointer to the rcu_tasks structure.
*/
struct rcu_tasks_percpu {
struct rcu_segcblist cblist;
raw_spinlock_t __private lock;
unsigned long rtp_jiffies;
unsigned long rtp_n_lock_retries;
struct work_struct rtp_work;
struct irq_work rtp_irq_work;
struct rcu_head barrier_q_head;
int cpu;
struct rcu_tasks *rtpp;
};
/**
* struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
* @cbs_wq: Wait queue allowing new callback to get kthread's attention.
* @cbs_gbl_lock: Lock protecting callback list.
* @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
* @gp_func: This flavor's grace-period-wait function.
* @gp_state: Grace period's most recent state transition (debugging).
* @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
* @init_fract: Initial backoff sleep interval.
* @gp_jiffies: Time of last @gp_state transition.
* @gp_start: Most recent grace-period start in jiffies.
* @tasks_gp_seq: Number of grace periods completed since boot.
* @n_ipis: Number of IPIs sent to encourage grace periods to end.
* @n_ipis_fails: Number of IPI-send failures.
* @pregp_func: This flavor's pre-grace-period function (optional).
* @pertask_func: This flavor's per-task scan function (optional).
* @postscan_func: This flavor's post-task scan function (optional).
* @holdouts_func: This flavor's holdout-list scan function (optional).
* @postgp_func: This flavor's post-grace-period function (optional).
* @call_func: This flavor's call_rcu()-equivalent function.
* @rtpcpu: This flavor's rcu_tasks_percpu structure.
* @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
* @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
* @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
* @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
* @barrier_q_mutex: Serialize barrier operations.
* @barrier_q_count: Number of queues being waited on.
* @barrier_q_completion: Barrier wait/wakeup mechanism.
* @barrier_q_seq: Sequence number for barrier operations.
* @name: This flavor's textual name.
* @kname: This flavor's kthread name.
*/
struct rcu_tasks {
struct wait_queue_head cbs_wq;
raw_spinlock_t cbs_gbl_lock;
int gp_state;
int gp_sleep;
int init_fract;
unsigned long gp_jiffies;
unsigned long gp_start;
unsigned long tasks_gp_seq;
unsigned long n_ipis;
unsigned long n_ipis_fails;
struct task_struct *kthread_ptr;
rcu_tasks_gp_func_t gp_func;
pregp_func_t pregp_func;
pertask_func_t pertask_func;
postscan_func_t postscan_func;
holdouts_func_t holdouts_func;
postgp_func_t postgp_func;
call_rcu_func_t call_func;
struct rcu_tasks_percpu __percpu *rtpcpu;
int percpu_enqueue_shift;
int percpu_enqueue_lim;
int percpu_dequeue_lim;
unsigned long percpu_dequeue_gpseq;
struct mutex barrier_q_mutex;
atomic_t barrier_q_count;
struct completion barrier_q_completion;
unsigned long barrier_q_seq;
char *name;
char *kname;
};
static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
#define DEFINE_RCU_TASKS(rt_name, gp, call, n) \
static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = { \
.lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock), \
.rtp_irq_work = IRQ_WORK_INIT(call_rcu_tasks_iw_wakeup), \
}; \
static struct rcu_tasks rt_name = \
{ \
.cbs_wq = __WAIT_QUEUE_HEAD_INITIALIZER(rt_name.cbs_wq), \
.cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock), \
.gp_func = gp, \
.call_func = call, \
.rtpcpu = &rt_name ## __percpu, \
.name = n, \
.percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS), \
.percpu_enqueue_lim = 1, \
.percpu_dequeue_lim = 1, \
.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex), \
.barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT, \
.kname = #rt_name, \
}
/* Track exiting tasks in order to allow them to be waited for. */
DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
/* Avoid IPIing CPUs early in the grace period. */
#define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
module_param(rcu_task_ipi_delay, int, 0644);
/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
module_param(rcu_task_stall_timeout, int, 0644);
static int rcu_task_enqueue_lim __read_mostly = -1;
module_param(rcu_task_enqueue_lim, int, 0444);
static bool rcu_task_cb_adjust;
static int rcu_task_contend_lim __read_mostly = 100;
module_param(rcu_task_contend_lim, int, 0444);
static int rcu_task_collapse_lim __read_mostly = 10;
module_param(rcu_task_collapse_lim, int, 0444);
/* RCU tasks grace-period state for debugging. */
#define RTGS_INIT 0
#define RTGS_WAIT_WAIT_CBS 1
#define RTGS_WAIT_GP 2
#define RTGS_PRE_WAIT_GP 3
#define RTGS_SCAN_TASKLIST 4
#define RTGS_POST_SCAN_TASKLIST 5
#define RTGS_WAIT_SCAN_HOLDOUTS 6
#define RTGS_SCAN_HOLDOUTS 7
#define RTGS_POST_GP 8
#define RTGS_WAIT_READERS 9
#define RTGS_INVOKE_CBS 10
#define RTGS_WAIT_CBS 11
#ifndef CONFIG_TINY_RCU
static const char * const rcu_tasks_gp_state_names[] = {
"RTGS_INIT",
"RTGS_WAIT_WAIT_CBS",
"RTGS_WAIT_GP",
"RTGS_PRE_WAIT_GP",
"RTGS_SCAN_TASKLIST",
"RTGS_POST_SCAN_TASKLIST",
"RTGS_WAIT_SCAN_HOLDOUTS",
"RTGS_SCAN_HOLDOUTS",
"RTGS_POST_GP",
"RTGS_WAIT_READERS",
"RTGS_INVOKE_CBS",
"RTGS_WAIT_CBS",
};
#endif /* #ifndef CONFIG_TINY_RCU */
////////////////////////////////////////////////////////////////////////
//
// Generic code.
static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);
/* Record grace-period phase and time. */
static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
{
rtp->gp_state = newstate;
rtp->gp_jiffies = jiffies;
}
#ifndef CONFIG_TINY_RCU
/* Return state name. */
static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
{
int i = data_race(rtp->gp_state); // Let KCSAN detect update races
int j = READ_ONCE(i); // Prevent the compiler from reading twice
if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
return "???";
return rcu_tasks_gp_state_names[j];
}
#endif /* #ifndef CONFIG_TINY_RCU */
// Initialize per-CPU callback lists for the specified flavor of
// Tasks RCU.
static void cblist_init_generic(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
int lim;
int shift;
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rcu_task_enqueue_lim < 0) {
rcu_task_enqueue_lim = 1;
rcu_task_cb_adjust = true;
pr_info("%s: Setting adjustable number of callback queues.\n", __func__);
} else if (rcu_task_enqueue_lim == 0) {
rcu_task_enqueue_lim = 1;
}
lim = rcu_task_enqueue_lim;
if (lim > nr_cpu_ids)
lim = nr_cpu_ids;
shift = ilog2(nr_cpu_ids / lim);
if (((nr_cpu_ids - 1) >> shift) >= lim)
shift++;
WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
smp_store_release(&rtp->percpu_enqueue_lim, lim);
for_each_possible_cpu(cpu) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
WARN_ON_ONCE(!rtpcp);
if (cpu)
raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
if (rcu_segcblist_empty(&rtpcp->cblist))
rcu_segcblist_init(&rtpcp->cblist);
INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
rtpcp->cpu = cpu;
rtpcp->rtpp = rtp;
raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
pr_info("%s: Setting shift to %d and lim to %d.\n", __func__, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim));
}
// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
rtp = rtpcp->rtpp;
wake_up(&rtp->cbs_wq);
}
// Enqueue a callback for the specified flavor of Tasks RCU.
static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
struct rcu_tasks *rtp)
{
unsigned long flags;
unsigned long j;
bool needadjust = false;
bool needwake;
struct rcu_tasks_percpu *rtpcp;
rhp->next = NULL;
rhp->func = func;
local_irq_save(flags);
rcu_read_lock();
rtpcp = per_cpu_ptr(rtp->rtpcpu,
smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift));
if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
j = jiffies;
if (rtpcp->rtp_jiffies != j) {
rtpcp->rtp_jiffies = j;
rtpcp->rtp_n_lock_retries = 0;
}
if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
needadjust = true; // Defer adjustment to avoid deadlock.
}
if (!rcu_segcblist_is_enabled(&rtpcp->cblist)) {
raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
cblist_init_generic(rtp);
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
}
needwake = rcu_segcblist_empty(&rtpcp->cblist);
rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
if (unlikely(needadjust)) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
rcu_read_unlock();
/* We can't create the thread unless interrupts are enabled. */
if (needwake && READ_ONCE(rtp->kthread_ptr))
irq_work_queue(&rtpcp->rtp_irq_work);
}
// Wait for a grace period for the specified flavor of Tasks RCU.
static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
{
/* Complain if the scheduler has not started. */
RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
"synchronize_rcu_tasks called too soon");
/* Wait for the grace period. */
wait_rcu_gp(rtp->call_func);
}
// RCU callback function for rcu_barrier_tasks_generic().
static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp;
rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
rtp = rtpcp->rtpp;
if (atomic_dec_and_test(&rtp->barrier_q_count))
complete(&rtp->barrier_q_completion);
}
// Wait for all in-flight callbacks for the specified RCU Tasks flavor.
// Operates in a manner similar to rcu_barrier().
static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
struct rcu_tasks_percpu *rtpcp;
unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);
mutex_lock(&rtp->barrier_q_mutex);
if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
smp_mb();
mutex_unlock(&rtp->barrier_q_mutex);
return;
}
rcu_seq_start(&rtp->barrier_q_seq);
init_completion(&rtp->barrier_q_completion);
atomic_set(&rtp->barrier_q_count, 2);
for_each_possible_cpu(cpu) {
if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
break;
rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
atomic_inc(&rtp->barrier_q_count);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
if (atomic_sub_and_test(2, &rtp->barrier_q_count))
complete(&rtp->barrier_q_completion);
wait_for_completion(&rtp->barrier_q_completion);
rcu_seq_end(&rtp->barrier_q_seq);
mutex_unlock(&rtp->barrier_q_mutex);
}
// Advance callbacks and indicate whether either a grace period or
// callback invocation is needed.
static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
long n;
long ncbs = 0;
long ncbsnz = 0;
int needgpcb = 0;
for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_dequeue_lim); cpu++) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
/* Advance and accelerate any new callbacks. */
if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
continue;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
// Should we shrink down to a single callback queue?
n = rcu_segcblist_n_cbs(&rtpcp->cblist);
if (n) {
ncbs += n;
if (cpu > 0)
ncbsnz += n;
}
rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
if (rcu_segcblist_pend_cbs(&rtpcp->cblist))
needgpcb |= 0x3;
if (!rcu_segcblist_empty(&rtpcp->cblist))
needgpcb |= 0x1;
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
// Shrink down to a single callback queue if appropriate.
// This is done in two stages: (1) If there are no more than
// rcu_task_collapse_lim callbacks on CPU 0 and none on any other
// CPU, limit enqueueing to CPU 0. (2) After an RCU grace period,
// if there has not been an increase in callbacks, limit dequeuing
// to CPU 0. Note the matching RCU read-side critical section in
// call_rcu_tasks_generic().
if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim > 1) {
WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
smp_store_release(&rtp->percpu_enqueue_lim, 1);
rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
if (rcu_task_cb_adjust && !ncbsnz &&
poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq)) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
return needgpcb;
}
// Advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
{
int cpu;
int cpunext;
unsigned long flags;
int len;
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
struct rcu_tasks_percpu *rtpcp_next;
cpu = rtpcp->cpu;
cpunext = cpu * 2 + 1;
if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
cpunext++;
if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
}
}
if (rcu_segcblist_empty(&rtpcp->cblist))
return;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
len = rcl.len;
for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
local_bh_disable();
rhp->func(rhp);
local_bh_enable();
cond_resched();
}
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
rcu_segcblist_add_len(&rtpcp->cblist, -len);
(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}
// Workqueue flood to advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
{
struct rcu_tasks *rtp;
struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);
rtp = rtpcp->rtpp;
rcu_tasks_invoke_cbs(rtp, rtpcp);
}
/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
static int __noreturn rcu_tasks_kthread(void *arg)
{
int needgpcb;
struct rcu_tasks *rtp = arg;
/* Run on housekeeping CPUs by default. Sysadm can move if desired. */
housekeeping_affine(current, HK_TYPE_RCU);
WRITE_ONCE(rtp->kthread_ptr, current); // Let GPs start!
/*
* Each pass through the following loop makes one check for
* newly arrived callbacks, and, if there are some, waits for
* one RCU-tasks grace period and then invokes the callbacks.
* This loop is terminated by the system going down. ;-)
*/
for (;;) {
set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
/* If there were none, wait a bit and start over. */
wait_event_idle(rtp->cbs_wq, (needgpcb = rcu_tasks_need_gpcb(rtp)));
if (needgpcb & 0x2) {
// Wait for one grace period.
set_tasks_gp_state(rtp, RTGS_WAIT_GP);
rtp->gp_start = jiffies;
rcu_seq_start(&rtp->tasks_gp_seq);
rtp->gp_func(rtp);
rcu_seq_end(&rtp->tasks_gp_seq);
}
/* Invoke callbacks. */
set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
/* Paranoid sleep to keep this from entering a tight loop */
schedule_timeout_idle(rtp->gp_sleep);
}
}
/* Spawn RCU-tasks grace-period kthread. */
static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
{
struct task_struct *t;
t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
return;
smp_mb(); /* Ensure others see full kthread. */
}
#ifndef CONFIG_TINY_RCU
/*
* Print any non-default Tasks RCU settings.
*/
static void __init rcu_tasks_bootup_oddness(void)
{
#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RCU
pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RUDE_RCU
pr_info("\tRude variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
#ifdef CONFIG_TASKS_TRACE_RCU
pr_info("\tTracing variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}
#endif /* #ifndef CONFIG_TINY_RCU */
#ifndef CONFIG_TINY_RCU
/* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
{
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, 0); // for_each...
pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c %s\n",
rtp->kname,
tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
jiffies - data_race(rtp->gp_jiffies),
data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
".k"[!!data_race(rtp->kthread_ptr)],
".C"[!data_race(rcu_segcblist_empty(&rtpcp->cblist))],
s);
}
#endif // #ifndef CONFIG_TINY_RCU
static void exit_tasks_rcu_finish_trace(struct task_struct *t);
#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
////////////////////////////////////////////////////////////////////////
//
// Shared code between task-list-scanning variants of Tasks RCU.
/* Wait for one RCU-tasks grace period. */
static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
{
struct task_struct *g, *t;
unsigned long lastreport;
LIST_HEAD(holdouts);
int fract;
set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
rtp->pregp_func();
/*
* There were callbacks, so we need to wait for an RCU-tasks
* grace period. Start off by scanning the task list for tasks
* that are not already voluntarily blocked. Mark these tasks
* and make a list of them in holdouts.
*/
set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
rcu_read_lock();
for_each_process_thread(g, t)
rtp->pertask_func(t, &holdouts);
rcu_read_unlock();
set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
rtp->postscan_func(&holdouts);
/*
* Each pass through the following loop scans the list of holdout
* tasks, removing any that are no longer holdouts. When the list
* is empty, we are done.
*/
lastreport = jiffies;
// Start off with initial wait and slowly back off to 1 HZ wait.
fract = rtp->init_fract;
while (!list_empty(&holdouts)) {
bool firstreport;
bool needreport;
int rtst;
/* Slowly back off waiting for holdouts */
set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
schedule_timeout_idle(fract);
if (fract < HZ)
fract++;
rtst = READ_ONCE(rcu_task_stall_timeout);
needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
if (needreport)
lastreport = jiffies;
firstreport = true;
WARN_ON(signal_pending(current));
set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
rtp->holdouts_func(&holdouts, needreport, &firstreport);
}
set_tasks_gp_state(rtp, RTGS_POST_GP);
rtp->postgp_func(rtp);
}
#endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU) */
#ifdef CONFIG_TASKS_RCU
////////////////////////////////////////////////////////////////////////
//
// Simple variant of RCU whose quiescent states are voluntary context
// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle.
// As such, grace periods can take one good long time. There are no
// read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
// because this implementation is intended to get the system into a safe
// state for some of the manipulations involved in tracing and the like.
// Finally, this implementation does not support high call_rcu_tasks()
// rates from multiple CPUs. If this is required, per-CPU callback lists
// will be needed.
//
// The implementation uses rcu_tasks_wait_gp(), which relies on function
// pointers in the rcu_tasks structure. The rcu_spawn_tasks_kthread()
// function sets these function pointers up so that rcu_tasks_wait_gp()
// invokes these functions in this order:
//
// rcu_tasks_pregp_step():
// Invokes synchronize_rcu() in order to wait for all in-flight
// t->on_rq and t->nvcsw transitions to complete. This works because
// all such transitions are carried out with interrupts disabled.
// rcu_tasks_pertask(), invoked on every non-idle task:
// For every runnable non-idle task other than the current one, use
// get_task_struct() to pin down that task, snapshot that task's
// number of voluntary context switches, and add that task to the
// holdout list.
// rcu_tasks_postscan():
// Invoke synchronize_srcu() to ensure that all tasks that were
// in the process of exiting (and which thus might not know to
// synchronize with this RCU Tasks grace period) have completed
// exiting.
// check_all_holdout_tasks(), repeatedly until holdout list is empty:
// Scans the holdout list, attempting to identify a quiescent state
// for each task on the list. If there is a quiescent state, the
// corresponding task is removed from the holdout list.
// rcu_tasks_postgp():
// Invokes synchronize_rcu() in order to ensure that all prior
// t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
// to have happened before the end of this RCU Tasks grace period.
// Again, this works because all such transitions are carried out
// with interrupts disabled.
//
// For each exiting task, the exit_tasks_rcu_start() and
// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
// read-side critical sections waited for by rcu_tasks_postscan().
//
// Pre-grace-period update-side code is ordered before the grace
// via the raw_spin_lock.*rcu_node(). Pre-grace-period read-side code
// is ordered before the grace period via synchronize_rcu() call in
// rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
// disabling.
/* Pre-grace-period preparation. */
static void rcu_tasks_pregp_step(void)
{
/*
* Wait for all pre-existing t->on_rq and t->nvcsw transitions
* to complete. Invoking synchronize_rcu() suffices because all
* these transitions occur with interrupts disabled. Without this
* synchronize_rcu(), a read-side critical section that started
* before the grace period might be incorrectly seen as having
* started after the grace period.
*
* This synchronize_rcu() also dispenses with the need for a
* memory barrier on the first store to t->rcu_tasks_holdout,
* as it forces the store to happen after the beginning of the
* grace period.
*/
synchronize_rcu();
}
/* Per-task initial processing. */
static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
{
if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
get_task_struct(t);
t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
WRITE_ONCE(t->rcu_tasks_holdout, true);
list_add(&t->rcu_tasks_holdout_list, hop);
}
}
/* Processing between scanning taskslist and draining the holdout list. */
static void rcu_tasks_postscan(struct list_head *hop)
{
/*
* Wait for tasks that are in the process of exiting. This
* does only part of the job, ensuring that all tasks that were
* previously exiting reach the point where they have disabled
* preemption, allowing the later synchronize_rcu() to finish
* the job.
*/
synchronize_srcu(&tasks_rcu_exit_srcu);
}
/* See if tasks are still holding out, complain if so. */
static void check_holdout_task(struct task_struct *t,
bool needreport, bool *firstreport)
{
int cpu;
if (!READ_ONCE(t->rcu_tasks_holdout) ||
t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
!READ_ONCE(t->on_rq) ||
(IS_ENABLED(CONFIG_NO_HZ_FULL) &&
!is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
WRITE_ONCE(t->rcu_tasks_holdout, false);
list_del_init(&t->rcu_tasks_holdout_list);
put_task_struct(t);
return;
}
rcu_request_urgent_qs_task(t);
if (!needreport)
return;
if (*firstreport) {
pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
*firstreport = false;
}
cpu = task_cpu(t);
pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
t, ".I"[is_idle_task(t)],
"N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
t->rcu_tasks_idle_cpu, cpu);
sched_show_task(t);
}
/* Scan the holdout lists for tasks no longer holding out. */
static void check_all_holdout_tasks(struct list_head *hop,
bool needreport, bool *firstreport)
{
struct task_struct *t, *t1;
list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
check_holdout_task(t, needreport, firstreport);
cond_resched();
}
}
/* Finish off the Tasks-RCU grace period. */
static void rcu_tasks_postgp(struct rcu_tasks *rtp)
{
/*
* Because ->on_rq and ->nvcsw are not guaranteed to have a full
* memory barriers prior to them in the schedule() path, memory
* reordering on other CPUs could cause their RCU-tasks read-side
* critical sections to extend past the end of the grace period.
* However, because these ->nvcsw updates are carried out with
* interrupts disabled, we can use synchronize_rcu() to force the
* needed ordering on all such CPUs.
*
* This synchronize_rcu() also confines all ->rcu_tasks_holdout
* accesses to be within the grace period, avoiding the need for
* memory barriers for ->rcu_tasks_holdout accesses.
*
* In addition, this synchronize_rcu() waits for exiting tasks
* to complete their final preempt_disable() region of execution,
* cleaning up after the synchronize_srcu() above.
*/
synchronize_rcu();
}
void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
/**
* call_rcu_tasks() - Queue an RCU for invocation task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a full grace
* period elapses, in other words after all currently executing RCU
* read-side critical sections have completed. call_rcu_tasks() assumes
* that the read-side critical sections end at a voluntary context
* switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle,
* or transition to usermode execution. As such, there are no read-side
* primitives analogous to rcu_read_lock() and rcu_read_unlock() because
* this primitive is intended to determine that all tasks have passed
* through a safe state, not so much for data-structure synchronization.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks);
/**
* synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
*
* Control will return to the caller some time after a full rcu-tasks
* grace period has elapsed, in other words after all currently
* executing rcu-tasks read-side critical sections have elapsed. These
* read-side critical sections are delimited by calls to schedule(),
* cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
* to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function
* preambles and profiling hooks. The synchronize_rcu_tasks() function
* is not (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks(void)
{
synchronize_rcu_tasks_generic(&rcu_tasks);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
/**
* rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks(void)
{
rcu_barrier_tasks_generic(&rcu_tasks);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
static int __init rcu_spawn_tasks_kthread(void)
{
cblist_init_generic(&rcu_tasks);
rcu_tasks.gp_sleep = HZ / 10;
rcu_tasks.init_fract = HZ / 10;
rcu_tasks.pregp_func = rcu_tasks_pregp_step;
rcu_tasks.pertask_func = rcu_tasks_pertask;
rcu_tasks.postscan_func = rcu_tasks_postscan;
rcu_tasks.holdouts_func = check_all_holdout_tasks;
rcu_tasks.postgp_func = rcu_tasks_postgp;
rcu_spawn_tasks_kthread_generic(&rcu_tasks);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_classic_gp_kthread(void)
{
show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
/* Do the srcu_read_lock() for the above synchronize_srcu(). */
void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
{
preempt_disable();
current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
preempt_enable();
}
/* Do the srcu_read_unlock() for the above synchronize_srcu(). */
void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
{
struct task_struct *t = current;
preempt_disable();
__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
preempt_enable();
exit_tasks_rcu_finish_trace(t);
}
#else /* #ifdef CONFIG_TASKS_RCU */
void exit_tasks_rcu_start(void) { }
void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
#endif /* #else #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RUDE_RCU
////////////////////////////////////////////////////////////////////////
//
// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
// passing an empty function to schedule_on_each_cpu(). This approach
// provides an asynchronous call_rcu_tasks_rude() API and batching of
// concurrent calls to the synchronous synchronize_rcu_tasks_rude() API.
// This invokes schedule_on_each_cpu() in order to send IPIs far and wide
// and induces otherwise unnecessary context switches on all online CPUs,
// whether idle or not.
//
// Callback handling is provided by the rcu_tasks_kthread() function.
//
// Ordering is provided by the scheduler's context-switch code.
// Empty function to allow workqueues to force a context switch.
static void rcu_tasks_be_rude(struct work_struct *work)
{
}
// Wait for one rude RCU-tasks grace period.
static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
{
rtp->n_ipis += cpumask_weight(cpu_online_mask);
schedule_on_each_cpu(rcu_tasks_be_rude);
}
void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
"RCU Tasks Rude");
/**
* call_rcu_tasks_rude() - Queue a callback rude task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a full grace
* period elapses, in other words after all currently executing RCU
* read-side critical sections have completed. call_rcu_tasks_rude()
* assumes that the read-side critical sections end at context switch,
* cond_resched_tasks_rcu_qs(), or transition to usermode execution (as
* usermode execution is schedulable). As such, there are no read-side
* primitives analogous to rcu_read_lock() and rcu_read_unlock() because
* this primitive is intended to determine that all tasks have passed
* through a safe state, not so much for data-structure synchronization.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
/**
* synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
*
* Control will return to the caller some time after a rude rcu-tasks
* grace period has elapsed, in other words after all currently
* executing rcu-tasks read-side critical sections have elapsed. These
* read-side critical sections are delimited by calls to schedule(),
* cond_resched_tasks_rcu_qs(), userspace execution (which is a schedulable
* context), and (in theory, anyway) cond_resched().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function preambles
* and profiling hooks. The synchronize_rcu_tasks_rude() function is not
* (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks_rude(void)
{
synchronize_rcu_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
/**
* rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks_rude(void)
{
rcu_barrier_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
static int __init rcu_spawn_tasks_rude_kthread(void)
{
cblist_init_generic(&rcu_tasks_rude);
rcu_tasks_rude.gp_sleep = HZ / 10;
rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_rude_gp_kthread(void)
{
show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
////////////////////////////////////////////////////////////////////////
//
// Tracing variant of Tasks RCU. This variant is designed to be used
// to protect tracing hooks, including those of BPF. This variant
// therefore:
//
// 1. Has explicit read-side markers to allow finite grace periods
// in the face of in-kernel loops for PREEMPT=n builds.
//
// 2. Protects code in the idle loop, exception entry/exit, and
// CPU-hotplug code paths, similar to the capabilities of SRCU.
//
// 3. Avoids expensive read-side instructions, having overhead similar
// to that of Preemptible RCU.
//
// There are of course downsides. The grace-period code can send IPIs to
// CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.
// It is necessary to scan the full tasklist, much as for Tasks RCU. There
// is a single callback queue guarded by a single lock, again, much as for
// Tasks RCU. If needed, these downsides can be at least partially remedied.
//
// Perhaps most important, this variant of RCU does not affect the vanilla
// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
// readers can operate from idle, offline, and exception entry/exit in no
// way allows rcu_preempt and rcu_sched readers to also do so.
//
// The implementation uses rcu_tasks_wait_gp(), which relies on function
// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread()
// function sets these function pointers up so that rcu_tasks_wait_gp()
// invokes these functions in this order:
//
// rcu_tasks_trace_pregp_step():
// Initialize the count of readers and block CPU-hotplug operations.
// rcu_tasks_trace_pertask(), invoked on every non-idle task:
// Initialize per-task state and attempt to identify an immediate
// quiescent state for that task, or, failing that, attempt to
// set that task's .need_qs flag so that task's next outermost
// rcu_read_unlock_trace() will report the quiescent state (in which
// case the count of readers is incremented). If both attempts fail,
// the task is added to a "holdout" list. Note that IPIs are used
// to invoke trc_read_check_handler() in the context of running tasks
// in order to avoid ordering overhead on common-case shared-variable
// accessses.
// rcu_tasks_trace_postscan():
// Initialize state and attempt to identify an immediate quiescent
// state as above (but only for idle tasks), unblock CPU-hotplug
// operations, and wait for an RCU grace period to avoid races with
// tasks that are in the process of exiting.
// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
// Scans the holdout list, attempting to identify a quiescent state
// for each task on the list. If there is a quiescent state, the
// corresponding task is removed from the holdout list.
// rcu_tasks_trace_postgp():
// Wait for the count of readers do drop to zero, reporting any stalls.
// Also execute full memory barriers to maintain ordering with code
// executing after the grace period.
//
// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
//
// Pre-grace-period update-side code is ordered before the grace
// period via the ->cbs_lock and barriers in rcu_tasks_kthread().
// Pre-grace-period read-side code is ordered before the grace period by
// atomic_dec_and_test() of the count of readers (for IPIed readers) and by
// scheduler context-switch ordering (for locked-down non-running readers).
// The lockdep state must be outside of #ifdef to be useful.
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_trace_key;
struct lockdep_map rcu_trace_lock_map =
STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
#ifdef CONFIG_TASKS_TRACE_RCU
static atomic_t trc_n_readers_need_end; // Number of waited-for readers.
static DECLARE_WAIT_QUEUE_HEAD(trc_wait); // List of holdout tasks.
// Record outstanding IPIs to each CPU. No point in sending two...
static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
// The number of detections of task quiescent state relying on
// heavyweight readers executing explicit memory barriers.
static unsigned long n_heavy_reader_attempts;
static unsigned long n_heavy_reader_updates;
static unsigned long n_heavy_reader_ofl_updates;
void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
"RCU Tasks Trace");
/*
* This irq_work handler allows rcu_read_unlock_trace() to be invoked
* while the scheduler locks are held.
*/
static void rcu_read_unlock_iw(struct irq_work *iwp)
{
wake_up(&trc_wait);
}
static DEFINE_IRQ_WORK(rcu_tasks_trace_iw, rcu_read_unlock_iw);
/* If we are the last reader, wake up the grace-period kthread. */
void rcu_read_unlock_trace_special(struct task_struct *t)
{
int nq = READ_ONCE(t->trc_reader_special.b.need_qs);
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) &&
t->trc_reader_special.b.need_mb)
smp_mb(); // Pairs with update-side barriers.
// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
if (nq)
WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
WRITE_ONCE(t->trc_reader_nesting, 0);
if (nq && atomic_dec_and_test(&trc_n_readers_need_end))
irq_work_queue(&rcu_tasks_trace_iw);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
/* Add a task to the holdout list, if it is not already on the list. */
static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
{
if (list_empty(&t->trc_holdout_list)) {
get_task_struct(t);
list_add(&t->trc_holdout_list, bhp);
}
}
/* Remove a task from the holdout list, if it is in fact present. */
static void trc_del_holdout(struct task_struct *t)
{
if (!list_empty(&t->trc_holdout_list)) {
list_del_init(&t->trc_holdout_list);
put_task_struct(t);
}
}
/* IPI handler to check task state. */
static void trc_read_check_handler(void *t_in)
{
struct task_struct *t = current;
struct task_struct *texp = t_in;
// If the task is no longer running on this CPU, leave.
if (unlikely(texp != t)) {
goto reset_ipi; // Already on holdout list, so will check later.
}
// If the task is not in a read-side critical section, and
// if this is the last reader, awaken the grace-period kthread.
if (likely(!READ_ONCE(t->trc_reader_nesting))) {
WRITE_ONCE(t->trc_reader_checked, true);
goto reset_ipi;
}
// If we are racing with an rcu_read_unlock_trace(), try again later.
if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0))
goto reset_ipi;
WRITE_ONCE(t->trc_reader_checked, true);
// Get here if the task is in a read-side critical section. Set
// its state so that it will awaken the grace-period kthread upon
// exit from that critical section.
atomic_inc(&trc_n_readers_need_end); // One more to wait on.
WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
reset_ipi:
// Allow future IPIs to be sent on CPU and for task.
// Also order this IPI handler against any later manipulations of
// the intended task.
smp_store_release(per_cpu_ptr(&trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
}
/* Callback function for scheduler to check locked-down task. */
static int trc_inspect_reader(struct task_struct *t, void *arg)
{
int cpu = task_cpu(t);
int nesting;
bool ofl = cpu_is_offline(cpu);
if (task_curr(t)) {
WARN_ON_ONCE(ofl && !is_idle_task(t));
// If no chance of heavyweight readers, do it the hard way.
if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
return -EINVAL;
// If heavyweight readers are enabled on the remote task,
// we can inspect its state despite its currently running.
// However, we cannot safely change its state.
n_heavy_reader_attempts++;
if (!ofl && // Check for "running" idle tasks on offline CPUs.
!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
return -EINVAL; // No quiescent state, do it the hard way.
n_heavy_reader_updates++;
if (ofl)
n_heavy_reader_ofl_updates++;
nesting = 0;
} else {
// The task is not running, so C-language access is safe.
nesting = t->trc_reader_nesting;
}
// If not exiting a read-side critical section, mark as checked
// so that the grace-period kthread will remove it from the
// holdout list.
t->trc_reader_checked = nesting >= 0;
if (nesting <= 0)
return nesting ? -EINVAL : 0; // If in QS, done, otherwise try again later.
// The task is in a read-side critical section, so set up its
// state so that it will awaken the grace-period kthread upon exit
// from that critical section.
atomic_inc(&trc_n_readers_need_end); // One more to wait on.
WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
return 0;
}
/* Attempt to extract the state for the specified task. */
static void trc_wait_for_one_reader(struct task_struct *t,
struct list_head *bhp)
{
int cpu;
// If a previous IPI is still in flight, let it complete.
if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
return;
// The current task had better be in a quiescent state.
if (t == current) {
t->trc_reader_checked = true;
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
return;
}
// Attempt to nail down the task for inspection.
get_task_struct(t);
if (!task_call_func(t, trc_inspect_reader, NULL)) {
put_task_struct(t);
return;
}
put_task_struct(t);
// If this task is not yet on the holdout list, then we are in
// an RCU read-side critical section. Otherwise, the invocation of
// trc_add_holdout() that added it to the list did the necessary
// get_task_struct(). Either way, the task cannot be freed out
// from under this code.
// If currently running, send an IPI, either way, add to list.
trc_add_holdout(t, bhp);
if (task_curr(t) &&
time_after(jiffies + 1, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
// The task is currently running, so try IPIing it.
cpu = task_cpu(t);
// If there is already an IPI outstanding, let it happen.
if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
return;
per_cpu(trc_ipi_to_cpu, cpu) = true;
t->trc_ipi_to_cpu = cpu;
rcu_tasks_trace.n_ipis++;
if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
// Just in case there is some other reason for
// failure than the target CPU being offline.
WARN_ONCE(1, "%s(): smp_call_function_single() failed for CPU: %d\n",
__func__, cpu);
rcu_tasks_trace.n_ipis_fails++;
per_cpu(trc_ipi_to_cpu, cpu) = false;
t->trc_ipi_to_cpu = -1;
}
}
}
/* Initialize for a new RCU-tasks-trace grace period. */
static void rcu_tasks_trace_pregp_step(void)
{
int cpu;
// Allow for fast-acting IPIs.
atomic_set(&trc_n_readers_need_end, 1);
// There shouldn't be any old IPIs, but...
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
// Disable CPU hotplug across the tasklist scan.
// This also waits for all readers in CPU-hotplug code paths.
cpus_read_lock();
}
/* Do first-round processing for the specified task. */
static void rcu_tasks_trace_pertask(struct task_struct *t,
struct list_head *hop)
{
// During early boot when there is only the one boot CPU, there
// is no idle task for the other CPUs. Just return.
if (unlikely(t == NULL))
return;
WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
WRITE_ONCE(t->trc_reader_checked, false);
t->trc_ipi_to_cpu = -1;
trc_wait_for_one_reader(t, hop);
}
/*
* Do intermediate processing between task and holdout scans and
* pick up the idle tasks.
*/
static void rcu_tasks_trace_postscan(struct list_head *hop)
{
int cpu;
for_each_possible_cpu(cpu)
rcu_tasks_trace_pertask(idle_task(cpu), hop);
// Re-enable CPU hotplug now that the tasklist scan has completed.
cpus_read_unlock();
// Wait for late-stage exiting tasks to finish exiting.
// These might have passed the call to exit_tasks_rcu_finish().
synchronize_rcu();
// Any tasks that exit after this point will set ->trc_reader_checked.
}
/* Communicate task state back to the RCU tasks trace stall warning request. */
struct trc_stall_chk_rdr {
int nesting;
int ipi_to_cpu;
u8 needqs;
};
static int trc_check_slow_task(struct task_struct *t, void *arg)
{
struct trc_stall_chk_rdr *trc_rdrp = arg;
if (task_curr(t))
return false; // It is running, so decline to inspect it.
trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
trc_rdrp->needqs = READ_ONCE(t->trc_reader_special.b.need_qs);
return true;
}
/* Show the state of a task stalling the current RCU tasks trace GP. */
static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
{
int cpu;
struct trc_stall_chk_rdr trc_rdr;
bool is_idle_tsk = is_idle_task(t);
if (*firstreport) {
pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
*firstreport = false;
}
cpu = task_cpu(t);
if (!task_call_func(t, trc_check_slow_task, &trc_rdr))
pr_alert("P%d: %c\n",
t->pid,
".i"[is_idle_tsk]);
else
pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d\n",
t->pid,
".I"[trc_rdr.ipi_to_cpu >= 0],
".i"[is_idle_tsk],
".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
trc_rdr.nesting,
" N"[!!trc_rdr.needqs],
cpu);
sched_show_task(t);
}
/* List stalled IPIs for RCU tasks trace. */
static void show_stalled_ipi_trace(void)
{
int cpu;
for_each_possible_cpu(cpu)
if (per_cpu(trc_ipi_to_cpu, cpu))
pr_alert("\tIPI outstanding to CPU %d\n", cpu);
}
/* Do one scan of the holdout list. */
static void check_all_holdout_tasks_trace(struct list_head *hop,
bool needreport, bool *firstreport)
{
struct task_struct *g, *t;
// Disable CPU hotplug across the holdout list scan.
cpus_read_lock();
list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
// If safe and needed, try to check the current task.
if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
!READ_ONCE(t->trc_reader_checked))
trc_wait_for_one_reader(t, hop);
// If check succeeded, remove this task from the list.
if (smp_load_acquire(&t->trc_ipi_to_cpu) == -1 &&
READ_ONCE(t->trc_reader_checked))
trc_del_holdout(t);
else if (needreport)
show_stalled_task_trace(t, firstreport);
}
// Re-enable CPU hotplug now that the holdout list scan has completed.
cpus_read_unlock();
if (needreport) {
if (*firstreport)
pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
show_stalled_ipi_trace();
}
}
static void rcu_tasks_trace_empty_fn(void *unused)
{
}
/* Wait for grace period to complete and provide ordering. */
static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
{
int cpu;
bool firstreport;
struct task_struct *g, *t;
LIST_HEAD(holdouts);
long ret;
// Wait for any lingering IPI handlers to complete. Note that
// if a CPU has gone offline or transitioned to userspace in the
// meantime, all IPI handlers should have been drained beforehand.
// Yes, this assumes that CPUs process IPIs in order. If that ever
// changes, there will need to be a recheck and/or timed wait.
for_each_online_cpu(cpu)
if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
// Remove the safety count.
smp_mb__before_atomic(); // Order vs. earlier atomics
atomic_dec(&trc_n_readers_need_end);
smp_mb__after_atomic(); // Order vs. later atomics
// Wait for readers.
set_tasks_gp_state(rtp, RTGS_WAIT_READERS);
for (;;) {
ret = wait_event_idle_exclusive_timeout(
trc_wait,
atomic_read(&trc_n_readers_need_end) == 0,
READ_ONCE(rcu_task_stall_timeout));
if (ret)
break; // Count reached zero.
// Stall warning time, so make a list of the offenders.
rcu_read_lock();
for_each_process_thread(g, t)
if (READ_ONCE(t->trc_reader_special.b.need_qs))
trc_add_holdout(t, &holdouts);
rcu_read_unlock();
firstreport = true;
list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list) {
if (READ_ONCE(t->trc_reader_special.b.need_qs))
show_stalled_task_trace(t, &firstreport);
trc_del_holdout(t); // Release task_struct reference.
}
if (firstreport)
pr_err("INFO: rcu_tasks_trace detected stalls? (Counter/taskslist mismatch?)\n");
show_stalled_ipi_trace();
pr_err("\t%d holdouts\n", atomic_read(&trc_n_readers_need_end));
}
smp_mb(); // Caller's code must be ordered after wakeup.
// Pairs with pretty much every ordering primitive.
}
/* Report any needed quiescent state for this exiting task. */
static void exit_tasks_rcu_finish_trace(struct task_struct *t)
{
WRITE_ONCE(t->trc_reader_checked, true);
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
WRITE_ONCE(t->trc_reader_nesting, 0);
if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
rcu_read_unlock_trace_special(t);
}
/**
* call_rcu_tasks_trace() - Queue a callback trace task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
* @func: actual callback function to be invoked after the grace period
*
* The callback function will be invoked some time after a trace rcu-tasks
* grace period elapses, in other words after all currently executing
* trace rcu-tasks read-side critical sections have completed. These
* read-side critical sections are delimited by calls to rcu_read_lock_trace()
* and rcu_read_unlock_trace().
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
{
call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);
/**
* synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
*
* Control will return to the caller some time after a trace rcu-tasks
* grace period has elapsed, in other words after all currently executing
* trace rcu-tasks read-side critical sections have elapsed. These read-side
* critical sections are delimited by calls to rcu_read_lock_trace()
* and rcu_read_unlock_trace().
*
* This is a very specialized primitive, intended only for a few uses in
* tracing and other situations requiring manipulation of function preambles
* and profiling hooks. The synchronize_rcu_tasks_trace() function is not
* (yet) intended for heavy use from multiple CPUs.
*
* See the description of synchronize_rcu() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu_tasks_trace(void)
{
RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
synchronize_rcu_tasks_generic(&rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);
/**
* rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() callbacks.
*
* Although the current implementation is guaranteed to wait, it is not
* obligated to, for example, if there are no pending callbacks.
*/
void rcu_barrier_tasks_trace(void)
{
rcu_barrier_tasks_generic(&rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
static int __init rcu_spawn_tasks_trace_kthread(void)
{
cblist_init_generic(&rcu_tasks_trace);
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
rcu_tasks_trace.gp_sleep = HZ / 10;
rcu_tasks_trace.init_fract = HZ / 10;
} else {
rcu_tasks_trace.gp_sleep = HZ / 200;
if (rcu_tasks_trace.gp_sleep <= 0)
rcu_tasks_trace.gp_sleep = 1;
rcu_tasks_trace.init_fract = HZ / 200;
if (rcu_tasks_trace.init_fract <= 0)
rcu_tasks_trace.init_fract = 1;
}
rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
rcu_tasks_trace.pertask_func = rcu_tasks_trace_pertask;
rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
return 0;
}
#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_trace_gp_kthread(void)
{
char buf[64];
sprintf(buf, "N%d h:%lu/%lu/%lu", atomic_read(&trc_n_readers_need_end),
data_race(n_heavy_reader_ofl_updates),
data_race(n_heavy_reader_updates),
data_race(n_heavy_reader_attempts));
show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_trace_gp_kthread);
#endif // !defined(CONFIG_TINY_RCU)
#else /* #ifdef CONFIG_TASKS_TRACE_RCU */
static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
#ifndef CONFIG_TINY_RCU
void show_rcu_tasks_gp_kthreads(void)
{
show_rcu_tasks_classic_gp_kthread();
show_rcu_tasks_rude_gp_kthread();
show_rcu_tasks_trace_gp_kthread();
}
#endif /* #ifndef CONFIG_TINY_RCU */
#ifdef CONFIG_PROVE_RCU
struct rcu_tasks_test_desc {
struct rcu_head rh;
const char *name;
bool notrun;
};
static struct rcu_tasks_test_desc tests[] = {
{
.name = "call_rcu_tasks()",
/* If not defined, the test is skipped. */
.notrun = !IS_ENABLED(CONFIG_TASKS_RCU),
},
{
.name = "call_rcu_tasks_rude()",
/* If not defined, the test is skipped. */
.notrun = !IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
},
{
.name = "call_rcu_tasks_trace()",
/* If not defined, the test is skipped. */
.notrun = !IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
}
};
static void test_rcu_tasks_callback(struct rcu_head *rhp)
{
struct rcu_tasks_test_desc *rttd =
container_of(rhp, struct rcu_tasks_test_desc, rh);
pr_info("Callback from %s invoked.\n", rttd->name);
rttd->notrun = true;
}
static void rcu_tasks_initiate_self_tests(void)
{
pr_info("Running RCU-tasks wait API self tests\n");
#ifdef CONFIG_TASKS_RCU
synchronize_rcu_tasks();
call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
#endif
#ifdef CONFIG_TASKS_RUDE_RCU
synchronize_rcu_tasks_rude();
call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
#endif
#ifdef CONFIG_TASKS_TRACE_RCU
synchronize_rcu_tasks_trace();
call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
#endif
}
static int rcu_tasks_verify_self_tests(void)
{
int ret = 0;
int i;
for (i = 0; i < ARRAY_SIZE(tests); i++) {
if (!tests[i].notrun) { // still hanging.
pr_err("%s has been failed.\n", tests[i].name);
ret = -1;
}
}
if (ret)
WARN_ON(1);
return ret;
}
late_initcall(rcu_tasks_verify_self_tests);
#else /* #ifdef CONFIG_PROVE_RCU */
static void rcu_tasks_initiate_self_tests(void) { }
#endif /* #else #ifdef CONFIG_PROVE_RCU */
void __init rcu_init_tasks_generic(void)
{
#ifdef CONFIG_TASKS_RCU
rcu_spawn_tasks_kthread();
#endif
#ifdef CONFIG_TASKS_RUDE_RCU
rcu_spawn_tasks_rude_kthread();
#endif
#ifdef CONFIG_TASKS_TRACE_RCU
rcu_spawn_tasks_trace_kthread();
#endif
// Run the self-tests.
rcu_tasks_initiate_self_tests();
}
#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
static inline void rcu_tasks_bootup_oddness(void) {}
#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */