linux/fs/btrfs/async-thread.c
Tejun Heo 58e814fcac btrfs: use alloc_ordered_workqueue() to create ordered workqueues
BACKGROUND
==========

When multiple work items are queued to a workqueue, their execution order
doesn't match the queueing order. They may get executed in any order and
simultaneously. When fully serialized execution - one by one in the queueing
order - is needed, an ordered workqueue should be used which can be created
with alloc_ordered_workqueue().

However, alloc_ordered_workqueue() was a later addition. Before it, an
ordered workqueue could be obtained by creating an UNBOUND workqueue with
@max_active==1. This originally was an implementation side-effect which was
broken by 4c16bd327c ("workqueue: restore WQ_UNBOUND/max_active==1 to be
ordered"). Because there were users that depended on the ordered execution,
5c0338c687 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")
made workqueue allocation path to implicitly promote UNBOUND workqueues w/
@max_active==1 to ordered workqueues.

While this has worked okay, overloading the UNBOUND allocation interface
this way creates other issues. It's difficult to tell whether a given
workqueue actually needs to be ordered and users that legitimately want a
min concurrency level wq unexpectedly gets an ordered one instead. With
planned UNBOUND workqueue updates to improve execution locality and more
prevalence of chiplet designs which can benefit from such improvements, this
isn't a state we wanna be in forever.

This patch series audits all call sites that create an UNBOUND workqueue w/
@max_active==1 and converts them to alloc_ordered_workqueue() as necessary.

BTRFS
=====

* fs_info->scrub_workers initialized in scrub_workers_get() was setting
  @max_active to 1 when @is_dev_replace is set and it seems that the
  workqueue actually needs to be ordered if @is_dev_replace. Update the code
  so that alloc_ordered_workqueue() is used if @is_dev_replace.

* fs_info->discard_ctl.discard_workers initialized in
  btrfs_init_workqueues() was directly using alloc_workqueue() w/
  @max_active==1. Converted to alloc_ordered_workqueue().

* fs_info->fixup_workers and fs_info->qgroup_rescan_workers initialized in
  btrfs_queue_work() use the btrfs's workqueue wrapper, btrfs_workqueue,
  which are allocated with btrfs_alloc_workqueue().

  btrfs_workqueue implements automatic @max_active adjustment which is
  disabled when the specified max limit is below a certain threshold, so
  calling btrfs_alloc_workqueue() with @limit_active==1 yields an ordered
  workqueue whose @max_active won't be changed as the auto-tuning is
  disabled.

  This is rather brittle in that nothing clearly indicates that the two
  workqueues should be ordered or btrfs_alloc_workqueue() must disable
  auto-tuning when @limit_active==1.

  This patch factors out the common btrfs_workqueue init code into
  btrfs_init_workqueue() and add explicit btrfs_alloc_ordered_workqueue().
  The two workqueues are converted to use the new ordered allocation
  interface.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>
2023-06-19 13:59:30 +02:00

376 lines
9.7 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (C) 2007 Oracle. All rights reserved.
* Copyright (C) 2014 Fujitsu. All rights reserved.
*/
#include <linux/kthread.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/freezer.h>
#include "async-thread.h"
#include "ctree.h"
enum {
WORK_DONE_BIT,
WORK_ORDER_DONE_BIT,
};
#define NO_THRESHOLD (-1)
#define DFT_THRESHOLD (32)
struct btrfs_workqueue {
struct workqueue_struct *normal_wq;
/* File system this workqueue services */
struct btrfs_fs_info *fs_info;
/* List head pointing to ordered work list */
struct list_head ordered_list;
/* Spinlock for ordered_list */
spinlock_t list_lock;
/* Thresholding related variants */
atomic_t pending;
/* Up limit of concurrency workers */
int limit_active;
/* Current number of concurrency workers */
int current_active;
/* Threshold to change current_active */
int thresh;
unsigned int count;
spinlock_t thres_lock;
};
struct btrfs_fs_info * __pure btrfs_workqueue_owner(const struct btrfs_workqueue *wq)
{
return wq->fs_info;
}
struct btrfs_fs_info * __pure btrfs_work_owner(const struct btrfs_work *work)
{
return work->wq->fs_info;
}
bool btrfs_workqueue_normal_congested(const struct btrfs_workqueue *wq)
{
/*
* We could compare wq->pending with num_online_cpus()
* to support "thresh == NO_THRESHOLD" case, but it requires
* moving up atomic_inc/dec in thresh_queue/exec_hook. Let's
* postpone it until someone needs the support of that case.
*/
if (wq->thresh == NO_THRESHOLD)
return false;
return atomic_read(&wq->pending) > wq->thresh * 2;
}
static void btrfs_init_workqueue(struct btrfs_workqueue *wq,
struct btrfs_fs_info *fs_info)
{
wq->fs_info = fs_info;
atomic_set(&wq->pending, 0);
INIT_LIST_HEAD(&wq->ordered_list);
spin_lock_init(&wq->list_lock);
spin_lock_init(&wq->thres_lock);
}
struct btrfs_workqueue *btrfs_alloc_workqueue(struct btrfs_fs_info *fs_info,
const char *name, unsigned int flags,
int limit_active, int thresh)
{
struct btrfs_workqueue *ret = kzalloc(sizeof(*ret), GFP_KERNEL);
if (!ret)
return NULL;
btrfs_init_workqueue(ret, fs_info);
ret->limit_active = limit_active;
if (thresh == 0)
thresh = DFT_THRESHOLD;
/* For low threshold, disabling threshold is a better choice */
if (thresh < DFT_THRESHOLD) {
ret->current_active = limit_active;
ret->thresh = NO_THRESHOLD;
} else {
/*
* For threshold-able wq, let its concurrency grow on demand.
* Use minimal max_active at alloc time to reduce resource
* usage.
*/
ret->current_active = 1;
ret->thresh = thresh;
}
ret->normal_wq = alloc_workqueue("btrfs-%s", flags, ret->current_active,
name);
if (!ret->normal_wq) {
kfree(ret);
return NULL;
}
trace_btrfs_workqueue_alloc(ret, name);
return ret;
}
struct btrfs_workqueue *btrfs_alloc_ordered_workqueue(
struct btrfs_fs_info *fs_info, const char *name,
unsigned int flags)
{
struct btrfs_workqueue *ret;
ret = kzalloc(sizeof(*ret), GFP_KERNEL);
if (!ret)
return NULL;
btrfs_init_workqueue(ret, fs_info);
/* Ordered workqueues don't allow @max_active adjustments. */
ret->limit_active = 1;
ret->current_active = 1;
ret->thresh = NO_THRESHOLD;
ret->normal_wq = alloc_ordered_workqueue("btrfs-%s", flags, name);
if (!ret->normal_wq) {
kfree(ret);
return NULL;
}
trace_btrfs_workqueue_alloc(ret, name);
return ret;
}
/*
* Hook for threshold which will be called in btrfs_queue_work.
* This hook WILL be called in IRQ handler context,
* so workqueue_set_max_active MUST NOT be called in this hook
*/
static inline void thresh_queue_hook(struct btrfs_workqueue *wq)
{
if (wq->thresh == NO_THRESHOLD)
return;
atomic_inc(&wq->pending);
}
/*
* Hook for threshold which will be called before executing the work,
* This hook is called in kthread content.
* So workqueue_set_max_active is called here.
*/
static inline void thresh_exec_hook(struct btrfs_workqueue *wq)
{
int new_current_active;
long pending;
int need_change = 0;
if (wq->thresh == NO_THRESHOLD)
return;
atomic_dec(&wq->pending);
spin_lock(&wq->thres_lock);
/*
* Use wq->count to limit the calling frequency of
* workqueue_set_max_active.
*/
wq->count++;
wq->count %= (wq->thresh / 4);
if (!wq->count)
goto out;
new_current_active = wq->current_active;
/*
* pending may be changed later, but it's OK since we really
* don't need it so accurate to calculate new_max_active.
*/
pending = atomic_read(&wq->pending);
if (pending > wq->thresh)
new_current_active++;
if (pending < wq->thresh / 2)
new_current_active--;
new_current_active = clamp_val(new_current_active, 1, wq->limit_active);
if (new_current_active != wq->current_active) {
need_change = 1;
wq->current_active = new_current_active;
}
out:
spin_unlock(&wq->thres_lock);
if (need_change) {
workqueue_set_max_active(wq->normal_wq, wq->current_active);
}
}
static void run_ordered_work(struct btrfs_workqueue *wq,
struct btrfs_work *self)
{
struct list_head *list = &wq->ordered_list;
struct btrfs_work *work;
spinlock_t *lock = &wq->list_lock;
unsigned long flags;
bool free_self = false;
while (1) {
spin_lock_irqsave(lock, flags);
if (list_empty(list))
break;
work = list_entry(list->next, struct btrfs_work,
ordered_list);
if (!test_bit(WORK_DONE_BIT, &work->flags))
break;
/*
* Orders all subsequent loads after reading WORK_DONE_BIT,
* paired with the smp_mb__before_atomic in btrfs_work_helper
* this guarantees that the ordered function will see all
* updates from ordinary work function.
*/
smp_rmb();
/*
* we are going to call the ordered done function, but
* we leave the work item on the list as a barrier so
* that later work items that are done don't have their
* functions called before this one returns
*/
if (test_and_set_bit(WORK_ORDER_DONE_BIT, &work->flags))
break;
trace_btrfs_ordered_sched(work);
spin_unlock_irqrestore(lock, flags);
work->ordered_func(work);
/* now take the lock again and drop our item from the list */
spin_lock_irqsave(lock, flags);
list_del(&work->ordered_list);
spin_unlock_irqrestore(lock, flags);
if (work == self) {
/*
* This is the work item that the worker is currently
* executing.
*
* The kernel workqueue code guarantees non-reentrancy
* of work items. I.e., if a work item with the same
* address and work function is queued twice, the second
* execution is blocked until the first one finishes. A
* work item may be freed and recycled with the same
* work function; the workqueue code assumes that the
* original work item cannot depend on the recycled work
* item in that case (see find_worker_executing_work()).
*
* Note that different types of Btrfs work can depend on
* each other, and one type of work on one Btrfs
* filesystem may even depend on the same type of work
* on another Btrfs filesystem via, e.g., a loop device.
* Therefore, we must not allow the current work item to
* be recycled until we are really done, otherwise we
* break the above assumption and can deadlock.
*/
free_self = true;
} else {
/*
* We don't want to call the ordered free functions with
* the lock held.
*/
work->ordered_free(work);
/* NB: work must not be dereferenced past this point. */
trace_btrfs_all_work_done(wq->fs_info, work);
}
}
spin_unlock_irqrestore(lock, flags);
if (free_self) {
self->ordered_free(self);
/* NB: self must not be dereferenced past this point. */
trace_btrfs_all_work_done(wq->fs_info, self);
}
}
static void btrfs_work_helper(struct work_struct *normal_work)
{
struct btrfs_work *work = container_of(normal_work, struct btrfs_work,
normal_work);
struct btrfs_workqueue *wq = work->wq;
int need_order = 0;
/*
* We should not touch things inside work in the following cases:
* 1) after work->func() if it has no ordered_free
* Since the struct is freed in work->func().
* 2) after setting WORK_DONE_BIT
* The work may be freed in other threads almost instantly.
* So we save the needed things here.
*/
if (work->ordered_func)
need_order = 1;
trace_btrfs_work_sched(work);
thresh_exec_hook(wq);
work->func(work);
if (need_order) {
/*
* Ensures all memory accesses done in the work function are
* ordered before setting the WORK_DONE_BIT. Ensuring the thread
* which is going to executed the ordered work sees them.
* Pairs with the smp_rmb in run_ordered_work.
*/
smp_mb__before_atomic();
set_bit(WORK_DONE_BIT, &work->flags);
run_ordered_work(wq, work);
} else {
/* NB: work must not be dereferenced past this point. */
trace_btrfs_all_work_done(wq->fs_info, work);
}
}
void btrfs_init_work(struct btrfs_work *work, btrfs_func_t func,
btrfs_func_t ordered_func, btrfs_func_t ordered_free)
{
work->func = func;
work->ordered_func = ordered_func;
work->ordered_free = ordered_free;
INIT_WORK(&work->normal_work, btrfs_work_helper);
INIT_LIST_HEAD(&work->ordered_list);
work->flags = 0;
}
void btrfs_queue_work(struct btrfs_workqueue *wq, struct btrfs_work *work)
{
unsigned long flags;
work->wq = wq;
thresh_queue_hook(wq);
if (work->ordered_func) {
spin_lock_irqsave(&wq->list_lock, flags);
list_add_tail(&work->ordered_list, &wq->ordered_list);
spin_unlock_irqrestore(&wq->list_lock, flags);
}
trace_btrfs_work_queued(work);
queue_work(wq->normal_wq, &work->normal_work);
}
void btrfs_destroy_workqueue(struct btrfs_workqueue *wq)
{
if (!wq)
return;
destroy_workqueue(wq->normal_wq);
trace_btrfs_workqueue_destroy(wq);
kfree(wq);
}
void btrfs_workqueue_set_max(struct btrfs_workqueue *wq, int limit_active)
{
if (wq)
wq->limit_active = limit_active;
}
void btrfs_flush_workqueue(struct btrfs_workqueue *wq)
{
flush_workqueue(wq->normal_wq);
}