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rcu: Move expedited grace period (GP) work to RT kthread_worker
Enabling CONFIG_RCU_BOOST did not reduce RCU expedited grace-period latency because its workqueues run at SCHED_OTHER, and thus can be delayed by normal processes. This commit avoids these delays by moving the expedited GP work items to a real-time-priority kthread_worker. This option is controlled by CONFIG_RCU_EXP_KTHREAD and disabled by default on PREEMPT_RT=y kernels which disable expedited grace periods after boot by unconditionally setting rcupdate.rcu_normal_after_boot=1. The results were evaluated on arm64 Android devices (6GB ram) running 5.10 kernel, and capturing trace data in critical user-level code. The table below shows the resulting order-of-magnitude improvements in synchronize_rcu_expedited() latency: ------------------------------------------------------------------------ | | workqueues | kthread_worker | Diff | ------------------------------------------------------------------------ | Count | 725 | 688 | | ------------------------------------------------------------------------ | Min Duration (ns) | 326 | 447 | 37.12% | ------------------------------------------------------------------------ | Q1 (ns) | 39,428 | 38,971 | -1.16% | ------------------------------------------------------------------------ | Q2 - Median (ns) | 98,225 | 69,743 | -29.00% | ------------------------------------------------------------------------ | Q3 (ns) | 342,122 | 126,638 | -62.98% | ------------------------------------------------------------------------ | Max Duration (ns) | 372,766,967 | 2,329,671 | -99.38% | ------------------------------------------------------------------------ | Avg Duration (ns) | 2,746,353 | 151,242 | -94.49% | ------------------------------------------------------------------------ | Standard Deviation (ns) | 19,327,765 | 294,408 | | ------------------------------------------------------------------------ The below table show the range of maximums/minimums for synchronize_rcu_expedited() latency from all experiments: ------------------------------------------------------------------------ | | workqueues | kthread_worker | Diff | ------------------------------------------------------------------------ | Total No. of Experiments | 25 | 23 | | ------------------------------------------------------------------------ | Largest Maximum (ns) | 372,766,967 | 2,329,671 | -99.38% | ------------------------------------------------------------------------ | Smallest Maximum (ns) | 38,819 | 86,954 | 124.00% | ------------------------------------------------------------------------ | Range of Maximums (ns) | 372,728,148 | 2,242,717 | | ------------------------------------------------------------------------ | Largest Minimum (ns) | 88,623 | 27,588 | -68.87% | ------------------------------------------------------------------------ | Smallest Minimum (ns) | 326 | 447 | 37.12% | ------------------------------------------------------------------------ | Range of Minimums (ns) | 88,297 | 27,141 | | ------------------------------------------------------------------------ Cc: "Paul E. McKenney" <paulmck@kernel.org> Cc: Tejun Heo <tj@kernel.org> Reported-by: Tim Murray <timmurray@google.com> Reported-by: Wei Wang <wvw@google.com> Tested-by: Kyle Lin <kylelin@google.com> Tested-by: Chunwei Lu <chunweilu@google.com> Tested-by: Lulu Wang <luluw@google.com> Signed-off-by: Kalesh Singh <kaleshsingh@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Kalesh Singh
Paul E. McKenney
parent
28b3ae4265
commit
9621fbee44
@ -195,6 +195,20 @@ config RCU_BOOST_DELAY
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Accept the default if unsure.
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config RCU_EXP_KTHREAD
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bool "Perform RCU expedited work in a real-time kthread"
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depends on RCU_BOOST && RCU_EXPERT
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default !PREEMPT_RT && NR_CPUS <= 32
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help
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Use this option to further reduce the latencies of expedited
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grace periods at the expense of being more disruptive.
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This option is disabled by default on PREEMPT_RT=y kernels which
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disable expedited grace periods after boot by unconditionally
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setting rcupdate.rcu_normal_after_boot=1.
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Accept the default if unsure.
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config RCU_NOCB_CPU
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bool "Offload RCU callback processing from boot-selected CPUs"
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depends on TREE_RCU
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@ -536,7 +536,12 @@ int rcu_get_gp_kthreads_prio(void);
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void rcu_fwd_progress_check(unsigned long j);
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void rcu_force_quiescent_state(void);
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extern struct workqueue_struct *rcu_gp_wq;
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#ifdef CONFIG_RCU_EXP_KTHREAD
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extern struct kthread_worker *rcu_exp_gp_kworker;
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extern struct kthread_worker *rcu_exp_par_gp_kworker;
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#else /* !CONFIG_RCU_EXP_KTHREAD */
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extern struct workqueue_struct *rcu_par_gp_wq;
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#endif /* CONFIG_RCU_EXP_KTHREAD */
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#endif /* #else #ifdef CONFIG_TINY_RCU */
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#ifdef CONFIG_RCU_NOCB_CPU
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@ -4471,6 +4471,51 @@ static int rcu_pm_notify(struct notifier_block *self,
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return NOTIFY_OK;
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}
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#ifdef CONFIG_RCU_EXP_KTHREAD
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struct kthread_worker *rcu_exp_gp_kworker;
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struct kthread_worker *rcu_exp_par_gp_kworker;
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static void __init rcu_start_exp_gp_kworkers(void)
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{
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const char *par_gp_kworker_name = "rcu_exp_par_gp_kthread_worker";
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const char *gp_kworker_name = "rcu_exp_gp_kthread_worker";
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struct sched_param param = { .sched_priority = kthread_prio };
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rcu_exp_gp_kworker = kthread_create_worker(0, gp_kworker_name);
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if (IS_ERR_OR_NULL(rcu_exp_gp_kworker)) {
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pr_err("Failed to create %s!\n", gp_kworker_name);
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return;
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}
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rcu_exp_par_gp_kworker = kthread_create_worker(0, par_gp_kworker_name);
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if (IS_ERR_OR_NULL(rcu_exp_par_gp_kworker)) {
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pr_err("Failed to create %s!\n", par_gp_kworker_name);
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kthread_destroy_worker(rcu_exp_gp_kworker);
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return;
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}
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sched_setscheduler_nocheck(rcu_exp_gp_kworker->task, SCHED_FIFO, ¶m);
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sched_setscheduler_nocheck(rcu_exp_par_gp_kworker->task, SCHED_FIFO,
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¶m);
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}
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static inline void rcu_alloc_par_gp_wq(void)
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{
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}
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#else /* !CONFIG_RCU_EXP_KTHREAD */
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struct workqueue_struct *rcu_par_gp_wq;
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static void __init rcu_start_exp_gp_kworkers(void)
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{
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}
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static inline void rcu_alloc_par_gp_wq(void)
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{
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rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
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WARN_ON(!rcu_par_gp_wq);
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}
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#endif /* CONFIG_RCU_EXP_KTHREAD */
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/*
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* Spawn the kthreads that handle RCU's grace periods.
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*/
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@ -4500,6 +4545,8 @@ static int __init rcu_spawn_gp_kthread(void)
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rcu_spawn_nocb_kthreads();
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rcu_spawn_boost_kthreads();
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rcu_spawn_core_kthreads();
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/* Create kthread worker for expedited GPs */
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rcu_start_exp_gp_kworkers();
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return 0;
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}
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early_initcall(rcu_spawn_gp_kthread);
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@ -4745,7 +4792,6 @@ static void __init rcu_dump_rcu_node_tree(void)
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}
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struct workqueue_struct *rcu_gp_wq;
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struct workqueue_struct *rcu_par_gp_wq;
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static void __init kfree_rcu_batch_init(void)
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{
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@ -4811,8 +4857,7 @@ void __init rcu_init(void)
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/* Create workqueue for Tree SRCU and for expedited GPs. */
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rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
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WARN_ON(!rcu_gp_wq);
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rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
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WARN_ON(!rcu_par_gp_wq);
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rcu_alloc_par_gp_wq();
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/* Fill in default value for rcutree.qovld boot parameter. */
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/* -After- the rcu_node ->lock fields are initialized! */
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@ -10,6 +10,7 @@
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*/
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#include <linux/cache.h>
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#include <linux/kthread.h>
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#include <linux/spinlock.h>
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#include <linux/rtmutex.h>
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#include <linux/threads.h>
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@ -23,7 +24,11 @@
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/* Communicate arguments to a workqueue handler. */
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struct rcu_exp_work {
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unsigned long rew_s;
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#ifdef CONFIG_RCU_EXP_KTHREAD
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struct kthread_work rew_work;
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#else
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struct work_struct rew_work;
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#endif /* CONFIG_RCU_EXP_KTHREAD */
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};
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/* RCU's kthread states for tracing. */
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@ -334,15 +334,13 @@ fastpath:
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* Select the CPUs within the specified rcu_node that the upcoming
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* expedited grace period needs to wait for.
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*/
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static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
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static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
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{
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int cpu;
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unsigned long flags;
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unsigned long mask_ofl_test;
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unsigned long mask_ofl_ipi;
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int ret;
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struct rcu_exp_work *rewp =
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container_of(wp, struct rcu_exp_work, rew_work);
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struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
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raw_spin_lock_irqsave_rcu_node(rnp, flags);
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@ -417,13 +415,119 @@ retry_ipi:
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rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
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}
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static void rcu_exp_sel_wait_wake(unsigned long s);
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#ifdef CONFIG_RCU_EXP_KTHREAD
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static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
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{
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struct rcu_exp_work *rewp =
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container_of(wp, struct rcu_exp_work, rew_work);
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__sync_rcu_exp_select_node_cpus(rewp);
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}
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static inline bool rcu_gp_par_worker_started(void)
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{
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return !!READ_ONCE(rcu_exp_par_gp_kworker);
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}
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static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
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{
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kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
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/*
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* Use rcu_exp_par_gp_kworker, because flushing a work item from
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* another work item on the same kthread worker can result in
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* deadlock.
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*/
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kthread_queue_work(rcu_exp_par_gp_kworker, &rnp->rew.rew_work);
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}
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static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
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{
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kthread_flush_work(&rnp->rew.rew_work);
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}
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/*
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* Work-queue handler to drive an expedited grace period forward.
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*/
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static void wait_rcu_exp_gp(struct kthread_work *wp)
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{
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struct rcu_exp_work *rewp;
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rewp = container_of(wp, struct rcu_exp_work, rew_work);
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rcu_exp_sel_wait_wake(rewp->rew_s);
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}
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static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
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{
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kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
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kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
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}
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static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
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{
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}
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#else /* !CONFIG_RCU_EXP_KTHREAD */
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static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
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{
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struct rcu_exp_work *rewp =
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container_of(wp, struct rcu_exp_work, rew_work);
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__sync_rcu_exp_select_node_cpus(rewp);
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}
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static inline bool rcu_gp_par_worker_started(void)
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{
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return !!READ_ONCE(rcu_par_gp_wq);
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}
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static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
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{
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int cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
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INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
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/* If all offline, queue the work on an unbound CPU. */
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if (unlikely(cpu > rnp->grphi - rnp->grplo))
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cpu = WORK_CPU_UNBOUND;
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else
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cpu += rnp->grplo;
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queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
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}
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static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
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{
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flush_work(&rnp->rew.rew_work);
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}
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/*
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* Work-queue handler to drive an expedited grace period forward.
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*/
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static void wait_rcu_exp_gp(struct work_struct *wp)
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{
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struct rcu_exp_work *rewp;
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rewp = container_of(wp, struct rcu_exp_work, rew_work);
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rcu_exp_sel_wait_wake(rewp->rew_s);
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}
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static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
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{
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INIT_WORK_ONSTACK(&rew->rew_work, wait_rcu_exp_gp);
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queue_work(rcu_gp_wq, &rew->rew_work);
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}
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static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
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{
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destroy_work_on_stack(&rew->rew_work);
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}
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#endif /* CONFIG_RCU_EXP_KTHREAD */
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/*
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* Select the nodes that the upcoming expedited grace period needs
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* to wait for.
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*/
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static void sync_rcu_exp_select_cpus(void)
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{
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int cpu;
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struct rcu_node *rnp;
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trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
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@ -435,28 +539,21 @@ static void sync_rcu_exp_select_cpus(void)
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rnp->exp_need_flush = false;
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if (!READ_ONCE(rnp->expmask))
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continue; /* Avoid early boot non-existent wq. */
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if (!READ_ONCE(rcu_par_gp_wq) ||
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if (!rcu_gp_par_worker_started() ||
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rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
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rcu_is_last_leaf_node(rnp)) {
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/* No workqueues yet or last leaf, do direct call. */
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/* No worker started yet or last leaf, do direct call. */
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sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
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continue;
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}
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INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
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cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
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/* If all offline, queue the work on an unbound CPU. */
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if (unlikely(cpu > rnp->grphi - rnp->grplo))
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cpu = WORK_CPU_UNBOUND;
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else
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cpu += rnp->grplo;
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queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
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sync_rcu_exp_select_cpus_queue_work(rnp);
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rnp->exp_need_flush = true;
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}
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/* Wait for workqueue jobs (if any) to complete. */
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/* Wait for jobs (if any) to complete. */
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rcu_for_each_leaf_node(rnp)
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if (rnp->exp_need_flush)
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flush_work(&rnp->rew.rew_work);
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sync_rcu_exp_select_cpus_flush_work(rnp);
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}
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/*
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@ -622,17 +719,6 @@ static void rcu_exp_sel_wait_wake(unsigned long s)
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rcu_exp_wait_wake(s);
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}
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/*
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* Work-queue handler to drive an expedited grace period forward.
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*/
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static void wait_rcu_exp_gp(struct work_struct *wp)
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{
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struct rcu_exp_work *rewp;
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rewp = container_of(wp, struct rcu_exp_work, rew_work);
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rcu_exp_sel_wait_wake(rewp->rew_s);
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}
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#ifdef CONFIG_PREEMPT_RCU
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/*
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@ -848,20 +934,19 @@ void synchronize_rcu_expedited(void)
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} else {
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/* Marshall arguments & schedule the expedited grace period. */
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rew.rew_s = s;
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INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
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queue_work(rcu_gp_wq, &rew.rew_work);
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synchronize_rcu_expedited_queue_work(&rew);
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}
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/* Wait for expedited grace period to complete. */
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rnp = rcu_get_root();
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wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
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sync_exp_work_done(s));
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smp_mb(); /* Workqueue actions happen before return. */
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smp_mb(); /* Work actions happen before return. */
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/* Let the next expedited grace period start. */
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mutex_unlock(&rcu_state.exp_mutex);
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if (likely(!boottime))
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destroy_work_on_stack(&rew.rew_work);
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synchronize_rcu_expedited_destroy_work(&rew);
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}
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EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
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