Date:   Mon, 11 Apr 2022 17:57:36 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     surenb@...gle.com, kernel-team@...roid.com,
        Tejun Heo <tj@...nel.org>, Tim Murray <timmurray@...gle.com>,
        Wei Wang <wvw@...gle.com>, Kyle Lin <kylelin@...gle.com>,
        Chunwei Lu <chunweilu@...gle.com>,
        Lulu Wang <luluw@...gle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] EXP rcu: Move expedited grace period (GP) work to RT
 kthread_worker

On Fri, Apr 08, 2022 at 05:35:27PM -0700, Kalesh Singh wrote:
> 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@...nel.org>
> Cc: Tejun Heo <tj@...nel.org>
> Reported-by: Tim Murray <timmurray@...gle.com>
> Reported-by: Wei Wang <wvw@...gle.com>
> Tested-by: Kyle Lin <kylelin@...gle.com>
> Tested-by: Chunwei Lu <chunweilu@...gle.com>
> Tested-by: Lulu Wang <luluw@...gle.com>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>

And I have placed this on -rcu experimental branch fastexp.2022.04.11a.
If testing goes well, I will merge it in!

Thank you all!

							Thanx, Paul

> ---
> 
> Changes in v3:
>   - Update Kconfig and commit text describing default RCU_EXP_KTHREAD value
>     for PREEMT_RT kernels; per Steve
> 
> Changes in v2:
>   - Add stats on variation: standard deviation, range of maxs, etc; per Paul
>   - Make this optimization configurable (CONFIG_RCU_EXP_KTHREAD); per Paul
> 
> 
>  kernel/rcu/Kconfig    |  14 ++++
>  kernel/rcu/rcu.h      |   5 ++
>  kernel/rcu/tree.c     |  51 ++++++++++++++-
>  kernel/rcu/tree.h     |   5 ++
>  kernel/rcu/tree_exp.h | 147 +++++++++++++++++++++++++++++++++---------
>  5 files changed, 188 insertions(+), 34 deletions(-)
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index bf8e341e75b4..fd64a75823cb 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -195,6 +195,20 @@ config RCU_BOOST_DELAY
>  
>  	  Accept the default if unsure.
>  
> +config RCU_EXP_KTHREAD
> +	bool "Perform RCU expedited work in a real-time kthread"
> +	depends on RCU_BOOST && RCU_EXPERT
> +	default !PREEMPT_RT && NR_CPUS <= 32
> +	help
> +	  Use this option to further reduce the latencies of expedited
> +	  grace periods at the expense of being more disruptive.
> +
> +	  This option is disabled by default on PREEMPT_RT=y kernels which
> +	  disable expedited grace periods after boot by unconditionally
> +	  setting rcupdate.rcu_normal_after_boot=1.
> +
> +	  Accept the default if unsure.
> +
>  config RCU_NOCB_CPU
>  	bool "Offload RCU callback processing from boot-selected CPUs"
>  	depends on TREE_RCU
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 24b5f2c2de87..5510d2231c32 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -534,7 +534,12 @@ int rcu_get_gp_kthreads_prio(void);
>  void rcu_fwd_progress_check(unsigned long j);
>  void rcu_force_quiescent_state(void);
>  extern struct workqueue_struct *rcu_gp_wq;
> +#ifdef CONFIG_RCU_EXP_KTHREAD
> +extern struct kthread_worker *rcu_exp_gp_kworker;
> +extern struct kthread_worker *rcu_exp_par_gp_kworker;
> +#else /* !CONFIG_RCU_EXP_KTHREAD */
>  extern struct workqueue_struct *rcu_par_gp_wq;
> +#endif /* CONFIG_RCU_EXP_KTHREAD */
>  #endif /* #else #ifdef CONFIG_TINY_RCU */
>  
>  #ifdef CONFIG_RCU_NOCB_CPU
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a4b8189455d5..763e45fdf49b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4471,6 +4471,51 @@ static int rcu_pm_notify(struct notifier_block *self,
>  	return NOTIFY_OK;
>  }
>  
> +#ifdef CONFIG_RCU_EXP_KTHREAD
> +struct kthread_worker *rcu_exp_gp_kworker;
> +struct kthread_worker *rcu_exp_par_gp_kworker;
> +
> +static void __init rcu_start_exp_gp_kworkers(void)
> +{
> +	const char *par_gp_kworker_name = "rcu_exp_par_gp_kthread_worker";
> +	const char *gp_kworker_name = "rcu_exp_gp_kthread_worker";
> +	struct sched_param param = { .sched_priority = kthread_prio };
> +
> +	rcu_exp_gp_kworker = kthread_create_worker(0, gp_kworker_name);
> +	if (IS_ERR_OR_NULL(rcu_exp_gp_kworker)) {
> +		pr_err("Failed to create %s!\n", gp_kworker_name);
> +		return;
> +	}
> +
> +	rcu_exp_par_gp_kworker = kthread_create_worker(0, par_gp_kworker_name);
> +	if (IS_ERR_OR_NULL(rcu_exp_par_gp_kworker)) {
> +		pr_err("Failed to create %s!\n", par_gp_kworker_name);
> +		kthread_destroy_worker(rcu_exp_gp_kworker);
> +		return;
> +	}
> +
> +	sched_setscheduler_nocheck(rcu_exp_gp_kworker->task, SCHED_FIFO, &param);
> +	sched_setscheduler_nocheck(rcu_exp_par_gp_kworker->task, SCHED_FIFO,
> +				   &param);
> +}
> +
> +static inline void rcu_alloc_par_gp_wq(void)
> +{
> +}
> +#else /* !CONFIG_RCU_EXP_KTHREAD */
> +struct workqueue_struct *rcu_par_gp_wq;
> +
> +static void __init rcu_start_exp_gp_kworkers(void)
> +{
> +}
> +
> +static inline void rcu_alloc_par_gp_wq(void)
> +{
> +	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
> +	WARN_ON(!rcu_par_gp_wq);
> +}
> +#endif /* CONFIG_RCU_EXP_KTHREAD */
> +
>  /*
>   * Spawn the kthreads that handle RCU's grace periods.
>   */
> @@ -4500,6 +4545,8 @@ static int __init rcu_spawn_gp_kthread(void)
>  	rcu_spawn_nocb_kthreads();
>  	rcu_spawn_boost_kthreads();
>  	rcu_spawn_core_kthreads();
> +	/* Create kthread worker for expedited GPs */
> +	rcu_start_exp_gp_kworkers();
>  	return 0;
>  }
>  early_initcall(rcu_spawn_gp_kthread);
> @@ -4745,7 +4792,6 @@ static void __init rcu_dump_rcu_node_tree(void)
>  }
>  
>  struct workqueue_struct *rcu_gp_wq;
> -struct workqueue_struct *rcu_par_gp_wq;
>  
>  static void __init kfree_rcu_batch_init(void)
>  {
> @@ -4811,8 +4857,7 @@ void __init rcu_init(void)
>  	/* Create workqueue for Tree SRCU and for expedited GPs. */
>  	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
>  	WARN_ON(!rcu_gp_wq);
> -	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
> -	WARN_ON(!rcu_par_gp_wq);
> +	rcu_alloc_par_gp_wq();
>  
>  	/* Fill in default value for rcutree.qovld boot parameter. */
>  	/* -After- the rcu_node ->lock fields are initialized! */
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 926673ebe355..b577cdfdc851 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -10,6 +10,7 @@
>   */
>  
>  #include <linux/cache.h>
> +#include <linux/kthread.h>
>  #include <linux/spinlock.h>
>  #include <linux/rtmutex.h>
>  #include <linux/threads.h>
> @@ -23,7 +24,11 @@
>  /* Communicate arguments to a workqueue handler. */
>  struct rcu_exp_work {
>  	unsigned long rew_s;
> +#ifdef CONFIG_RCU_EXP_KTHREAD
> +	struct kthread_work rew_work;
> +#else
>  	struct work_struct rew_work;
> +#endif /* CONFIG_RCU_EXP_KTHREAD */
>  };
>  
>  /* RCU's kthread states for tracing. */
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 60197ea24ceb..2c6c0e0bc83a 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -334,15 +334,13 @@ static bool exp_funnel_lock(unsigned long s)
>   * Select the CPUs within the specified rcu_node that the upcoming
>   * expedited grace period needs to wait for.
>   */
> -static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> +static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
>  {
>  	int cpu;
>  	unsigned long flags;
>  	unsigned long mask_ofl_test;
>  	unsigned long mask_ofl_ipi;
>  	int ret;
> -	struct rcu_exp_work *rewp =
> -		container_of(wp, struct rcu_exp_work, rew_work);
>  	struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
>  
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> @@ -417,13 +415,119 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>  		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
>  }
>  
> +static void rcu_exp_sel_wait_wake(unsigned long s);
> +
> +#ifdef CONFIG_RCU_EXP_KTHREAD
> +static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
> +{
> +	struct rcu_exp_work *rewp =
> +		container_of(wp, struct rcu_exp_work, rew_work);
> +
> +	__sync_rcu_exp_select_node_cpus(rewp);
> +}
> +
> +static inline bool rcu_gp_par_worker_started(void)
> +{
> +	return !!READ_ONCE(rcu_exp_par_gp_kworker);
> +}
> +
> +static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
> +{
> +	kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> +	/*
> +	 * Use rcu_exp_par_gp_kworker, because flushing a work item from
> +	 * another work item on the same kthread worker can result in
> +	 * deadlock.
> +	 */
> +	kthread_queue_work(rcu_exp_par_gp_kworker, &rnp->rew.rew_work);
> +}
> +
> +static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
> +{
> +	kthread_flush_work(&rnp->rew.rew_work);
> +}
> +
> +/*
> + * Work-queue handler to drive an expedited grace period forward.
> + */
> +static void wait_rcu_exp_gp(struct kthread_work *wp)
> +{
> +	struct rcu_exp_work *rewp;
> +
> +	rewp = container_of(wp, struct rcu_exp_work, rew_work);
> +	rcu_exp_sel_wait_wake(rewp->rew_s);
> +}
> +
> +static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
> +{
> +	kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
> +	kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
> +}
> +
> +static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
> +{
> +}
> +#else /* !CONFIG_RCU_EXP_KTHREAD */
> +static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> +{
> +	struct rcu_exp_work *rewp =
> +		container_of(wp, struct rcu_exp_work, rew_work);
> +
> +	__sync_rcu_exp_select_node_cpus(rewp);
> +}
> +
> +static inline bool rcu_gp_par_worker_started(void)
> +{
> +	return !!READ_ONCE(rcu_par_gp_wq);
> +}
> +
> +static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
> +{
> +	int cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
> +
> +	INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> +	/* If all offline, queue the work on an unbound CPU. */
> +	if (unlikely(cpu > rnp->grphi - rnp->grplo))
> +		cpu = WORK_CPU_UNBOUND;
> +	else
> +		cpu += rnp->grplo;
> +	queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> +}
> +
> +static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
> +{
> +	flush_work(&rnp->rew.rew_work);
> +}
> +
> +/*
> + * Work-queue handler to drive an expedited grace period forward.
> + */
> +static void wait_rcu_exp_gp(struct work_struct *wp)
> +{
> +	struct rcu_exp_work *rewp;
> +
> +	rewp = container_of(wp, struct rcu_exp_work, rew_work);
> +	rcu_exp_sel_wait_wake(rewp->rew_s);
> +}
> +
> +static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
> +{
> +	INIT_WORK_ONSTACK(&rew->rew_work, wait_rcu_exp_gp);
> +	queue_work(rcu_gp_wq, &rew->rew_work);
> +}
> +
> +static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
> +{
> +	destroy_work_on_stack(&rew->rew_work);
> +}
> +#endif /* CONFIG_RCU_EXP_KTHREAD */
> +
>  /*
>   * Select the nodes that the upcoming expedited grace period needs
>   * to wait for.
>   */
>  static void sync_rcu_exp_select_cpus(void)
>  {
> -	int cpu;
>  	struct rcu_node *rnp;
>  
>  	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
> @@ -435,28 +539,21 @@ static void sync_rcu_exp_select_cpus(void)
>  		rnp->exp_need_flush = false;
>  		if (!READ_ONCE(rnp->expmask))
>  			continue; /* Avoid early boot non-existent wq. */
> -		if (!READ_ONCE(rcu_par_gp_wq) ||
> +		if (!rcu_gp_par_worker_started() ||
>  		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
>  		    rcu_is_last_leaf_node(rnp)) {
> -			/* No workqueues yet or last leaf, do direct call. */
> +			/* No worker started yet or last leaf, do direct call. */
>  			sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
>  			continue;
>  		}
> -		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> -		cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
> -		/* If all offline, queue the work on an unbound CPU. */
> -		if (unlikely(cpu > rnp->grphi - rnp->grplo))
> -			cpu = WORK_CPU_UNBOUND;
> -		else
> -			cpu += rnp->grplo;
> -		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> +		sync_rcu_exp_select_cpus_queue_work(rnp);
>  		rnp->exp_need_flush = true;
>  	}
>  
> -	/* Wait for workqueue jobs (if any) to complete. */
> +	/* Wait for jobs (if any) to complete. */
>  	rcu_for_each_leaf_node(rnp)
>  		if (rnp->exp_need_flush)
> -			flush_work(&rnp->rew.rew_work);
> +			sync_rcu_exp_select_cpus_flush_work(rnp);
>  }
>  
>  /*
> @@ -622,17 +719,6 @@ static void rcu_exp_sel_wait_wake(unsigned long s)
>  	rcu_exp_wait_wake(s);
>  }
>  
> -/*
> - * Work-queue handler to drive an expedited grace period forward.
> - */
> -static void wait_rcu_exp_gp(struct work_struct *wp)
> -{
> -	struct rcu_exp_work *rewp;
> -
> -	rewp = container_of(wp, struct rcu_exp_work, rew_work);
> -	rcu_exp_sel_wait_wake(rewp->rew_s);
> -}
> -
>  #ifdef CONFIG_PREEMPT_RCU
>  
>  /*
> @@ -848,20 +934,19 @@ void synchronize_rcu_expedited(void)
>  	} else {
>  		/* Marshall arguments & schedule the expedited grace period. */
>  		rew.rew_s = s;
> -		INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
> -		queue_work(rcu_gp_wq, &rew.rew_work);
> +		synchronize_rcu_expedited_queue_work(&rew);
>  	}
>  
>  	/* Wait for expedited grace period to complete. */
>  	rnp = rcu_get_root();
>  	wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
>  		   sync_exp_work_done(s));
> -	smp_mb(); /* Workqueue actions happen before return. */
> +	smp_mb(); /* Work actions happen before return. */
>  
>  	/* Let the next expedited grace period start. */
>  	mutex_unlock(&rcu_state.exp_mutex);
>  
>  	if (likely(!boottime))
> -		destroy_work_on_stack(&rew.rew_work);
> +		synchronize_rcu_expedited_destroy_work(&rew);
>  }
>  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
> 
> base-commit: 1a3b1bba7c7a5eb8a11513cf88427cb9d77bc60a
> -- 
> 2.35.1.1178.g4f1659d476-goog
> 

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