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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Hillf Danton <hdanton@sina.com>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
	dipankar@in.ibm.com, akpm@linux-foundation.org,
	mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
	tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
	oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH RFC v2 tip/core/rcu 04/22] rcu-tasks: Move Tasks RCU to its own file
Date: Fri, 20 Mar 2020 12:14:38 -0700
Message-ID: <20200320191438.GS3199@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200320071228.9740-1-hdanton@sina.com>

On Fri, Mar 20, 2020 at 03:12:28PM +0800, Hillf Danton wrote:
> 
> On Wed, 18 Mar 2020 17:10:42 -0700 "Paul E. McKenney" wrote:
> > 
> > +/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
> > +static int __noreturn rcu_tasks_kthread(void *arg)
> > +{
> > +	unsigned long flags;
> > +	struct task_struct *g, *t;
> > +	unsigned long lastreport;
> > +	struct rcu_head *list;
> > +	struct rcu_head *next;
> > +	LIST_HEAD(rcu_tasks_holdouts);
> > +	int fract;
> > +
> > +	/* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
> > +	housekeeping_affine(current, HK_FLAG_RCU);
> > +
> > +	/*
> > +	 * 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 (;;) {
> > +
> > +		/* Pick up any new callbacks. */
> > +		raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> > +		list = rcu_tasks_cbs_head;
> > +		rcu_tasks_cbs_head = NULL;
> > +		rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
> > +		raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> > +
> > +		/* If there were none, wait a bit and start over. */
> > +		if (!list) {
> > +			wait_event_interruptible(rcu_tasks_cbs_wq,
> > +						 READ_ONCE(rcu_tasks_cbs_head));
> > +			if (!rcu_tasks_cbs_head) {
> > +				WARN_ON(signal_pending(current));
> > +				schedule_timeout_interruptible(HZ/10);
> > +			}
> > +			continue;
> > +		}
> > +
> > +		/*
> > +		 * 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
> > +		 * ->rcu_tasks_holdout, as it forces the store to happen
> > +		 * after the beginning of the grace period.
> > +		 */
> > +		synchronize_rcu();
> > +
> > +		/*
> > +		 * 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 rcu_tasks_holdouts.
> > +		 */
> > +		rcu_read_lock();
> > +		for_each_process_thread(g, t) {
> > +			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,
> > +					 &rcu_tasks_holdouts);
> > +			}
> > +		}
> 
> Nit: report stall if it would take a jiffy longer than
> rcu_task_stall_timeout to collect the tasks?

Fair point!

That said, the wait time below is in hundreds of milliseconds and the
stall time defaults to ten minutes, so it is not clear that such a check
out constitute non-dead code.

							Thanx, Paul

> > +		rcu_read_unlock();
> > +
> > +		/*
> > +		 * 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);
> > +
> > +		/*
> > +		 * 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 HZ/10 wait and slowly back off to 1 HZ wait*/
> > +		fract = 10;
> > +
> > +		for (;;) {
> > +			bool firstreport;
> > +			bool needreport;
> > +			int rtst;
> > +			struct task_struct *t1;
> > +
> > +			if (list_empty(&rcu_tasks_holdouts))
> > +				break;
> > +
> > +			/* Slowly back off waiting for holdouts */
> > +			schedule_timeout_interruptible(HZ/fract);
> > +
> > +			if (fract > 1)
> > +				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));
> > +			list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
> > +						rcu_tasks_holdout_list) {
> > +				check_holdout_task(t, needreport, &firstreport);
> > +				cond_resched();
> > +			}
> > +		}
> > +
> > +		/*
> > +		 * 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();
> > +
> > +		/* Invoke the callbacks. */
> > +		while (list) {
> > +			next = list->next;
> > +			local_bh_disable();
> > +			list->func(list);
> > +			local_bh_enable();
> > +			list = next;
> > +			cond_resched();
> > +		}
> > +		/* Paranoid sleep to keep this from entering a tight loop */
> > +		schedule_timeout_uninterruptible(HZ/10);
> > +	}
> > +}
> 

  parent reply index

Thread overview: 123+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-12 18:16 [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle, exception, offline Paul E. McKenney
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 01/16] sched/core: Add function to sample state of non-running function paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 02/16] rcu: Add per-task state to RCU CPU stall warnings paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 03/16] rcutorture: Add flag to produce non-busy-wait task stalls paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 04/16] rcu-tasks: Move Tasks RCU to its own file paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 05/16] rcu-tasks: Create struct to hold state information paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 06/16] rcu: Reinstate synchronize_rcu_mult() paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 07/16] rcutorture: Add a test for synchronize_rcu_mult() paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 08/16] rcu-tasks: Refactor RCU-tasks to allow variants to be added paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant paulmck
2020-03-16 19:47   ` Joel Fernandes
2020-03-16 20:17     ` Joel Fernandes
2020-03-16 20:32       ` Paul E. McKenney
2020-03-16 21:32         ` Steven Rostedt
2020-03-16 21:45           ` Joel Fernandes
2020-03-16 22:03             ` Steven Rostedt
2020-03-16 22:11               ` Paul E. McKenney
2020-03-16 20:29     ` Paul E. McKenney
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 10/16] rcutorture: Add torture tests for RCU Tasks Rude paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 11/16] rcu-tasks: Use unique names for RCU-Tasks kthreads and messages paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 12/16] rcu-tasks: Further refactor RCU-tasks to allow adding more variants paulmck
2020-03-12 18:16 ` [PATCH RFC tip/core/rcu 13/16] rcu-tasks: Code movement to allow more Tasks RCU variants paulmck
2020-03-12 18:17 ` [PATCH RFC tip/core/rcu 14/16] rcu: Add an RCU Tasks Trace to simplify protection of tracing hooks paulmck
2020-03-12 18:17 ` [PATCH RFC tip/core/rcu 15/16] rcutorture: Add torture tests for RCU Tasks Trace paulmck
2020-03-12 18:17 ` [PATCH RFC tip/core/rcu 16/16] rcu-tasks: Add stall warnings " paulmck
2020-03-13 14:41 ` [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle, exception, offline Frederic Weisbecker
2020-03-13 15:42   ` Paul E. McKenney
2020-03-15 17:45     ` Mathieu Desnoyers
2020-03-15 17:59       ` Paul E. McKenney
2020-03-16 18:36         ` Steven Rostedt
2020-03-16 18:52           ` Paul E. McKenney
2020-03-16 14:45     ` Frederic Weisbecker
2020-03-16 15:39       ` Paul E. McKenney
2020-03-19  0:10 ` [PATCH RFC v2 tip/core/rcu 0/22] " Paul E. McKenney
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task paulmck
2020-03-19 17:22     ` Steven Rostedt
2020-03-19 17:35       ` Paul E. McKenney
2020-03-20  2:49         ` Paul E. McKenney
2020-03-20  3:09           ` Steven Rostedt
2020-03-20 16:27             ` Paul E. McKenney
2020-03-24  0:06           ` Joel Fernandes
2020-03-24  0:15             ` Joel Fernandes
2020-03-24 16:26               ` Paul E. McKenney
2020-03-24 15:48             ` Paul E. McKenney
2020-03-24 16:52               ` Joel Fernandes
2020-03-24 17:20                 ` Paul E. McKenney
2020-03-24 18:19                   ` Joel Fernandes
2020-03-25  0:58                     ` Paul E. McKenney
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 02/22] rcu: Add per-task state to RCU CPU stall warnings paulmck
2020-03-19 17:27     ` Steven Rostedt
2020-03-19 17:41       ` Paul E. McKenney
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 03/22] rcutorture: Add flag to produce non-busy-wait task stalls paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 04/22] rcu-tasks: Move Tasks RCU to its own file paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 05/22] rcu-tasks: Create struct to hold state information paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 06/22] rcu: Reinstate synchronize_rcu_mult() paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 07/22] rcutorture: Add a test for synchronize_rcu_mult() paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 08/22] rcu-tasks: Refactor RCU-tasks to allow variants to be added paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 09/22] rcu-tasks: Add an RCU-tasks rude variant paulmck
2020-03-19 19:04     ` Steven Rostedt
2020-03-19 23:58       ` Paul E. McKenney
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 10/22] rcutorture: Add torture tests for RCU Tasks Rude paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 11/22] rcu-tasks: Use unique names for RCU-Tasks kthreads and messages paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 12/22] rcu-tasks: Further refactor RCU-tasks to allow adding more variants paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 13/22] rcu-tasks: Code movement to allow more Tasks RCU variants paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 14/22] rcu-tasks: Add an RCU Tasks Trace to simplify protection of tracing hooks paulmck
2020-03-19  1:37     ` Joel Fernandes
2020-03-19  1:58       ` Joel Fernandes
2020-03-19  3:40         ` Paul E. McKenney
2020-03-19 19:42     ` Steven Rostedt
2020-03-20  0:28       ` Paul E. McKenney
2020-03-20  0:48         ` Steven Rostedt
2020-03-20  2:41           ` Paul E. McKenney
2020-03-28 14:06             ` Joel Fernandes
2020-03-28 15:34               ` Paul E. McKenney
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 15/22] rcutorture: Add torture tests for RCU Tasks Trace paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 16/22] rcu-tasks: Add stall warnings " paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 17/22] rcu-tasks: Move #ifdef into tasks.h paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 18/22] rcu-tasks: Add RCU tasks to rcutorture writer stall output paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 19/22] rcu-tasks: Make rcutorture writer stall output include GP state paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 20/22] rcu-tasks: Make RCU Tasks Trace make use of RCU scheduler hooks paulmck
2020-03-19  0:10   ` [PATCH RFC v2 tip/core/rcu 21/22] rcu-tasks: Add a grace-period start time for throttling and debug paulmck
2020-03-19  0:11   ` [PATCH RFC v2 tip/core/rcu 22/22] rcu-tasks: Provide boot parameter to delay IPIs until late in grace period paulmck
2020-03-19 11:31   ` [PATCH RFC v2 tip/core/rcu 0/22] Prototype RCU usable from idle, exception, offline Mathieu Desnoyers
2020-03-19 13:13     ` Paul E. McKenney
     [not found]   ` <20200319104614.11444-1-hdanton@sina.com>
2020-03-19 12:38     ` [PATCH RFC v2 tip/core/rcu 03/22] rcutorture: Add flag to produce non-busy-wait task stalls Paul E. McKenney
     [not found]     ` <20200319133947.12172-1-hdanton@sina.com>
2020-03-19 15:22       ` Paul E. McKenney
     [not found]       ` <20200320040329.9840-1-hdanton@sina.com>
2020-03-20 16:11         ` Paul E. McKenney
     [not found]   ` <20200320071228.9740-1-hdanton@sina.com>
2020-03-20 19:14     ` Paul E. McKenney [this message]
2020-03-27 22:23   ` [PATCH RFC v3 tip/core/rcu 0/34] Prototype RCU usable from idle, exception, offline Paul E. McKenney
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 01/34] sched/core: Add function to sample state of locked-down task paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 02/34] rcu: Add per-task state to RCU CPU stall warnings paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 03/34] rcutorture: Add flag to produce non-busy-wait task stalls paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 04/34] rcu-tasks: Move Tasks RCU to its own file paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 05/34] rcu-tasks: Create struct to hold state information paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 06/34] rcu: Reinstate synchronize_rcu_mult() paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 07/34] rcutorture: Add a test for synchronize_rcu_mult() paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 08/34] rcu-tasks: Refactor RCU-tasks to allow variants to be added paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 09/34] rcu-tasks: Add an RCU-tasks rude variant paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 10/34] rcutorture: Add torture tests for RCU Tasks Rude paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 11/34] rcu-tasks: Use unique names for RCU-Tasks kthreads and messages paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 12/34] rcu-tasks: Further refactor RCU-tasks to allow adding more variants paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 13/34] rcu-tasks: Code movement to allow more Tasks RCU variants paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 14/34] rcu-tasks: Add an RCU Tasks Trace to simplify protection of tracing hooks paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 15/34] rcutorture: Add torture tests for RCU Tasks Trace paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 16/34] rcu-tasks: Add stall warnings " paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 17/34] rcu-tasks: Move #ifdef into tasks.h paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 18/34] rcu-tasks: Add RCU tasks to rcutorture writer stall output paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 19/34] rcu-tasks: Make rcutorture writer stall output include GP state paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 20/34] rcu-tasks: Make RCU Tasks Trace make use of RCU scheduler hooks paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 21/34] rcu-tasks: Add a grace-period start time for throttling and debug paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 22/34] rcu-tasks: Provide boot parameter to delay IPIs until late in grace period paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 23/34] rcu-tasks: Split ->trc_reader_need_end paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 24/34] rcu-tasks: Add grace-period and IPI counts to statistics paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 25/34] rcu-tasks: Add Kconfig option to mediate smp_mb() vs. IPI paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 26/34] rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 27/34] rcu-tasks: Allow rcu_read_unlock_trace() under scheduler locks paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 28/34] rcu-tasks: Disable CPU hotplug across RCU tasks trace scans paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 29/34] rcu-tasks: Handle the running-offline idle-task special case paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 30/34] rcu-tasks: Make RCU tasks trace also wait for idle tasks paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 31/34] rcu-tasks: Add rcu_dynticks_zero_in_eqs() effectiveness statistics paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 32/34] rcu-tasks: Add count for idle tasks on offline CPUs paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 33/34] rcutorture: Add TRACE02 scenario enabling RCU Tasks Trace IPIs paulmck
2020-03-27 22:24     ` [PATCH v3 tip/core/rcu 34/34] rcu-tasks: Add IPI failure count to statistics paulmck

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