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From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	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, dhowells@redhat.com,
	edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task
Date: Mon, 23 Mar 2020 20:06:39 -0400
Message-ID: <20200324000639.GA29340@google.com> (raw)
In-Reply-To: <20200320024943.GA29649@paulmck-ThinkPad-P72>

On Thu, Mar 19, 2020 at 07:49:43PM -0700, Paul E. McKenney wrote:
[...] 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit e26a234c1205bf02b62b62cd7f15f8086fc0b13b
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Thu Mar 19 15:33:12 2020 -0700
> 
>     rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built
>     
>     Systems running CPU-bound real-time task do not want IPIs sent to CPUs
>     executing nohz_full userspace tasks.  Battery-powered systems don't
>     want IPIs sent to idle CPUs in low-power mode.  Unfortunately, RCU tasks
>     trace can and will send such IPIs in some cases.
>     
>     Both of these situations occur only when the target CPU is in RCU
>     dyntick-idle mode, in other words, when RCU is not watching the
>     target CPU.  This suggests that CPUs in dyntick-idle mode should use
>     memory barriers in outermost invocations of rcu_read_lock_trace()
>     and rcu_read_unlock_trace(), which would allow the RCU tasks trace
>     grace period to directly read out the target CPU's read-side state.
>     One challenge is that RCU tasks trace is not targeting a specific
>     CPU, but rather a task.  And that task could switch from one CPU to
>     another at any time.
>     
>     This commit therefore uses try_invoke_on_locked_down_task()
>     and checks for task_curr() in trc_inspect_reader_notrunning().
>     When this condition holds, the target task is running and cannot move.
>     If CONFIG_TASKS_TRACE_RCU_READ_MB=y, the new rcu_dynticks_zero_in_eqs()
>     function can be used to check if the specified integer (in this case,
>     t->trc_reader_nesting) is zero while the target CPU remains in that same
>     dyntick-idle sojourn.  If so, the target task is in a quiescent state.
>     If not, trc_read_check_handler() must indicate failure so that the
>     grace-period kthread can take appropriate action or retry after an
>     appropriate delay, as the case may be.
>     
>     With this change, given CONFIG_TASKS_TRACE_RCU_READ_MB=y, if a given
>     CPU remains idle or a given task continues executing in nohz_full mode,
>     the RCU tasks trace grace-period kthread will detect this without the
>     need to send an IPI.
>     
>     Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index e1089fd..296f926 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -501,6 +501,7 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type,
>  #endif
>  
>  #ifdef CONFIG_TINY_RCU
> +static inline bool rcu_dynticks_zero_in_eqs(int cpu, int *vp) { return false; }
>  static inline unsigned long rcu_get_gp_seq(void) { return 0; }
>  static inline unsigned long rcu_exp_batches_completed(void) { return 0; }
>  static inline unsigned long
> @@ -510,6 +511,7 @@ static inline void show_rcu_gp_kthreads(void) { }
>  static inline int rcu_get_gp_kthreads_prio(void) { return 0; }
>  static inline void rcu_fwd_progress_check(unsigned long j) { }
>  #else /* #ifdef CONFIG_TINY_RCU */
> +bool rcu_dynticks_zero_in_eqs(int cpu, int *vp);
>  unsigned long rcu_get_gp_seq(void);
>  unsigned long rcu_exp_batches_completed(void);
>  unsigned long srcu_batches_completed(struct srcu_struct *sp);
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index d31ed74..36f03d3 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -802,22 +802,38 @@ static void trc_read_check_handler(void *t_in)
>  /* Callback function for scheduler to check non-running) task.  */
>  static bool trc_inspect_reader_notrunning(struct task_struct *t, void *arg)

This function name is a bit confusing. The task could be running when this
function is called. Below you are detecting that the task is running, by
calling task_curr().

Maybe just trc_inspect_reader() is better?

[..]

> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 44edd0a..43991a4 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -455,6 +455,8 @@ static void rcu_bind_gp_kthread(void);
>  static bool rcu_nohz_full_cpu(void);
>  static void rcu_dynticks_task_enter(void);
>  static void rcu_dynticks_task_exit(void);
> +static void rcu_dynticks_task_trace_enter(void);
> +static void rcu_dynticks_task_trace_exit(void);
>  
>  /* Forward declarations for tree_stall.h */
>  static void record_gp_stall_check_time(void);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 9355536..f4a344e 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2553,3 +2553,21 @@ static void rcu_dynticks_task_exit(void)
>  	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
>  #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
>  }
> +
> +/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
> +static void rcu_dynticks_task_trace_enter(void)
> +{
> +#ifdef CONFIG_TASKS_RCU_TRACE
> +	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> +		current->trc_reader_special.b.need_mb = true;

If this is every called from middle of a reader section (that is we
transition from IPI-mode to using heavier reader-sections), then is a memory
barrier needed here just to protect the reader section that already started?

thanks,

 - Joel


> +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> +}
> +
> +/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
> +static void rcu_dynticks_task_trace_exit(void)
> +{
> +#ifdef CONFIG_TASKS_RCU_TRACE
> +	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> +		current->trc_reader_special.b.need_mb = false;
> +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> +}

  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 [this message]
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     ` [PATCH RFC v2 tip/core/rcu 04/22] rcu-tasks: Move Tasks RCU to its own file Paul E. McKenney
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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