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Date:   Mon, 23 Mar 2020 20:06:39 -0400
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,
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        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
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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 */
> +}