Re: [PATCH v6 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Chengming Zhou]

On 2022/10/29 03:45, Suren Baghdasaryan wrote:
> Psi polling mechanism is trying to minimize the number of wakeups to
> run psi_poll_work and is currently relying on timer_pending() to detect
> when this work is already scheduled. This provides a window of opportunity
> for psi_group_change to schedule an immediate psi_poll_work after
> poll_timer_fn got called but before psi_poll_work could reschedule itself.
> Below is the depiction of this entire window:
> 
> poll_timer_fn
>   wake_up_interruptible(&group->poll_wait);
> 
> psi_poll_worker
>   wait_event_interruptible(group->poll_wait, ...)
>   psi_poll_work
>     psi_schedule_poll_work
>       if (timer_pending(&group->poll_timer)) return;
>       ...
>       mod_timer(&group->poll_timer, jiffies + delay);
> 
> Prior to 461daba06bdc we used to rely on poll_scheduled atomic which was
> reset and set back inside psi_poll_work and therefore this race window
> was much smaller.
> The larger window causes increased number of wakeups and our partners
> report visible power regression of ~10mA after applying 461daba06bdc.
> Bring back the poll_scheduled atomic and make this race window even
> narrower by resetting poll_scheduled only when we reach polling expiration
> time. This does not completely eliminate the possibility of extra wakeups
> caused by a race with psi_group_change however it will limit it to the
> worst case scenario of one extra wakeup per every tracking window (0.5s
> in the worst case).
> This patch also ensures correct ordering between clearing poll_scheduled
> flag and obtaining changed_states using memory barrier. Correct ordering
> between updating changed_states and setting poll_scheduled is ensured by
> atomic_xchg operation.
> By tracing the number of immediate rescheduling attempts performed by
> psi_group_change and the number of these attempts being blocked due to
> psi monitor being already active, we can assess the effects of this change:
> 
> Before the patch:
>                                            Run#1    Run#2      Run#3
> Immediate reschedules attempted:           684365   1385156    1261240
> Immediate reschedules blocked:             682846   1381654    1258682
> Immediate reschedules (delta):             1519     3502       2558
> Immediate reschedules (% of attempted):    0.22%    0.25%      0.20%
> 
> After the patch:
>                                            Run#1    Run#2      Run#3
> Immediate reschedules attempted:           882244   770298    426218
> Immediate reschedules blocked:             881996   769796    426074
> Immediate reschedules (delta):             248      502       144
> Immediate reschedules (% of attempted):    0.03%    0.07%     0.03%
> 
> The number of non-blocked immediate reschedules dropped from 0.22-0.25%
> to 0.03-0.07%. The drop is attributed to the decrease in the race window
> size and the fact that we allow this race only when psi monitors reach
> polling window expiration time.
> 
> Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
> Reported-by: Kathleen Chang <yt.chang@xxxxxxxxxxxx>
> Reported-by: Wenju Xu <wenju.xu@xxxxxxxxxxxx>
> Reported-by: Jonathan Chen <jonathan.jmchen@xxxxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Tested-by: SH Chen <show-hong.chen@xxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>

Thanks.

> ---
> Changes since v5 posted at
> https://lore.kernel.org/all/20221026233839.1934419-1-surenb@xxxxxxxxxx/
> - Fixed spelling in the comments, per Chengming Zhou
> 
>  include/linux/psi_types.h |  1 +
>  kernel/sched/psi.c        | 62 ++++++++++++++++++++++++++++++++-------
>  2 files changed, 53 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> index 6e4372735068..14a1ebb74e11 100644
> --- a/include/linux/psi_types.h
> +++ b/include/linux/psi_types.h
> @@ -177,6 +177,7 @@ struct psi_group {
>  	struct timer_list poll_timer;
>  	wait_queue_head_t poll_wait;
>  	atomic_t poll_wakeup;
> +	atomic_t poll_scheduled;
>  
>  	/* Protects data used by the monitor */
>  	struct mutex trigger_lock;
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index dbaeac915895..94c4c5143f23 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -189,6 +189,7 @@ static void group_init(struct psi_group *group)
>  	INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
>  	mutex_init(&group->avgs_lock);
>  	/* Init trigger-related members */
> +	atomic_set(&group->poll_scheduled, 0);
>  	mutex_init(&group->trigger_lock);
>  	INIT_LIST_HEAD(&group->triggers);
>  	group->poll_min_period = U32_MAX;
> @@ -580,18 +581,17 @@ static u64 update_triggers(struct psi_group *group, u64 now)
>  	return now + group->poll_min_period;
>  }
>  
> -/* Schedule polling if it's not already scheduled. */
> -static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
> +/* Schedule polling if it's not already scheduled or forced. */
> +static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay,
> +				   bool force)
>  {
>  	struct task_struct *task;
>  
>  	/*
> -	 * Do not reschedule if already scheduled.
> -	 * Possible race with a timer scheduled after this check but before
> -	 * mod_timer below can be tolerated because group->polling_next_update
> -	 * will keep updates on schedule.
> +	 * atomic_xchg should be called even when !force to provide a
> +	 * full memory barrier (see the comment inside psi_poll_work).
>  	 */
> -	if (timer_pending(&group->poll_timer))
> +	if (atomic_xchg(&group->poll_scheduled, 1) && !force)
>  		return;
>  
>  	rcu_read_lock();
> @@ -603,12 +603,15 @@ static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
>  	 */
>  	if (likely(task))
>  		mod_timer(&group->poll_timer, jiffies + delay);
> +	else
> +		atomic_set(&group->poll_scheduled, 0);
>  
>  	rcu_read_unlock();
>  }
>  
>  static void psi_poll_work(struct psi_group *group)
>  {
> +	bool force_reschedule = false;
>  	u32 changed_states;
>  	u64 now;
>  
> @@ -616,6 +619,43 @@ static void psi_poll_work(struct psi_group *group)
>  
>  	now = sched_clock();
>  
> +	if (now > group->polling_until) {
> +		/*
> +		 * We are either about to start or might stop polling if no
> +		 * state change was recorded. Resetting poll_scheduled leaves
> +		 * a small window for psi_group_change to sneak in and schedule
> +		 * an immediate poll_work before we get to rescheduling. One
> +		 * potential extra wakeup at the end of the polling window
> +		 * should be negligible and polling_next_update still keeps
> +		 * updates correctly on schedule.
> +		 */
> +		atomic_set(&group->poll_scheduled, 0);
> +		/*
> +		 * A task change can race with the poll worker that is supposed to
> +		 * report on it. To avoid missing events, ensure ordering between
> +		 * poll_scheduled and the task state accesses, such that if the poll
> +		 * worker misses the state update, the task change is guaranteed to
> +		 * reschedule the poll worker:
> +		 *
> +		 * poll worker:
> +		 *   atomic_set(poll_scheduled, 0)
> +		 *   smp_mb()
> +		 *   LOAD states
> +		 *
> +		 * task change:
> +		 *   STORE states
> +		 *   if atomic_xchg(poll_scheduled, 1) == 0:
> +		 *     schedule poll worker
> +		 *
> +		 * The atomic_xchg() implies a full barrier.
> +		 */
> +		smp_mb();
> +	} else {
> +		/* Polling window is not over, keep rescheduling */
> +		force_reschedule = true;
> +	}
> +
> +
>  	collect_percpu_times(group, PSI_POLL, &changed_states);
>  
>  	if (changed_states & group->poll_states) {
> @@ -641,7 +681,8 @@ static void psi_poll_work(struct psi_group *group)
>  		group->polling_next_update = update_triggers(group, now);
>  
>  	psi_schedule_poll_work(group,
> -		nsecs_to_jiffies(group->polling_next_update - now) + 1);
> +		nsecs_to_jiffies(group->polling_next_update - now) + 1,
> +		force_reschedule);
>  
>  out:
>  	mutex_unlock(&group->trigger_lock);
> @@ -802,7 +843,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  	write_seqcount_end(&groupc->seq);
>  
>  	if (state_mask & group->poll_states)
> -		psi_schedule_poll_work(group, 1);
> +		psi_schedule_poll_work(group, 1, false);
>  
>  	if (wake_clock && !delayed_work_pending(&group->avgs_work))
>  		schedule_delayed_work(&group->avgs_work, PSI_FREQ);
> @@ -956,7 +997,7 @@ void psi_account_irqtime(struct task_struct *task, u32 delta)
>  		write_seqcount_end(&groupc->seq);
>  
>  		if (group->poll_states & (1 << PSI_IRQ_FULL))
> -			psi_schedule_poll_work(group, 1);
> +			psi_schedule_poll_work(group, 1, false);
>  	} while ((group = group->parent));
>  }
>  #endif
> @@ -1342,6 +1383,7 @@ void psi_trigger_destroy(struct psi_trigger *t)
>  		 * can no longer be found through group->poll_task.
>  		 */
>  		kthread_stop(task_to_destroy);
> +		atomic_set(&group->poll_scheduled, 0);
>  	}
>  	kfree(t);
>  }