linux-kernel - Re: [PATCH v4] sched: fix incorrect wait time and wait count statistics

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Date:	Fri, 6 Nov 2015 14:57:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Joonwoo Park <joonwoop@...eaurora.org>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, ohaugan@...eaurora.org
Subject: Re: [PATCH v4] sched: fix incorrect wait time and wait count
 statistics

On Tue, Oct 27, 2015 at 09:46:53PM -0700, Joonwoo Park wrote:
> @@ -1272,6 +1272,15 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  	WARN_ON_ONCE(p->state != TASK_RUNNING && p->state != TASK_WAKING &&
>  			!p->on_rq);
>  
> +	/*
> +	 * Migrating fair class task must have p->on_rq = TASK_ON_RQ_MIGRATING,
> +	 * because schedstat_wait_{start,end} rebase migrating task's wait_start
> +	 * time relying on p->on_rq.
> +	 */
> +	WARN_ON_ONCE(p->state == TASK_RUNNING &&
> +		     p->sched_class == &fair_sched_class &&
> +		     (p->on_rq && !task_on_rq_migrating(p)));
> +

Why do we have to test p->on_rq? Would not ->state == RUNNING imply
that?

> +++ b/kernel/sched/fair.c
> @@ -737,41 +737,69 @@ static void update_curr_fair(struct rq *rq)
>  	update_curr(cfs_rq_of(&rq->curr->se));
>  }
>  
> +#ifdef CONFIG_SCHEDSTATS
>  static inline void
>  update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> +	u64 wait_start = rq_clock(rq_of(cfs_rq));
>  
> +	if (entity_is_task(se) && task_on_rq_migrating(task_of(se)) &&
> +	    likely(wait_start > se->statistics.wait_start))
> +		wait_start -= se->statistics.wait_start;
> +
> +	schedstat_set(se->statistics.wait_start, wait_start);
>  }
>  
>  static void
>  update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {

Since this is now all under CONFIG_SCHEDSTAT, would it not make sense
to do something like:

	u64 now = rq_clock(rq_of(cfs_rq));

to avoid the endless calling of that function?

Also, for that very same reason; would it not make sense to drop the
schedstat_set() usage below, that would greatly enhance readability.

> +	if (entity_is_task(se) && task_on_rq_migrating(task_of(se))) {
> +		/*
> +		 * Preserve migrating task's wait time so wait_start time stamp
> +		 * can be adjusted to accumulate wait time prior to migration.
> +		 */
> +		schedstat_set(se->statistics.wait_start,
> +			      rq_clock(rq_of(cfs_rq)) -
> +			      se->statistics.wait_start);
> +		return;
> +	}
> +
>  	schedstat_set(se->statistics.wait_max, max(se->statistics.wait_max,
>  			rq_clock(rq_of(cfs_rq)) - se->statistics.wait_start));
>  	schedstat_set(se->statistics.wait_count, se->statistics.wait_count + 1);
>  	schedstat_set(se->statistics.wait_sum, se->statistics.wait_sum +
>  			rq_clock(rq_of(cfs_rq)) - se->statistics.wait_start);
> +
>  	if (entity_is_task(se)) {
>  		trace_sched_stat_wait(task_of(se),
>  			rq_clock(rq_of(cfs_rq)) - se->statistics.wait_start);
>  	}

Is there no means of collapsing the two 'entity_is_task()' branches?

>  	schedstat_set(se->statistics.wait_start, 0);
>  }
> +#else
> +static inline void
> +update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +}
> +
> +static inline void
> +update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +}
> +#endif
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