linux-kernel - Re: [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock

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Message-ID: <20160511064929.GC3193@twins.programming.kicks-ass.net>
Date:	Wed, 11 May 2016 08:49:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	bsegall@...gle.com
Cc:	Xunlei Pang <xlpang@...hat.com>, linux-kernel@...r.kernel.org,
	Juri Lelli <juri.lelli@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>, pjt@...gle.com
Subject: Re: [PATCH] sched/fair: Fix the wrong throttled clock time for
 cfs_rq_clock_task()

On Tue, May 10, 2016 at 11:19:44AM -0700, bsegall@...gle.com wrote:
> Xunlei Pang <xlpang@...hat.com> writes:
> 
> > Two minor fixes for cfs_rq_clock_task().
> > 1) If cfs_rq is currently being throttled, we need to subtract the cfs
> >    throttled clock time.
> >
> > 2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
> >    need it as well.
> >
> > Signed-off-by: Xunlei Pang <xlpang@...hat.com>
> > ---
> >  kernel/sched/fair.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1708729e..fb80a12 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> >  static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
> >  {
> >  	if (unlikely(cfs_rq->throttle_count))
> > -		return cfs_rq->throttled_clock_task;
> > +		return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
> >  
> >  	return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
> >  }

The alternative is obviously to do the subtraction in
tg_throttle_down(), were we set ->throttled_clock_task.

> > @@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
> >  	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> >  
> >  	cfs_rq->throttle_count--;
> > -#ifdef CONFIG_SMP
> >  	if (!cfs_rq->throttle_count) {
> >  		/* adjust cfs_rq_clock_task() */
> >  		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
> >  					     cfs_rq->throttled_clock_task;
> >  	}
> > -#endif
> >  
> >  	return 0;
> >  }
> 
> [Cc: pjt@...gle.com]
> 
> This looks reasonable to me (at least the first part; I'm not
> certain why the CONFIG_SMP ifdef was put in place).

64660c864f46 ("sched: Prevent interactions with throttled entities")

Introduced it, because at that time it was about updating shares, which
is only present on SMP. Then:

f1b17280efbd ("sched: Maintain runnable averages across throttled periods")

Added the clock thing inside it, and:

82958366cfea ("sched: Replace update_shares weight distribution with per-entity computation")

took out the shares update and left the clock update, resulting in the
current code.