linux-kernel - Re: [PATCH v2] sched/fair: don't push cfs

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Date:   Tue, 11 Jun 2019 15:50:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     bsegall@...gle.com, linux-kernel@...r.kernel.org,
        Xunlei Pang <xlpang@...ux.alibaba.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] sched/fair: don't push cfs_bandwith slack timers
 forward

On Tue, Jun 11, 2019 at 09:04:17AM -0400, Phil Auld wrote:
> On Thu, Jun 06, 2019 at 10:21:01AM -0700 bsegall@...gle.com wrote:
> > When a cfs_rq sleeps and returns its quota, we delay for 5ms before
> > waking any throttled cfs_rqs to coalesce with other cfs_rqs going to
> > sleep, as this has to be done outside of the rq lock we hold.
> > 
> > The current code waits for 5ms without any sleeps, instead of waiting
> > for 5ms from the first sleep, which can delay the unthrottle more than
> > we want. Switch this around so that we can't push this forward forever.
> > 
> > This requires an extra flag rather than using hrtimer_active, since we
> > need to start a new timer if the current one is in the process of
> > finishing.
> > 
> > Signed-off-by: Ben Segall <bsegall@...gle.com>
> > Reviewed-by: Xunlei Pang <xlpang@...ux.alibaba.com>
> > ---
> >  kernel/sched/fair.c  | 7 +++++++
> >  kernel/sched/sched.h | 1 +
> >  2 files changed, 8 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 8213ff6e365d..2ead252cfa32 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4729,6 +4729,11 @@ static void start_cfs_slack_bandwidth(struct cfs_bandwidth *cfs_b)
> >  	if (runtime_refresh_within(cfs_b, min_left))
> >  		return;
> >  
> > +	/* don't push forwards an existing deferred unthrottle */
> > +	if (cfs_b->slack_started)
> > +		return;
> > +	cfs_b->slack_started = true;
> > +
> >  	hrtimer_start(&cfs_b->slack_timer,
> >  			ns_to_ktime(cfs_bandwidth_slack_period),
> >  			HRTIMER_MODE_REL);
> > @@ -4782,6 +4787,7 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
> >  
> >  	/* confirm we're still not at a refresh boundary */
> >  	raw_spin_lock_irqsave(&cfs_b->lock, flags);
> > +	cfs_b->slack_started = false;
> >  	if (cfs_b->distribute_running) {
> >  		raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
> >  		return;
> > @@ -4920,6 +4926,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >  	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> >  	cfs_b->slack_timer.function = sched_cfs_slack_timer;
> >  	cfs_b->distribute_running = 0;
> > +	cfs_b->slack_started = false;
> >  }
> >  
> >  static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index efa686eeff26..60219acda94b 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -356,6 +356,7 @@ struct cfs_bandwidth {
> >  	u64			throttled_time;
> >  
> >  	bool                    distribute_running;
> > +	bool                    slack_started;
> >  #endif
> >  };
> >  
> > -- 
> > 2.22.0.rc1.257.g3120a18244-goog
> > 
> 
> I think this looks good. I like not delaying that further even if it
> does not fix Dave's use case. 
> 
> It does make it glaring that I should have used false/true for setting
> distribute_running though :)
> 
> 
> Acked-by: Phil Auld <pauld@...hat.com>

Thanks!

Should this patch have a Fixes: tag?