linux-kernel - Re: [PATCH v2 2/2] sched/fair: Advance global expiration when period timer is restarted

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Date:   Wed, 20 Jun 2018 10:02:01 -0700
From:   bsegall@...gle.com
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Ben Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] sched/fair: Advance global expiration when period timer is restarted

Xunlei Pang <xlpang@...ux.alibaba.com> writes:

> When period gets restarted after some idle time, start_cfs_bandwidth()
> doesn't update the expiration information, expire_cfs_rq_runtime() will
> see cfs_rq->runtime_expires smaller than rq clock and go to the clock
> drift logic, wasting needless cpu cycles on the scheduler hot path.
>
> Update the global expiration in start_cfs_bandwidth() to avoid frequent
> expire_cfs_rq_runtime() calls once a new period begins.
>
> Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.com>
Reviewed-By: Ben Segall <bsegall@...gle.com>


> ---
>  kernel/sched/fair.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e6bb68d52962..f167aca066cc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5204,13 +5204,18 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>  
>  void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>  {
> +	u64 overrun;
> +
>  	lockdep_assert_held(&cfs_b->lock);
>  
> -	if (!cfs_b->period_active) {
> -		cfs_b->period_active = 1;
> -		hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> -		hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> -	}
> +	if (cfs_b->period_active)
> +		return;
> +
> +	cfs_b->period_active = 1;
> +	overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> +	cfs_b->runtime_expires += (overrun + 1) * ktime_to_ns(cfs_b->period);
> +	cfs_b->expires_seq++;
> +	hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>  }
>  
>  static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)