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

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Message-ID: <xm26o9g7q5ux.fsf@bsegall-linux.svl.corp.google.com>
Date:   Mon, 18 Jun 2018 11:58:30 -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 2/2] sched/fair: Advance global expiration when period timer is restarted

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

> I noticed the group frequently got throttled even it consumed
> low cpu usage, this caused some jitters on the response time
> to some of our business containers enabling cpu quota.
>
> It's very easy to reproduce:
> mkdir /sys/fs/cgroup/cpu/test
> cd /sys/fs/cgroup/cpu/test
> echo 100000 > cpu.cfs_quota_us
> echo $$ > tasks
> then repeat:
> cat cpu.stat |grep nr_throttled  // nr_throttled will increase
>
> After some analysis, we found that cfs_rq::runtime_remaining will
> be cleared by expire_cfs_rq_runtime() due to two equal but stale
> "cfs_{b|q}->runtime_expires" after period timer is re-armed.

If this is after the first patch, then this is no longer what should
happen, and instead it would incorrectly /keep/ old local cfs_rq
runtime, and not __refill global runtime until the period.


>
> The global expiration should be advanced accordingly when the
> bandwidth period timer is restarted.
>
> Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.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 9f384264e832..bb006e671609 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);

I think we actually want if (overrun) __refill_cfs_bandwidth_runtime(),
much like tg_set_cfs_bandwidth

> +	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)