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From:   bsegall@google.com
To:     Huaixin Chang <changhuaixin@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org, peterz@infradead.org,
        mingo@redhat.com, bsegall@google.com, chiluk+linux@indeed.com,
        vincent.guittot@linaro.org, pauld@redhead.com
Subject: Re: [PATCH 2/2] sched/fair: Refill bandwidth before scaling
References: <20200420024421.22442-1-changhuaixin@linux.alibaba.com>
        <20200420024421.22442-3-changhuaixin@linux.alibaba.com>
Date:   Mon, 20 Apr 2020 10:54:53 -0700
In-Reply-To: <20200420024421.22442-3-changhuaixin@linux.alibaba.com> (Huaixin
        Chang's message of "Mon, 20 Apr 2020 10:44:21 +0800")
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Huaixin Chang <changhuaixin@linux.alibaba.com> writes:

> In order to prevent possible hardlockup of sched_cfs_period_timer()
> loop, loop count is introduced to denote whether to scale quota and
> period or not. However, scale is done between forwarding period timer
> and refilling cfs bandwidth runtime, which means that period timer is
> forwarded with old "period" while runtime is refilled with scaled
> "quota".
>
> Move do_sched_cfs_period_timer() before scaling to solve this.

Reviewed-by: Ben Segall <bsegall@google.com>

>
> Fixes: 2e8e19226398 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
> Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com>
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..9ace1c5c73a5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5152,6 +5152,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  		if (!overrun)
>  			break;
>  
> +		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
> +
>  		if (++count > 3) {
>  			u64 new, old = ktime_to_ns(cfs_b->period);
>  
> @@ -5181,8 +5183,6 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  			/* reset count so we don't come right back in here */
>  			count = 0;
>  		}
> -
> -		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
>  	}
>  	if (idle)
>  		cfs_b->period_active = 0;