linux-kernel - Re: [PATCH] sched/fair: Fix CFS bandwidth hrtimer expiry type

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Date:   Tue, 29 Jun 2021 13:55:20 -0700
From:   Benjamin Segall <bsegall@...gle.com>
To:     Odin Ugedal <odin@...d.al>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Fix CFS bandwidth hrtimer expiry type

Odin Ugedal <odin@...d.al> writes:

> The time remaining until expiry of the refresh_timer can be negative.
> Casting the type to an unsigned 64-bit value will cause integer
> underflow, making the runtime_refresh_within return false instead of
> true. These situations are rare, but they do happen.
>
> This does not cause user-facing issues or errors; other than
> possibly unthrottling cfs_rq's using runtime from the previous period(s),
> making the CFS bandwidth enforcement less strict in those (special)
> situations.

Yeah, extremely rare, not any real sort of problem when it does happen,
but no reason not to fix it and get the slight win in precision.

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

>
> Signed-off-by: Odin Ugedal <odin@...d.al>
> ---
>  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 23663318fb81..62446c052efb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5108,7 +5108,7 @@ static const u64 cfs_bandwidth_slack_period = 5 * NSEC_PER_MSEC;
>  static int runtime_refresh_within(struct cfs_bandwidth *cfs_b, u64 min_expire)
>  {
>  	struct hrtimer *refresh_timer = &cfs_b->period_timer;
> -	u64 remaining;
> +	s64 remaining;
>  
>  	/* if the call-back is running a quota refresh is already occurring */
>  	if (hrtimer_callback_running(refresh_timer))
> @@ -5116,7 +5116,7 @@ static int runtime_refresh_within(struct cfs_bandwidth *cfs_b, u64 min_expire)
>  
>  	/* is a quota refresh about to occur? */
>  	remaining = ktime_to_ns(hrtimer_expires_remaining(refresh_timer));
> -	if (remaining < min_expire)
> +	if (remaining < (s64)min_expire)
>  		return 1;
>  
>  	return 0;