From: Benjamin Segall <bsegall@google.com>
To: Odin Ugedal <odin@uged.al>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair: Fix CFS bandwidth hrtimer expiry type
Date: Tue, 29 Jun 2021 13:55:20 -0700 [thread overview]
Message-ID: <xm26bl7o9yrr.fsf@google.com> (raw)
In-Reply-To: <20210629121452.18429-1-odin@uged.al> (Odin Ugedal's message of "Tue, 29 Jun 2021 14:14:52 +0200")
Odin Ugedal <odin@uged.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@google.com>
>
> Signed-off-by: Odin Ugedal <odin@uged.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;
next prev parent reply other threads:[~2021-06-29 20:55 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-29 12:14 [PATCH] sched/fair: Fix CFS bandwidth hrtimer expiry type Odin Ugedal
2021-06-29 20:55 ` Benjamin Segall [this message]
2021-07-05 7:53 ` [tip: sched/urgent] " tip-bot2 for Odin Ugedal
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