linux-kernel - Re: [PATCH] nohz: Fix possible missing clock reprog after tick soft restart

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Date:   Wed, 8 Feb 2017 09:56:13 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpeng.li@...mail.com>, stable@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] nohz: Fix possible missing clock reprog after tick soft restart

2017-02-08 0:44 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> ts->next_tick keeps track of the next tick deadline in order to optimize
> clock programmation on irq exit and avoid redundant clock device writes.
>
> Now if ts->next_tick missed an update, we may spuriously miss a clock
> reprog later as the nohz code is fooled by an obsolete next_tick value.
>
> This is what happens here on a specific path: when we observe an
> expired timer from the nohz update code on irq exit, we perform a soft
> tick restart which simply fires the closest possible tick without
> actually exiting the nohz mode and restoring a periodic state. But we
> forget to update ts->next_tick accordingly.
>
> As a result, after the next tick resulting from such soft tick restart,
> the nohz code sees a stale value on ts->next_tick which doesn't match
> the clock deadline that just expired. If that obsolete ts->next_tick
> value happens to collide with the actual next tick deadline to be
> scheduled, we may spuriously bypass the clock reprogramming. In the
> worst case, the tick may never fire again.
>
> Lets fix this with a ts->next_tick reset on soft tick restart.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Wanpeng Li <wanpeng.li@...mail.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: stable@...r.kernel.org

Reviewed-by: Wanpeng Li <wanpeng.li@...mail.com>

> ---
>  kernel/time/tick-sched.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 74e0388..fc6f740 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -725,6 +725,11 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>                  */
>                 if (delta == 0) {
>                         tick_nohz_restart(ts, now);
> +                       /*
> +                        * Make sure next tick stop doesn't get fooled by past
> +                        * clock deadline
> +                        */
> +                       ts->next_tick = 0;
>                         goto out;
>                 }
>         }
> --
> 2.7.4
>