linux-kernel - Re: [PATCH 2/2] tick: Make sure tick timer is active when bypassing reprogramming

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Message-ID: <alpine.DEB.2.20.1704201955320.2373@nanos>
Date:   Thu, 20 Apr 2017 19:56:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <fweisbec@...il.com>
cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        James Hartsock <hartsjc@...hat.com>, stable@...r.kernel.org,
        Tim Wright <tim@...bash.co.uk>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 2/2] tick: Make sure tick timer is active when bypassing
 reprogramming

On Thu, 20 Apr 2017, Frederic Weisbecker wrote:

> So far we have run into too much troubles with the optimization path
> that skips reprogramming the clock on IRQ exit when the expiration
> deadline hasn't changed. If by accident the cached deadline happens to
> be out of sync with the hardware deadline, the buggy result and its
> cause are hard to investigate. So lets detect and warn about the issue
> early.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Tim Wright <tim@...bash.co.uk>
> Cc: Pavel Machek <pavel@....cz>
> Cc: James Hartsock <hartsjc@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---
>  kernel/time/tick-sched.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 502b320..eb1366e 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -783,8 +783,10 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>  	tick = expires;
>  
>  	/* Skip reprogram of event if its not changed */
> -	if (ts->tick_stopped && (expires == ts->next_tick))
> +	if (ts->tick_stopped && (expires == ts->next_tick)) {
> +		WARN_ON_ONCE(dev->next_event > ts->next_tick);

What about handling it proper ? dev->next_event might be KTIME_MAX,
i.e. no more event for the next 500+ years.

Thanks,

	tglx