linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Thomas Gleixner <tglx@linutronix.de>
To: Waiman Long <longman@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>,
	linux-kernel@vger.kernel.org, Stephen Boyd <sboyd@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] clocksource: Warn if too many missing ticks are detected
Date: Tue, 18 Sep 2018 23:07:21 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1809182259110.1468@nanos.tec.linutronix.de> (raw)
In-Reply-To: <1537295774-17975-1-git-send-email-longman@redhat.com>

Waiman,

On Tue, 18 Sep 2018, Waiman Long wrote:

> The clocksource watchdog, when running, is scheduled on all the CPUs in
> the system sequentially on a round-robin fashion with a period of 0.5s.
> A bug in the 4.18 kernel is causing missing ticks when nohz_full
> is specified. Under some circumstances, this causes the watchdog to
> incorrectly state that the TSC is unstable because of counter overflow
> in the hpet watchdog clock source after a few minutes delay.
> 
> That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched:
> idle: Avoid retaining the tick when it has been stopped"). To make it
> easier to catch this kind of bug in the future, a check is added to see
> if there is too much delay in the watchdog invocation and print a
> warning once if it happens.

I like the idea.

> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/time/clocksource.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 0e6e97a..2ea5db0 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -140,6 +140,7 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags)
>   * Interval: 0.5sec Threshold: 0.0625s
>   */
>  #define WATCHDOG_INTERVAL (HZ >> 1)
> +#define WATCHDOG_INTERNVAL_NS (NSEC_PER_SEC >> 1)
>  #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
>  
>  static void clocksource_watchdog_work(struct work_struct *work)
> @@ -242,6 +243,18 @@ static void clocksource_watchdog(struct timer_list *unused)
>  		wd_nsec = clocksource_cyc2ns(delta, watchdog->mult,
>  					     watchdog->shift);
>  
> +		/*
> +		 * When the timer tick is incorrectly stopped on a CPU with
> +		 * pending events, for example, it is possible that the
> +		 * clocksource watchdog will stop running for a sufficiently
> +		 * long enough time to cause overflow in the delta
> +		 * computation leading to incorrect report of unstable clock
> +		 * source. So print a warning if there is unusually large
> +		 * delay (> 0.5s) in the invocation of the watchdog. That
> +		 * can indicate a hidden bug in the timer tick code.
> +		 */
> +		WARN_ON_ONCE(!wd_nsec || wd_nsec > 2*WATCHDOG_INTERNVAL_NS);

But this is using the watchdog delta to check. If that wrapped the
detection is broken.

I'd rather use watchdog_timer.expires and check against jiffies. That tells
you how late the timer callback actually is and does not suffer any
wraparound issues.

Thanks,

	tglx



  reply	other threads:[~2018-09-18 21:07 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-18 18:36 Waiman Long
2018-09-18 21:07 ` Thomas Gleixner [this message]
2018-09-18 21:54   ` Waiman Long
2018-09-18 22:06     ` Thomas Gleixner

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=alpine.DEB.2.21.1809182259110.1468@nanos.tec.linutronix.de \
    --to=tglx@linutronix.de \
    --cc=john.stultz@linaro.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=longman@redhat.com \
    --cc=peterz@infradead.org \
    --cc=sboyd@kernel.org \
    --subject='Re: [PATCH] clocksource: Warn if too many missing ticks are detected' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).