linux-kernel - Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup

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Message-ID: <9ae2ee4d-7b67-50ff-e736-1d51753c5ccd@redhat.com>
Date:   Thu, 16 Jan 2020 12:34:09 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Robert Richter <rrichter@...vell.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup

On 1/16/20 11:57 AM, Thomas Gleixner wrote:
>> So your theory the MONOTONIC clock runs differently/wrongly could
>> explain that (assuming this drives the sched clock). Though, I am
> No. sched_clock() is separate. It uses a raw timestamp (in your case
> from the ARM arch timer) and converts it to something which is close to
> proper time. So my assumption was based on the printout Waiman had:
>
>  [ 1... ] CPU.... watchdog_fn now  170000000
>  [ 25.. ] CPU.... watchdog_fn now 4170000000
>
> I assumed that now comes from ktime_get() or something like
> that. Waiman?

I printed out the now parameter of the  __hrtimer_run_queues() call. So
from the timer perspective, it is losing time. For watchdog, the soft
expiry time is 4s. The watchdog function won't be called until the
timer's time advances 4s or more. That corresponds to about 24s in
timestamp time for that particular class of systems.

Cheers,
Longman