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

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Message-ID: <7168b0aa-4904-4246-0de5-3906df13b5c8@redhat.com>
Date:   Thu, 16 Jan 2020 14:13:49 -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 2:10 PM, Thomas Gleixner wrote:
> Waiman Long <longman@...hat.com> writes:
>
>> 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.
> Yes. That's clock MONOTONIC.
>
>> 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.
> Right. And assumed that the firmware call is the culprit this has an
> explanation.
>
> Could you please take sched_clock() timestamps before and after the
> firmware call which kicks the secondary CPUs into life to verify that?
>
> They should sum up to the amount of time which gets lost accross
> smp_init().

Sure, I will do that after I get hold of the arm64 system that can
reproduce the issue. That system is currently used by another engineer.

Cheers,
Longman