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Subject: Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup
To:     Thomas Gleixner <tglx@linutronix.de>,
        Robert Richter <rrichter@marvell.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Kees Cook <keescook@chromium.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
References: <20200103151032.19590-1-longman@redhat.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <9ae2ee4d-7b67-50ff-e736-1d51753c5ccd@redhat.com>
Date:   Thu, 16 Jan 2020 12:34:09 -0500
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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=A0 __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