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

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Message-ID: <87blr3wrqw.fsf@nanos.tec.linutronix.de>
Date:   Thu, 16 Jan 2020 12:44:07 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Waiman Long <longman@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robert Richter <rrichter@...vell.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup

Thomas Gleixner <tglx@...utronix.de> writes:

Added ARM64 and ThunderX folks 

> Waiman Long <longman@...hat.com> writes:
>> By adding some instrumentation code, it was found that for cpu 14,
>> watchdog_enable() was called early with a timestamp of 1. That activates
>> the watchdog time checking logic. It was also found that the monotonic
>> time measured during the smp_init() phase runs much slower than the
>> real elapsed time as shown by the below debug printf output:
>>
>>   [    1.138522] run_queues, watchdog_timer_fn: now =  170000000
>>   [   25.519391] run_queues, watchdog_timer_fn: now = 4170000000
>>
>> In this particular case, it took about 24.4s of elapsed time for the
>> clock to advance 4s which is the soft expiration time that is required
>> to trigger the calling of watchdog_timer_fn(). That clock slowdown
>> stopped once the smp_init() call was done and the clock time ran at
>> the same rate as the elapsed time afterward.

And looking at this with a more awake brain, the root cause is pretty
obvious.

sched_clock() advances by 24 seconds, but clock MONOTONIC on which the
watchdog timer is based does not. As the timestamps you printed have 7
trailing zeros, it's pretty clear that timekeeping is still jiffies
based at this point and HZ is set to 100.

So while bringing up the non-boot CPUs the boot CPU loses ~2000 timer
interrupts. That needs to be fixed and not papered over.

Thanks,

        tglx