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

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Message-ID: <044ac2d7-1dee-46a9-93ba-31645ec2f8a1@redhat.com>
Date:   Thu, 2 Jan 2020 15:12:36 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] watchdog: Fix possible soft lockup warning at bootup

On 1/2/20 3:08 PM, Andrew Morton wrote:
> On Thu,  2 Jan 2020 10:41:49 -0500 Waiman Long <longman@...hat.com> wrote:
>
>> It was found that watchdog soft lockup warning was displayed on some
>> arm64 server systems at bootup time:
>>
>> ...
>>
>> Further analysis of the situation revealed that the smp_init() call
>> itself took more than 20s for that 2-socket 56-core and 224-thread
>> server.
>>
>>  [    0.115632] CPU1: Booted secondary processor 0x0000000100 [0x431f0af1]
>>    :
>>  [   27.177282] CPU223: Booted secondary processor 0x0000011b03 [0x431f0af1]
>>
>> By adding some instrumentation code, it was found that for cpu 14,
>> watchdog_enable() was called early with a timestamp of 1. The first
>> watchdog timer callback for that cpu, however, happened really late at
>> the above 25s timestamp mark causing the watchdog logic to treat the
>> delay as a soft lockup.
>>
>> On another arm64 system that doesn't show the soft lockup warning, the
>> watchdog timer callback happened earlier at the 5s timestamp mark with
>> the watchdog thread invoked shortly after that.
>>
>> The reason why there was such a delay in the first watchdog timer
>> callback for that particular system wasn't fully known yet.
> Mysteries are unwelcome.  Are you continuing to investigate this?
Yes, I will do some more investigation as to why it took so long.
>> Given
>> the fact that smp_init() can run for a long time on some systems,
>> it is probably more appropriate to enable the watchdog function after
>> smp_init() instead of before it.
>>
>> Another way is to leave watchdog_touch_ts at 0 in watchdog_enable()
>> while the system is at the booting stage. Either one of those should
>> be able to eliminate the soft lockup warning on bootup.
>>
>> ...
>>
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -496,7 +496,9 @@ static void watchdog_enable(unsigned int cpu)
>>  		      HRTIMER_MODE_REL_PINNED_HARD);
>>  
>>  	/* Initialize timestamp */
>> -	__touch_watchdog();
>> +	if (system_state != SYSTEM_BOOTING)
>> +		__touch_watchdog();
> A comment which explains the system_state test would be appropriate
> here.

Will do so.

>>  	/* Enable the perf event */
>>  	if (watchdog_enabled & NMI_WATCHDOG_ENABLED)
>>  		watchdog_nmi_enable(cpu);

Cheers,
Longman