linux-kernel - Re: [RFC 0/3] watchdog/softlockup: Make softlockup reports more reliable and useful

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Message-ID: <alpine.DEB.2.21.1906172307260.1963@nanos.tec.linutronix.de>
Date:   Mon, 17 Jun 2019 23:08:54 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurence Oberman <loberman@...hat.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/3] watchdog/softlockup: Make softlockup reports more
 reliable and useful

On Wed, 5 Jun 2019, Petr Mladek wrote:

> Hi,
> 
> we were analyzing logs with several softlockup reports in flush_tlb_kernel_range().
> They were confusing. Especially it was not clear whether it was deadlock,
> livelock, or separate softlockups.
> 
> It went out that even a simple busy loop:
> 
> 	while (true)
> 	      cpu_relax();
> 
> is able to produce several softlockups reports:
> 
> [  168.277520] watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [cat:4865]
> [  196.277604] watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [cat:4865]
> [  236.277522] watchdog: BUG: soft lockup - CPU#1 stuck for 23s! [cat:4865]
> 
> 
> I tried to understand the tricky watchdog code and produced two patches
> that would be helpful to debug the original real bug:
> 
>    1st patch prevents restart of the watchdog from unrelated locations.
> 
>    2nd patch helps to distinguish several possible situations by
>    regular reports.
> 
>    3rd patch can be used for testing the problem.
> 
> 
> The watchdog code might deserve even more clean up. Anyway, I would
> like to hear other's opinion first.

Anything which improves debugability is welcome. Unfortunately you missed
to add an example of the output after these patches are applied.

Thanks,

	tglx