linux-kernel - Re: [PATCH 1/3] watchdog/softlockup: Preserve original timestamp when touching watchdog externally

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Date:   Mon, 21 Oct 2019 14:42:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Laurence Oberman <loberman@...hat.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] watchdog/softlockup: Preserve original timestamp
 when touching watchdog externally

On Mon, Aug 19, 2019 at 12:47:30PM +0200, Petr Mladek wrote:
> Some bug report included the same softlockups in flush_tlb_kernel_range()
> in regular intervals. Unfortunately was not clear if there was a progress
> or not.
> 
> The situation can be simulated with a simply busy loop:
> 
> 	while (true)
> 	      cpu_relax();
> 
> The softlockup detector produces:
> 
> [  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]
> 
> One would expect only one softlockup report or several reports with
> an increased duration.

Let's just say our expectations differ.

> The result is that each softlockup is reported only once unless
> another process get scheduled:
> 
> [  320.248948] watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [cat:4916]

Which would greatly confuse me; as the above would have me think the
situation got resolved (no more lockups reported) even though it is
still very much stuck there.

IOW, I don't see how this makes anything better. You're removing
information.