linux-kernel - Re: [PATCH 2/3] watchdog/softlockup: Report the same softlockup regularly

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Date:   Mon, 21 Oct 2019 16:09:26 +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 2/3] watchdog/softlockup: Report the same softlockup
 regularly

On Mon, Oct 21, 2019 at 03:40:38PM +0200, Petr Mladek wrote:
> On Mon 2019-10-21 14:43:39, Peter Zijlstra wrote:
> > On Mon, Aug 19, 2019 at 12:47:31PM +0200, Petr Mladek wrote:
> > > Softlockup report means that there is no progress on the given CPU. It
> > > might be a "short" affair where the system gets recovered. But often
> > > the system stops being responsive and need to get rebooted.
> > > 
> > > The softlockup might be root of the problems or just a symptom. It might
> > > be a deadlock, livelock, or often repeated state.
> > > 
> > > Regular reports help to distinguish different situations. Fortunately,
> > > the watchdog is finally able to show correct information how long
> > > softlockup_fn() was not scheduled.
> > > 
> > > Report before this patch:
> > > 
> > > [  320.248948] watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [cat:4916]
> > > 
> > > And after this patch:
> > > 
> > > [  480.372418] watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [cat:4943]
> > > [  508.372359] watchdog: BUG: soft lockup - CPU#2 stuck for 52s! [cat:4943]
> > > [  548.372359] watchdog: BUG: soft lockup - CPU#2 stuck for 89s! [cat:4943]
> > > [  576.372351] watchdog: BUG: soft lockup - CPU#2 stuck for 115s! [cat:4943]
> > > 
> > > Note that the horrible code never really worked before the accounting
> > > was fixed. The last working timestamp was regularly lost by the many
> > > touch*watchdog() calls.
> > 
> > So what's the point of patch 1? Just confusing people?
> 
> I was not sure what was the expected behavior. The code actually
> looked like only the first report was wanted. But it probably never
> worked that way.

Not that I can remember at least :-) I normally don't bother with the
actual time, and if I do then I look at the printk timestamps to figure
out how long thing've been stuck.

But this is indeed nicer..

> Should I squash the two patches and send it again, please?

Probably makes sense to squash it. That also avoids having to ever
expose that ugleh :-)