linux-kernel - Re: [PATCH,RFC] smp,csd: throw an error if a CSD lock is stuck for too long

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Date:   Wed, 13 Sep 2023 18:17:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH,RFC] smp,csd: throw an error if a CSD lock is stuck for
 too long

On Wed, Sep 13, 2023 at 10:33:51AM -0400, Rik van Riel wrote:
> On Wed, 2023-09-13 at 15:22 +0200, Peter Zijlstra wrote:
> > On Mon, Aug 21, 2023 at 04:04:09PM -0400, Rik van Riel wrote:
> > > 
> > > +       /* How long since this CSD lock was stuck. */
> > > +       ts_delta = ts2 - ts0;
> > > 
> > > +       /*
> > > +        * If the CSD lock is still stuck after 5 minutes, it is
> > > unlikely
> > > +        * to become unstuck. Use a signed comparison to avoid
> > > triggering
> > > +        * on underflows when the TSC is out of sync between
> > > sockets.
> > > +        */
> > > +       BUG_ON((s64)ts_delta > 300000000000LL);
> > >         if (cpu_cur_csd && csd != cpu_cur_csd) {
> > >                 pr_alert("\tcsd: CSD lock (#%d) handling prior
> > > %pS(%ps) request.\n",
> > >                          *bug_id, READ_ONCE(per_cpu(cur_csd_func,
> > > cpux)),
> > 
> > How are you guys still seeing this? I thought the KVM APIC thing was
> > fixed a while ago?
> > 
> It's more fun than that. We're seeing this on bare metal.

Oh, 'fun' indeed, *groan*.

> Unfortunately, when a system gets wedged that way currently,
> it ends up being power cycled automatically, and we aren't
> getting crash dumps with clues on what causes the issue.
> 
> Doing a BUG_ON() + panic, followed by a kexec into the kdump
> kernel will hopefully give us some clues on what might be
> causing the issue.

I'm conflicted on the need to push such a debug patch upstream, otoh.
given the amount of debug code already in csd, why not.

But yeah, curious hear what comes out of this.