linux-kernel - Re: [PATCH] smp: print only local CPU info when sched

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Message-ID: <b108d8bb-8fe7-451c-8729-9439da221d0f@paulmck-laptop>
Date: Tue, 16 Jul 2024 06:47:41 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, neeraj.upadhyay@...nel.org, mingo@...nel.org,
	rostedt@...dmis.org, Leonardo Bras <leobras@...hat.com>
Subject: Re: [PATCH] smp: print only local CPU info when sched_clock goes
 backward

On Tue, Jul 16, 2024 at 11:04:43AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 15, 2024 at 01:49:41PM -0400, Rik van Riel wrote:
> > About 40% of all csd_lock warnings observed in our fleet appear to
> > be due to sched_clock() going backward in time (usually only a little
> > bit), resulting in ts0 being larger than ts2.
> > 
> > When the local CPU is at fault, we should print out a message reflecting
> > that, rather than trying to get the remote CPU's stack trace.
> > 
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > ---
> >  kernel/smp.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index f085ebcdf9e7..5656ef63ea82 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -237,6 +237,14 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
> >  	if (likely(ts_delta <= csd_lock_timeout_ns || csd_lock_timeout_ns == 0))
> >  		return false;
> >  
> > +	if (ts0 > ts2) {
> > +		/* Our own sched_clock went backward; don't blame another CPU. */
> > +		ts_delta = ts0 - ts2;
> > +		pr_alert("sched_clock on CPU %d went backward by %llu ns\n", raw_smp_processor_id(), ts_delta);
> > +		*ts1 = ts2;
> > +		return false;
> > +	}
> 
> So I've seen some chatter about this on IRC and was WTF, seeing this
> patch I'm still WTF. What is going on with those machines?!?!

I wish I knew, but for the record, I completely agree that both your
acronym and interrobang apply to this case.  ;-)

							Thanx, Paul