linux-kernel - Re: [PROBLEM] possible divide by 0 in kernel/sched/cputime.c scale

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Message-ID: <20131118141956.GB10022@twins.programming.kicks-ass.net>
Date:	Mon, 18 Nov 2013 15:19:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Christian Engelmayer <cengelma@....at>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PROBLEM] possible divide by 0 in kernel/sched/cputime.c
 scale_stime()

On Mon, Nov 18, 2013 at 03:02:24PM +0100, Stanislaw Gruszka wrote:
> > The x86 init check whether all booted CPUs have their TSC's synchronized, never
> > failed so far, however, the tsc clocksource is sporadically marked unstable.
> > 
> >    Clocksource tsc unstable (delta = -74994678 ns)

> >    Version 00020652:
> >    Type 0 - Original OEM
> >    Family 6 - Pentium Pro
> >    Model 5 - Pentium II Model 5/Xeon/Celeron
> >    Extended brand string: "Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz"

I'm not sure what tool you used to generate that, but its broken, that's
model 0x25 (37), it somehow truncates the upper model bits.

That said, its a westmere core and I've seen wsm-ep (dual socket)
machines loose their TSC sync quite regularly, but this would be the
first case a single socket wsm would loose its TSC sync.

That leads me to believe your BIOS is screwing you over with SMIs or the
like.

I would be tempted to say you should simply mark the tsc unstable on
boot and live with that -- we fully assume the sched_clock stuff is not
going backwards in an 'observable' way.

That said, it might be nice to not crash either.. 

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