linux-kernel - Re: gettimeofday() jumping into the future

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Message-ID: <alpine.LFD.1.10.0803311048170.3219@apollo.tec.linutronix.de>
Date:	Mon, 31 Mar 2008 10:55:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Tim Ricketts <tr@...th.li>
cc:	Michael Smith <msmith@...h.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Wingo <wingo@...endo.com>, Ingo Molnar <mingo@...e.hu>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: gettimeofday() jumping into the future

On Sun, 30 Mar 2008, Tim Ricketts wrote:
> On Thu, 23 Aug 2007, Michael Smith wrote:
> 
> > We've been seeing some strange behaviour on some of our applications
> > recently. I've tracked this down to gettimeofday() returning spurious
> > values occasionally.
> > 
> > Specifically, gettimeofday() will suddenly, for a single call, return
> > a value about 4398 seconds (~1 hour 13 minutes) in the future. The
> > following call goes back to a normal value.
> 
> I have also seen this.
> 
> > This seems to be occurring when the clock source goes slightly
> > backwards for a single call. In
> > kernel/time/timekeeping.c:__get_nsec_offset(), we have this:
> > cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> > 
> > So a small decrease in time here will (this is all unsigned
> > arithmetic) give us a very large cycle_delta. cyc2ns() then multiplies
> > this by some value, then right shifts by 22. The resulting value (in
> > nanoseconds) is approximately 4398 seconds; this gets added on to the
> > xtime value, giving us our jump into the future. The next call to
> > gettimeofday() returns to normal as we don't have this huge nanosecond
> > offset.
> 
> Indeed.  I don't know where the suggestion of off by 2^32us came in
> later in this thread.  As you've already pointed out, it's off by
> 2^42ns.
> 
> I've no idea why the TSC might go backwards, but perhaps we should not
> break horribly if it does.  How about treating it as zero?
 
> +	if (cycle_now < clock->cycle_last)
> +		return 0;
> +

No, this breaks wrapping clocksources e.g. pmtimer. We need a
different sanity check for that TSC crap.

Thanks,

	tglx
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