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To: jim.houston@attbi.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: LTP - gettimeofday02 FAIL
References: <200211190127.gAJ1RWg11023@linux.local>
From: Andi Kleen <ak@muc.de>
Date: 19 Nov 2002 13:03:48 +0100
In-Reply-To: <200211190127.gAJ1RWg11023@linux.local>
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Jim Houston <jim.houston@attbi.com> writes:

> I believe that this is the result of lost ticks.  It has gotten much
> easier to lose a tick since HZ was changed to 1000.  When the timer
> interrupt is delayed, the other processors will continue to keep reasonable
> time (based on the TSC), but when the timer interrupt eventually happens,
> it will add one tick's worth of nanoseconds to xtime.tv_nsec and set
> last_tsc_low to the current tsc value.  The other processors now base
> their time on this new last_tsc_low and  will see time go backwards.

It could be detected by keeping a per cpu last_tsc.

Best would be to use a global timer like HPET, but it's not available
everywhere and much slower than rdtsc too.

-Andi
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