Re: SMP time warps: count > LATCH - 1

From: Gregory Maxwell (greg@linuxpower.cx)
Date: Wed May 31 2000 - 00:01:35 EST


Not to be rude. but you arn't the nice person with the asymetricly clocked
SMP system?

On Tue, 30 May 2000, dave madden wrote:

> Hi. I have a long-standing problem with timekeeping on my 2xPIII
> machine: sooner or later after a reboot, successive calls to
> gettimeofday will return large negative deltas (i.e. time goes
> backwards). Several kernel gurus have offered help and patches, but
> to no avail; I usually end up just brute-forcing the kernel to never
> return a time earlier than one previously returned.
>
> I recently tried 2.3.99-pre7 to see if anything was different (time
> is still screwed up); but I added some printk's to see what was
> happening. I've found that in the delay_at_last_interrupt calculation
> near .../arch/i386/kernel/time.c:523:
>
> count = ((LATCH-1) - count) * TICK_SIZE;
>
> the value retrieved from the i8253 timer becomes greater than
> (LATCH-1) for some reason, and forever after the clock is unstable.
> The correlation seems to be good: the system is OK until the first
> time (count > (LATCH-1)), and thereafter both (count > (LATCH-1)) and
> negative time steps happen 10's to 100's of times per second.
>
> So I wonder:
>
> * Is the count in the 8253 *always* supposed to be less than LATCH-1
> (==11931)?
>
> * Is there anything besides flakey hardware that might cause this
> behaviour?
>
> regards,
> d.
>
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