linux-kernel - Re: Jiffies jumping with the x86 HPET

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Date:	Tue, 17 Nov 2009 06:57:59 -0800 (PST)
From:	just2ducky1234-1@...oo.com
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Jiffies jumping with the x86 HPET

Thanks for your reply, now this problem is with customers, so a patch would probably be best. But I was mainly hoping for confirmation of the scenario and the one-liner (one-byte!) fix, since we can cobble something together to implement a patch. Also I wanted to get information out on Google, in case someone else encounters the problem, since there don't seem to be any hits on this.

Regards,
Lee

________________________________
From: Michael Tokarev <mjt@....msk.ru>
To: Lee Merrill <lee_merrill@...oo.com>
Cc: LKML <linux-kernel@...r.kernel.org>
Sent: Tue, November 17, 2009 9:05:15 AM
Subject: Re: Jiffies jumping with the x86 HPET

Lee Merrill wrote:
> We are seeing jiffies go forward occasionally, by 300 seconds, this it appears is due to the following code in the 2.6.16 kernel:
> 
> mark_offset_tsc_hpet(void):
> ...
> 1       hpet_current = hpet_readl(HPET_COUNTER);
> 2       rdtsc(last_tsc_low, last_tsc_high);
> 3
> 4       /* lost tick compensation */
> 5       offset = hpet_readl(HPET_T0_CMP) - hpet_tick;
> 6       if (unlikely(((offset - hpet_last) > hpet_tick) && (hpet_last != 0))
> 7                                       && detect_lost_ticks) {
> 8               int lost_ticks = (offset - hpet_last) / hpet_tick;
> 9               jiffies_64 += lost_ticks;
> 10      }
> 11      hpet_last = hpet_current;

There is no such code in current (2.6.31 or even 2.6.27.y) kernel.
Both hpet support and timer/clock code received huge changes in
2.6.24+ kernels.  I guess your best bet is to upgrade.

/mjt
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