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Message-ID: <alpine.LFD.2.00.1009150015500.2416@localhost6.localdomain6>
Date: Wed, 15 Sep 2010 00:18:14 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Nix <nix@...eri.org.uk>
cc: Artur Skawina <art.08.09@...il.com>,
Venkatesh Pallipadi <venki@...gle.com>,
Damien Wyart <damien.wyart@...e.fr>,
John Drescher <drescherjm@...il.com>,
Suresh Siddha <suresh.b.siddha@...el.com>,
LKML <linux-kernel@...r.kernel.org>,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [BISECTED] 2.6.35.*: horrible (exponential? >linear) slowdown
to unusability (HPET)
On Tue, 14 Sep 2010, Nix wrote:
> On 14 Sep 2010, Thomas Gleixner said:
>
> > On Fri, 10 Sep 2010, Thomas Gleixner wrote:
> >> On Fri, 10 Sep 2010, Nix wrote:
> >> >
> >> > I suspect current_clocksource doesn't do what we think it does, or the
> >> > clocksource and 'clock event device' are not the same.)
> >>
> >> Right, they are not the same. clocksource provides us a read out
> >> device for timekeeping (usually a simple increasing counter). clock
> >> event device is used to generate timer interrupts.
> >>
> >> HPET provides both functionalities.
> >>
> >> The patch you bisected is affecting the clock events part of the
> >> HPET. So yes, it's not a clock source problem.
> >
> > Another question. If you add hpet=verbose (or revert that commit) does
> > the printk_once in hpet_set_next_event() show up in dmesg ?
> >
> > "hpet: compare register read back failed."
>
> Well, given that I reverted that commit in the past (during the
> bisection) and also added hpet=verbose[1], I'd have expected it to turn
> up in my past syslogs. There is no sign of it, in syslogs back to 2009.
>
>
> [1] and indeed am running 2.6.35.4 with hpet=verbose and no symptoms
> on the affected system *now* -- a mystery because I thought the
> hpet=verbose-turns-it-off code was removed, but turning on verbose
> still seems to quash the bug?! or perhaps it's slightly
> intermittent and every N boots I'm lucky enough to miss it. I'm
> slightly chary of rebooting to find out ;}
hpet=verbose forces the read back.
FYI, we got a reasonable explanation now what's going on in hardware
land. Patch is in the works.
Thanks,
tglx
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