linux-kernel - Re: [BISECTED] 2.6.35.*: horrible (exponential? >linear) slowdown to unusability (HPET)

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Message-ID: <878w34qdu0.fsf@spindle.srvr.nix>
Date:	Tue, 14 Sep 2010 21:17:59 +0100
From:	Nix <nix@...eri.org.uk>
To:	Thomas Gleixner <tglx@...utronix.de>
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 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 ;}
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