linux-kernel - Re: [PATCH] sched: do not stop ticks when cpu is not idle

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Message-ID: <alpine.LFD.1.10.0807212154420.4316@apollo.tec.linutronix.de>
Date:	Mon, 21 Jul 2008 22:25:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Philippe Troin <phil@...i.org>
cc:	eric miao <eric.y.miao@...il.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Jack Ren <jack.ren@...vell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle

On Mon, 21 Jul 2008, Philippe Troin wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> I've seen weird timer behavior on both i386 and x86_64 on SMP
> machines.  By weird I mean:
> 
>  - time stops for a few hours, then resumes as if nothing happened;
> 
>  - time flows too fast or slow (4x faster to 2x slower depending on
>    phase of the moon);
> 
>  - the last one I've seen (yesterday), was: 
>       sleep(1) sleeps for 1 second, but
>       select(0, NULL, NULL, NULL, 0.5) sleeps for nine seconds.
> 
> I have been trying to track this problem for a few weeks now, without
> success.  Booting a CONFIG_NO_HZ-enabled kernel with "highres=off
> nohz=off" does not make a difference.  However booting a kernel with
> CONFIG_NO_HZ and CONFIG_HIGH_RES_TIMERS disabled seems to be working
> (I cannot garantee that since I've been using that for 48h so far, but
> sometimes the problem takes a few days to manifest itself).
> 
> After a cursory reading of your patch, it looks to me that the race
> could happen on a kernel compiled with CONFIG_NO_HZ and
> CONFIG_HIGH_RES_TIMERS and booted with "nohz=off highres=off".  Can
> you confirm that?

No, I can not confirm that. With nohz=off / highres=off that code path
is not invoked.
 
> If you need more details (dmesg, lspci, etc), I have posted some
> details on LKML ( http://lkml.org/lkml/2008/7/9/330 ) and I have a bug
> posted on the Fedora/RH bugzilla (
> https://bugzilla.redhat.com/show_bug.cgi?id=451824 ).

Will have a look. 

Question: which clocksource is active ?

cat /sys/devices/system/clocksource/clocksource0/current_clocksource

Thanks,

	tglx

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