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

From: Thomas Gleixner
Date: Mon Jul 21 2008 - 16:25:56 EST

On Mon, 21 Jul 2008, Philippe Troin wrote:
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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 ( ) and I have a bug
> posted on the Fedora/RH bugzilla (
> ).

Will have a look.

Question: which clocksource is active ?

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



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