linux-kernel - Re: nohz and strange sleep latencies

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Message-ID: <20071122185121.GA1803@elf.ucw.cz>
Date:	Thu, 22 Nov 2007 19:51:21 +0100
From:	Pavel Machek <pavel@....cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: nohz and strange sleep latencies

Hi!

> > > Clock Event Device: hpet
> > >  set_next_event: hpet_legacy_next_event
> > >  set_mode:       hpet_legacy_set_mode
> > >  event_handler:  tick_handle_oneshot_broadcast
> > > 
> > > Clock Event Device: lapic
> > >  set_next_event: lapic_next_event
> > >  set_mode:       lapic_timer_setup
> > >  event_handler:  hrtimer_interrupt
> > > 
> > > Clock Event Device: lapic
> > >  set_next_event: lapic_next_event
> > >  set_mode:       lapic_timer_setup
> > >  event_handler:  hrtimer_interrupt
> > > 
> > > to me this has the feeling of lapic breakage in C2 mode. Does it get any 
> > > better if you boot with 'nolapic'? (but that might in turn turn off 
> > > high-res timers and nohz in essence) Thomas, any ideas?
> > 
> > Hmm, lapic is considered unstable in c2 by default. You have to tell 
> > the kernel that you trust it in C2 on the command line.
> 
> yeah, i was wondering about that too. ACPI enumerated them properly at a 
> certain stage:
> 
>  ACPI: CPU0 (power states: C1[C1] C2[C2] C3[C3])
>  ACPI: CPU1 (power states: C1[C1] C2[C2] C3[C3])
> 
> but perhaps somehow we miss this fact and fail to turn off the lapic 
> clockevents drivers?

I can confirm that setting max-cstate=1 helps. (Plus, problem gets
_way_ more visible with stripped down config. With usb and irda off,
time sleep .01 easily takes 2 seconds).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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