linux-kernel - Re: nohz and strange sleep latencies

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Message-ID: <20071123122254.GC2055@elf.ucw.cz>
Date:	Fri, 23 Nov 2007 13:22:54 +0100
From:	Pavel Machek <pavel@....cz>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: nohz and strange sleep latencies

Hi!

> > > but perhaps somehow we miss this fact and fail to turn off the lapic 
> > > clockevents drivers?
> > 
> > Ok, I guess I'm lost. If I offline second CPU, I immediately get
> > 1000Hz timer tick... is that expected?
> 
> Hmm. No. I have no idea why this is happening.
> 
> 34196 total events, 55.083 events/sec
> echo 0 >/sys/devices/system/cpu/cpu1/online
> 36073 total events, 54.679 events/sec

Strange.

> > I'm trying to decide when system is idle (lets say that means "no user
> > task is scheduled to wakeup within 10 seconds)... I added some
> > instrumentation to nohz subsystem, but it does not behave like I'd
> > expect: even if I run "while true; do sleep .01; done" loop, I see
> > nohz preparing for 5 seconds sleep... while it seems obvious that it
> > can only be 10msec sleep, and with max_cstate=1, it works that
> > way... Plus, nte->start_pid seems to contain some random numbers :-(.
> > 
> > What am I doing wrong?
> > 
> > (Patch for illustration, I can generate full diff against vanilla,
> > but...)
> 
> Just to make sure what we are hunting: Do you have the same problem
> with an non-pavel-tainted 2.6.24-rc3 ?

The strange sleep latencies were definitely there, I'll check for
"offline cpu and get 1000 interrupts", too.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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