linux-kernel - Re: nohz and strange sleep latencies

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Message-ID: <alpine.LFD.0.9999.0711222121500.25057@localhost.localdomain>
Date:	Thu, 22 Nov 2007 21:29:51 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Pavel Machek <pavel@....cz>
cc:	Ingo Molnar <mingo@...e.hu>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: nohz and strange sleep latencies

On Thu, 22 Nov 2007, Pavel Machek wrote:
> > 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

> 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 ?

     tglx
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