linux-kernel - Re: [BISECTED] "conservative" cpufreq governor broken

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Message-Id: <1254825795.31336.204.camel@eenurkka-desktop>
Date:	Tue, 06 Oct 2009 13:43:15 +0300
From:	Eero Nurkkala <ext-eero.nurkkala@...ia.com>
To:	ext Steven Noonan <steven@...inklabs.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: Re: [BISECTED] "conservative" cpufreq governor broken

On Tue, 2009-10-06 at 12:22 +0200, ext Steven Noonan wrote:
> 
> I would suspect you have to have CONFIG_NO_HZ enabled to be able to
> reproduce the issue (considering the title of the bisected commit and
> my own config). Do you have it enabled?
> 

Yes, it's enabled.

> > And another round:
> >
> > cpufreq stats: OP1:16,78%, OP2:0,24%, OP3:5,14%, OP4:77,83%  (72)
> >
> > Just once more after doing nothing:
> > OP1:7,41%, OP2:0,11%, OP3:2,38%, OP4:90,10%  (82)
> >
> > So I can't agree it's broken. The patch you bisected, actually filtered
> > out such phenomenon, in which an IRQ made the cpufreq framework
> > occasionally think we were idling, although we were not. So you got
> > "bonus" idle time that shouldn't been there in the first place. Now that
> > the "bonus" idle time is not there, your system load may indeed be so
> > high that the system never spends 80% or more time in idle? Could that
> > be the case? Of course, even though I can't agree it's broken, doesn't
> > mean it isn't somehow broken ;) It'd be nice to get info on other
> > systems as well...
> 
> Interestingly, "ondemand" (the governor fixed by the bisected commit)
> works fine. "conservative" is the only broken one.
> 

If you took timestamps in /arch/x86/kernel/process_**.c:
(let's assume process_64.c) in cpu_idle()
around enter_idle(); and __exit_idle(), took the diff,
added the diffs up, and compared it to system uptime, you could see how
much time you spend in idle()? I think it's possible that
even if the cpu load is near 0%, the system may idle only for a bare
moment (that translates to a buggy pm_idle()), and time is spent
elsewhere (less than 80% in idle). 

If I had similar findings like you, I'd be worried; but traces here and
there will point out where things go wrong ;)

- Eero

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