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

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Message-Id: <1254832609.31336.290.camel@eenurkka-desktop>
Date:	Tue, 06 Oct 2009 15:36:49 +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 13:22 +0200, ext Steven Noonan wrote:
> On Tue, Oct 6, 2009 at 3:43 AM, Eero Nurkkala
> <ext-eero.nurkkala@...ia.com> wrote:
> > 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).
> 
> This makes logical sense, but how should I test this? Is there a way
> to do this with existing tracers?

Tracers may by themselves add some load into the system.

If I were you, I'd add something like: (I have only one CPU BTW)

static ktime_t time_prior_idle;
static int64_t idle_total;

time_prior_idle = ktime_get();
<idle stuff>
idle_total += ktime_to_ns(ktime_sub(ktime_get(), time_prior_idle));

and have a sysfs hook (something already present, so you can just cat
it) with a trace:

printk("Times: %lld, %lld \n", idle_total, ktime_to_ns(ktime_get()));

Sample output:
374758812519, 386768249832

So I have 386768249832 / 386768249832 = 96.9 % time spent in idle in
this case.

(Right, this should provide somewhat descent info, hopefully ;) )

- Eero

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