Re: [BISECTED] "conservative" cpufreq governor broken

[Steven Noonan <steven@uplinklabs.net>]

On Tue, Oct 6, 2009 at 5:36 AM, Eero Nurkkala
<ext-eero.nurkkala@nokia.com> wrote:
> 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@nokia.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 ;) )
>

Well, I tried adding the code to cpu_idle() as suggested, but it never
printed anything. Apparently cpu_idle() isn't ever being called here.
Even added a 'BUG();' at the beginning of the function and it never
hit it. Of course, I'm probably missing something obvious. Is there a
separate cpu_idle()-esque function for SMP?

- Steven