Re: [linus:master] [timers] 7ee9887703: stress-ng.uprobe.ops_per_sec -17.1% regression

Anna-Maria Behnsen
2024-04-29T09:26:24Z

Hi,

Lukasz Luba <lukasz.luba@arm.com> writes:
> On 4/26/24 17:03, Rafael J. Wysocki wrote:
>> On Thu, Apr 25, 2024 at 10:23 AM Anna-Maria Behnsen
>> <anna-maria@linutronix.de> wrote:

[...]

>>> So my assumption here is, that cpuidle governors assume that a deeper
>>> idle state could be choosen and selecting the deeper idle state makes an
>>> overhead when returning from idle. But I have to notice here, that I'm
>>> still not familiar with cpuidle internals... So I would be happy about
>>> some hints how I can debug/trace cpuidle internals to falsify or verify
>>> this assumption.
>>
>> You can look at the "usage" and "time" numbers for idle states in
>>
>> /sys/devices/system/cpu/cpu*/cpuidle/state*/
>>
>> The "usage" value is the number of times the governor has selected the
>> given state and the "time" is the total idle time after requesting the
>> given state (ie. the sum of time intervals between selecting that
>> state by the governor and wakeup from it).
>>
>> If "usage" decreases for deeper (higher number) idle states relative
>> to its value for shallower (lower number) idle states after applying
>> the test patch, that will indicate that the theory is valid.
>
> I agree with Rafael here, this is the first thing to check, those
> statistics. Then, when you see difference in those stats in baseline
> vs. patched version, we can analyze the internal gov decisions
> with help of tracing.
>
> Please also share how many idle states is in those testing platforms.

Thanks Rafael and Lukasz, for the feedback here!

So I simply added the state usage values for all 112 CPUs and calculated
the diff before and after the stress-ng call. The values are from a
single run.

good bad bad+patch
---- --- ---------
state0 111 68 234
state1 419774 362549 408681
state2 3184799 2499565 3185723

good: 57e95a5c4117 ("timers: Introduce function to check timer base
is_idle flag")
bad: v6.9-rc4
bad+patch: v6.9-rc4 + patch

I choosed v6.9-rc4 for "bad", to make sure all the timer pull model fixes
are applied.

If I got Raphael right, the values indicate, that my theory is not
right...

Thanks,

Anna-Maria