linux-kernel - Re: [linus:master] [timers] 7ee9887703: stress-ng.uprobe.ops_per

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Date: Mon, 29 Apr 2024 12:40:52 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Lukasz Luba <lukasz.luba@....com>, "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Oliver Sang <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
 lkp@...el.com, linux-kernel@...r.kernel.org, Thomas Gleixner
 <tglx@...utronix.de>, ying.huang@...el.com, feng.tang@...el.com,
 fengwei.yin@...el.com, Frederic Weisbecker <frederic@...nel.org>, Daniel
 Lezcano <daniel.lezcano@...aro.org>, linux-pm@...r.kernel.org
Subject: Re: [linus:master] [timers] 7ee9887703:
 stress-ng.uprobe.ops_per_sec -17.1% regression

Anna-Maria Behnsen <anna-maria@...utronix.de> writes:

> Hi,
>
> Lukasz Luba <lukasz.luba@....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@...utronix.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.
>

Now here are the values of the states and the time because I forgot to
track also the time in the first run:

USAGE		good            bad		bad+patch
		----            ---             ---------
state0          115		137		234
state1          450680		354689		420904
state2          3092092		2687410		3169438


TIME		good            bad		bad+patch
		----            ---             ---------
state0		9347		9683		18378
state1		626029557	562678907	593350108
state2		6130557768	6201518541	6150403441


> 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...

.. but with the time values: CPUs are less often but in total longer in
state2.

Thanks,

	Anna-Maria