Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Fri, 25 Nov 2022 14:36:53 +0100 | Subject | Re: [RFC PATCH 2/3] cpuidle: ladder: Tune promotion/demotion threshold |
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On Fri, Nov 25, 2022 at 7:39 AM Zhang Rui <rui.zhang@intel.com> wrote: > > Hi, Rafael, > > thanks for reviewing the patch series. > > On Wed, 2022-11-23 at 18:50 +0100, Rafael J. Wysocki wrote: > > On Sat, Nov 5, 2022 at 6:40 PM Zhang Rui <rui.zhang@intel.com> wrote: > > > After fixing the bogus comparison between u64 and s64, the ladder > > > governor stops making promotion decisions errornously. > > > > > > However, after this, it is found that the ladder governor demotes > > > much > > > easier than promotes. > > > > "After fixing an error related to using signed and unsigned integers > > in the ladder governor in a previous patch, that governor turns out > > to > > demote much easier than promote" > > > > > Below is captured using turbostat after a 30 seconds runtime idle, > > > > > > Without previous patch, > > > Busy% IRQ POLL C1 C1E C3 C6 C7s C8 > > > C9 C10 CPU%c1 CPU%c3 CPU%c6 CPU%c7 PkgWatt > > > 0.30 2373 0 0 0 4 9 25 122 > > > 326 2857 0.36 0.04 0.57 98.73 1.48 > > > > Why is the above relevant? > > Just for comparison purpose. > For a pure idle scenario (Busy% < 0.5), with ladder governor, we used > to have 99% CPU%c7, but now, with patch 1/3, > > CPU%c1 CPU%c3 CPU%c6 CPU%c7 > 34.18 16.21 17.69 31.51 > This does not look like the correct behavior for any cpuidle governor.
It all depends on what the design goal was and I don't really know that in this particular case.
It looks like the plan was to make it promote less often than demote or the counts would have been chosen differently.
> > > > > With previous patch, > > > Busy% IRQ POLL C1 C1E C3 C6 C7s C8 > > > C9 C10 CPU%c1 CPU%c3 CPU%c6 CPU%c7 PkgWatt > > > 0.42 3071 0 771 838 447 327 336 382 > > > 299 344 34.18 16.21 17.69 31.51 2.00 > > > > > > And this is caused by the imbalanced > > > PROMOTION_COUNT/DEMOTION_COUNT. > > > > I would explain why/how the imbalanced PROMOTION_COUNT/DEMOTION_COUNT > > imbalance causes this. > > sure, how about something below. > > The PROMOTION_COUNT/DEMOTION_COUNT are used as the threshold between > the ladder governor detects it "should promote/demote", and the ladder > governor does a real promotion/demotion. > > Currently, PROMOTION_COUNT is set to 4 and DEMOTION_COUNT is set to 1. > This means that the ladder governor does real demotion immediately when > it "should demote", but it does real promotion only if it "should > promote" 4 times in a row, without a single "should demote" occur in > between. > > As a result, this lower the chance to do real promotion and the ladder > governor is more likely to choose a shallower state.
Sounds good and now the question is what's the behavior expected by users. Do we have any data?
> > > > I guess more residency in the deeper idle state is expected? Or > > desired?? > > > > > With this patch, > > > Busy% IRQ POLL C1 C1E C3 C6 C7s C8 > > > C9 C10 CPU%c1 CPU%c3 CPU%c6 CPU%c7 PkgWatt > > > 0.39 2436 0 1 72 177 51 194 243 > > > 799 1883 0.50 0.32 0.35 98.45 1.53 > > > > > > Note that this is an experimental patch to illustrate the problem, > > > and it is checked with idle scenario only for now. > > > I will try to evaluate with more scenarios, and if someone can help > > > evaluate with more scenarios at the same time and provide data for > > > the > > > benefit with different PROMOTION_COUNT/DEMOTION_COUNT values, that > > > would be great. > > > > So yes, this requires more work. > > > > Overall, I think that you are concerned that the previous change > > might > > be regarded as a regression and are trying to compensate for it with > > a > > PROMOTION_COUNT/DEMOTION_COUNT change. > > Exactly. > > > I'm not sure I can agree with that approach, because the shallower > > idle states might be preferred by the original ladder design > > intentionally, for performance reasons. > > > hmmm, even if there is only 30% c7/c8/c9/c10 residency in a pure idle > scenario?
Yes, even in that case. All boils down to the question regarding user expectations.
> And further more, since the imbalanced PROMOTION_COUNT/DEMOTION_COUNT > and the unsigned/signed integers problem are both there since the first > day the ladder governor was introduced, commit 4f86d3a8e297 ("cpuidle: > consolidate 2.6.22 cpuidle branch into one patch"), > > my guess is that > > the unsigned/signed integers problem introduces a lot of pseudo > promotions, and the PROMOTION_COUNT/DEMOTION_COUNT is introduced to > workaround this so that the ladder governor doesn't get stuck at deep > idle state.
That may be a good guess, so I would add it to the changelog of the patch.
> I don't have a solid proof for this. But at least for the pure idle > scenario, I don't think 30% deep idle residency is the right behavior, > and it needs to be tuned anyway.
Well, have you checked what happens if the counts are set to the same value, e.g. 2?
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