Messages in this thread | | | Subject | Re: [RFC PATCH 2/3] cpuidle: ladder: Tune promotion/demotion threshold | From | Zhang Rui <> | Date | Fri, 25 Nov 2022 14:38:59 +0800 |
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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.
> > > 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.
> > 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?
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.
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.
thanks, rui
> > Signed-off-by: Zhang Rui <rui.zhang@intel.com> > > --- > > drivers/cpuidle/governors/ladder.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/cpuidle/governors/ladder.c > > b/drivers/cpuidle/governors/ladder.c > > index fb61118aef37..4b47aa0a4da9 100644 > > --- a/drivers/cpuidle/governors/ladder.c > > +++ b/drivers/cpuidle/governors/ladder.c > > @@ -20,8 +20,8 @@ > > #include <asm/io.h> > > #include <linux/uaccess.h> > > > > -#define PROMOTION_COUNT 4 > > -#define DEMOTION_COUNT 1 > > +#define PROMOTION_COUNT 2 > > +#define DEMOTION_COUNT 4 > > > > struct ladder_device_state { > > struct { > > --
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