Messages in this thread Patch in this message | | | Date | Fri, 21 Jul 2023 18:09:19 +0200 | Subject | Re: [PATCH] sched/fair: remove util_est boosting | From | Dietmar Eggemann <> |
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On 12/07/2023 17:30, Vincent Guittot wrote: > On Tue, 11 Jul 2023 at 17:47, Qais Yousef <qyousef@layalina.io> wrote: >> >> On 07/06/23 15:51, Vincent Guittot wrote: >>> There is no need to use runnable_avg when estimating util_est and that >>> even generates wrong behavior because one includes blocked tasks whereas >>> the other one doesn't. This can lead to accounting twice the waking task p, >>> once with the blocked runnable_avg and another one when adding its >>> util_est.
... and we don't have this issue for the util_avg case since we have:
7317 } else if (p && task_cpu(p) != cpu && dst_cpu == cpu) ^^^^^^^^^^^^^^^^^^ 7318 util += task_util(p);
>>> cpu's runnable_avg is already used when computing util_avg which is then >>> compared with util_est.
We discussed why I have to use max(X, runnable) for X=util and X=util_est in v2:
https://lkml.kernel.org/r/251b524a-2c44-3892-1bae-03f879d6a64b@arm.com
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I need the util_est = max(util_est, runnable) further down as well. Just want to fetch runnable only once.
util = 50, task_util = 5, util_est = 60, task_util_est = 10, runnable = 70
max(70 + 5, 60 + 10) != max (70 + 5, 70 + 10) when dst_cpu == cpu
<--
But I assume your point is that:
7327 if (boost) 7328 util_est = max(util_est, runnable);
7356 if (dst_cpu == cpu) <-- (1) 7357 util_est += _task_util_est(p); 7358 else if (p && unlikely(task_on_rq_queued(p) || current == p)) 7359 lsub_positive(&util_est, _task_util_est(p)); 7360 7361 util = max(util, util_est);
--> (1) doesn't work anymore in case `util_est == runnable`.
It will break the assumption for the if condition depicted in cpu_util()'s comment:
7331 * During wake-up (2) @p isn't enqueued yet and doesn't contribute 7332 * to any cpu_rq(cpu)->cfs.avg.util_est.enqueued. 7333 * If @dst_cpu == @cpu add it to "simulate" cpu_util after @p 7334 * has been enqueued.
(2) eenv_pd_max_util() and find_energy_efficient_cpu() call-site.
<---
Rerunning Jankbench tests on Pix6 will tell if boosting util_avg instead of both will still show the anticipated results. Likelihood is high that it will since we do `util = max(util, util_est)` at the end of cpu_util().
>>> In some situation, feec will not select prev_cpu but another one on the >>> same performance domain because of higher max_util >>> >>> Fixes: 7d0583cf9ec7 ("sched/fair, cpufreq: Introduce 'runnable boosting'") >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>> --- >> >> Can we verify the numbers that introduced this magic boost are still valid >> please? > > TBH I don't expect it but I agree it's worth checking. Dietmar could > you rerun your tests with this change ?
Could do. But first lets understand the issue properly.
>> Otherwise LGTM. >> >> >> Thanks! >> >> -- >> >> Qais Yousef >> >>> kernel/sched/fair.c | 3 --- >>> 1 file changed, 3 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index a80a73909dc2..77c9f5816c31 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -7289,9 +7289,6 @@ cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost) >>> >>> util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued); >>> >>> - if (boost) >>> - util_est = max(util_est, runnable); >>> - >>> /* >>> * During wake-up @p isn't enqueued yet and doesn't contribute >>> * to any cpu_rq(cpu)->cfs.avg.util_est.enqueued. >>> -- >>> 2.34.1 >>>
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