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From: Qais Yousef <qyousef@layalina.io>
To: Valentin Schneider <vschneid@redhat.com>
Cc: Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	linux-kernel@vger.kernel.org, Xuewen Yan <xuewen.yan94@gmail.com>,
	Lukasz Luba <lukasz.luba@arm.com>, Wei Wang <wvw@google.com>,
	Jonathan JMChen <Jonathan.JMChen@mediatek.com>,
	Hank <han.lin@mediatek.com>
Subject: Re: [PATCH v2 1/9] sched/uclamp: Fix relationship between uclamp and migration margin
Date: Sat, 5 Nov 2022 19:24:52 +0000	[thread overview]
Message-ID: <20221105192452.e6lgq55uyiij7ecf@airbuntu> (raw)
In-Reply-To: <xhsmhy1sqvd0a.mognet@vschneid.remote.csb>

On 11/04/22 17:35, Valentin Schneider wrote:

> > +	/*
> > +	 * We must use capacity_orig_of() for comparing against uclamp_min and
> > +	 * uclamp_max. We only care about capacity pressure (by using
> > +	 * capacity_of()) for comparing against the real util.
> > +	 *
> > +	 * If a task is boosted to 1024 for example, we don't want a tiny
> > +	 * pressure to skew the check whether it fits a CPU or not.
> > +	 *
> > +	 * Similarly if a task is capped to capacity_orig_of(little_cpu), it
> > +	 * should fit a little cpu even if there's some pressure.
> > +	 *
> > +	 * Only exception is for thermal pressure since it has a direct impact
> > +	 * on available OPP of the system.
> > +	 *
> > +	 * We honour it for uclamp_min only as a drop in performance level
> > +	 * could result in not getting the requested minimum performance level.
> > +	 *
> 
> Why specifically care about OPPs here? Per our CPU capacity model, a task
> alone on a CPUx throttled to f=fmax/2 and a task coscheduled on a CPUy with
> RT/DL tasks and/or IRQs such that cpu_capacity(CPUy) = 50% are both getting
> (roughly) the same performance level.

Depends how you define performance level. What you call performance level,
I think is better called bandwidth. Uclamp is a performance and not a bandwidth
hint.

If a 10% task:

	p->util_avg = 10% * 1024

is requesting max performance level

	p->uclamp_min = 1024

This will translate to running at highest frequency and in case of big.LITTLE
system, the biggest CPU too.

RT/DL pressure has no impact in the task being able to achieve this; that is
running at max frequency and biggest cpu.

If the cpu has no bandwidth to fit this task, then our usual comparison of
util_avg with capacity_of() should fail as usual.

In the example above, the RT/DL pressure has to be pretty high for the 10% task
not to fit from bandwidth point of view. Which has nothing to do with
uclamp_min.  Only thermal pressure which drops OPPs can actually affect the
uclamp_min hint/request.

That is, when the task runs it will run at maximum frequency regardless of the
RT/DL pressure. The fact that the bandwidth of the CPU can be stolen has
nothing to do with uclamp_min hint.


Thanks!

--
Qais Yousef

  reply	other threads:[~2022-11-05 19:25 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-04 14:36 [PATCH v2 0/9] Fix relationship between uclamp and fits_capacity() Qais Yousef
2022-08-04 14:36 ` [PATCH v2 1/9] sched/uclamp: Fix relationship between uclamp and migration margin Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-11-04 17:35   ` [PATCH v2 1/9] " Valentin Schneider
2022-11-05 19:24     ` Qais Yousef [this message]
2022-11-07 18:58       ` Valentin Schneider
2022-11-08 11:33         ` Qais Yousef
2022-11-09 10:33   ` Dietmar Eggemann
2022-08-04 14:36 ` [PATCH v2 2/9] sched/uclamp: Make task_fits_capacity() use util_fits_cpu() Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 3/9] sched/uclamp: Fix fits_capacity() check in feec() Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 4/9] sched/uclamp: Make select_idle_capacity() use util_fits_cpu() Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 5/9] sched/uclamp: Make asym_fits_capacity() " Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 6/9] sched/uclamp: Make cpu_overutilized() " Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 7/9] sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s early exit condition Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-08-04 14:36 ` [PATCH v2 8/9] sched/fair: Detect capacity inversion Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-11-09 10:42   ` [PATCH v2 8/9] " Dietmar Eggemann
2022-11-12 19:35     ` Qais Yousef
2022-11-16 17:45       ` Dietmar Eggemann
2022-11-20 21:30         ` Qais Yousef
2022-08-04 14:36 ` [PATCH v2 9/9] sched/fair: Consider capacity inversion in util_fits_cpu() Qais Yousef
2022-10-28  6:42   ` [tip: sched/core] " tip-bot2 for Qais Yousef
2022-11-04 17:35   ` [PATCH v2 9/9] " Valentin Schneider
2022-11-05 20:41     ` Qais Yousef
2022-11-07 18:58       ` Valentin Schneider
2022-11-08 11:51         ` Qais Yousef
2022-11-09 10:43       ` Dietmar Eggemann

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