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: Tue, 8 Nov 2022 11:33:51 +0000 [thread overview]
Message-ID: <20221108113351.ne4kobarl5ooqffp@airbuntu> (raw)
In-Reply-To: <xhsmhpmdymw1n.mognet@vschneid.remote.csb>
On 11/07/22 18:58, Valentin Schneider wrote:
> On 05/11/22 19:24, Qais Yousef wrote:
> > 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.
> >
>
> Ok so we *do* have this with how the fitting criteria are combined (I
> didn't get that when I first scanned through the code); thanks for
> elaborating on that.
Oh yeah, this hasn't changed.
>
> > 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
>
next prev parent reply other threads:[~2022-11-08 11:34 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
2022-11-07 18:58 ` Valentin Schneider
2022-11-08 11:33 ` Qais Yousef [this message]
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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