Messages in this thread | | | Date | Sat, 5 Nov 2022 19:24:52 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2 1/9] sched/uclamp: Fix relationship between uclamp and migration margin |
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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
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