Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2 1/9] sched/uclamp: Fix relationship between uclamp and migration margin | Date | Mon, 07 Nov 2022 18:58:12 +0000 |
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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.
> 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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