Messages in this thread | | | Date | Thu, 16 Nov 2023 13:14:49 +0000 | Subject | Re: [PATCH v2 1/2] sched/schedutil: rework performance estimation | From | Lukasz Luba <> |
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Hi Vincent,
I know that there is v3, but just to respond to this below.
On 10/31/23 09:48, Vincent Guittot wrote: > Hi Lukasz, > > On Mon, 30 Oct 2023 at 18:45, Lukasz Luba <lukasz.luba@arm.com> wrote: >> >> Hi Vincent, >> >> On 10/26/23 18:09, Vincent Guittot wrote: >>> The current method to take into account uclamp hints when estimating the >>> target frequency can end into situation where the selected target >>> frequency is finally higher than uclamp hints whereas there are no real >>> needs. Such cases mainly happen because we are currently mixing the >>> traditional scheduler utilization signal with the uclamp performance >>> hints. By adding these 2 metrics, we loose an important information when >>> it comes to select the target frequency and we have to make some >>> assumptions which can't fit all cases. >>> >>> Rework the interface between the scheduler and schedutil governor in order >>> to propagate all information down to the cpufreq governor. >>> >>> effective_cpu_util() interface changes and now returns the actual >>> utilization of the CPU with 2 optional inputs: >>> - The minimum performance for this CPU; typically the capacity to handle >>> the deadline task and the interrupt pressure. But also uclamp_min >>> request when available. >>> - The maximum targeting performance for this CPU which reflects the >>> maximum level that we would like to not exceed. By default it will be >>> the CPU capacity but can be reduced because of some performance hints >>> set with uclamp. The value can be lower than actual utilization and/or >>> min performance level. >> >> You have probably missed my question in the last v1 patch set. > > Yes, sorry > >> >> The description above needs a bit of clarification, since looking at the >> patches some dark corners are introduced IMO: >> >> Currently, we have a less aggressive power saving policy than this >> proposal. >> >> The questions: >> What if the PD has 4 CPUs, the max util found is 500 and is from a CPU >> w/ uclamp_max, but there is another CPU with normal utilization 499? >> What should be the final frequency for that PD? > > We now follow the same sequence everywhere which can be summarized by: > > for each cpu sharing the same frequency domain: > util = cpu_util(cpu) > eff_util = effective_cpu_util(util, &min, &max) > eff_util = sugov_effective_cpu_perf(eff_util, min, max) which > applies the dvfs headroom if needed > max_util = max(max_util, eff_util); > > EAS anticipates the impact of the waking task on utilization and max > but the end result is the same as above once the task is enqueued so I > didn't show it for simplicity > > Coming back to your example > CPU0 has uclamp_max = 500 and an actual utilization above 500. Its > eff_util will be 500 > CPU1 doesn't have uclamp_max constraint and an actual utilization of > 499 which will be increase with dvfs headroom to 623 in > sugov_effective_cpu_perf() > > The final max util will be 623 > > With the current implementation we apply the dvfs headroom to the > final max_util (which is the CPU0 with uclamp_max == 500) whereas we > now apply the dvfs headroom on each CPU inside > sugov_effective_cpu_perf() > > The main difference is that if CPU1 has an actual utilization of 400, > the max_util of the frequency domain will be 500 whereas it is 625 > after applying dvfs headroom with current implementation > >> >> In current design, where we care more about 'delivered performance >> to the tasks' than power saving, the +20% would be applied for the >> frequency. Therefore if that CPU with 499 util doesn't have uclamp_max, >> it would get a decent amount of idle time for its tasks (to compensate >> some workload variation). > > CPU1 with 499 still gets its 25% margin or I missed something in your example ?
You understood this correctly. I don't have more questions. It should than work OK.
Thanks, Lukasz
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