Messages in this thread | | | Date | Thu, 4 Oct 2018 12:58:41 +0100 | From | Quentin Perret <> | Subject | Re: [PATCH v7 13/14] sched/topology: Make Energy Aware Scheduling depend on schedutil |
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On Thursday 04 Oct 2018 at 12:50:23 (+0200), Peter Zijlstra wrote: > On Wed, Sep 12, 2018 at 10:13:08AM +0100, Quentin Perret wrote: > > Energy Aware Scheduling (EAS) is designed with the assumption that > > frequencies of CPUs follow their utilization value. When using a CPUFreq > > governor other than schedutil, the chances of this assumption being true > > are small, if any. When schedutil is being used, EAS' predictions are at > > least consistent with the frequency requests. Although those requests > > have no guarantees to be honored by the hardware, they should at least > > guide DVFS in the right direction and provide some hope in regards to the > > EAS model being accurate. > > > > To make sure EAS is only used in a sane configuration, create a strong > > dependency on schedutil being used. Since having sugov compiled-in does > > not provide that guarantee, make CPUFreq call a scheduler function on > > on governor changes hence letting it rebuild the scheduling domains, > > check the governors of the online CPUs, and enable/disable EAS > > accordingly. > > So this patch disables EAS when we change cpufreq gov. How about we > force select schedutil and disallow changing it when EAS is enabled? > > Instead of silently disabling EAS when we frob the governor?
By doing it this way I think we have a better integration with things like that exclusive cpusets. In the end, EAS is enabled in a per-root_domain basis. So, say you have two root domains, A and B. EAS is enabled on A and disabled on B (because it doesn't have SD_ASYM_CPUCAPACITY or something). Should you be forced to use sugov on the CPUs of B to run EAS on A ? If there is a cpufreq policy covering only the CPUs of B, I'd say probably no.
Now I get the argument that this is happening silently as-is, so we can do better. How about we put a message in dmesg we EAS gets disabled in case of governor changes ?
Also, I'm currently working on a doc patch for EAS ATM (to be posted soon), so I can (and will) make it very clear there that you _must_ use sugov to use EAS.
Would that work ?
Thanks, Quentin
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