Messages in this thread | | | Date | Thu, 14 Mar 2019 15:46:06 +0530 | From | Viresh Kumar <> | Subject | Re: [PATCH 0/7] cpufreq: Call transition notifier only once for each policy |
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On 14-03-19, 10:28, Rafael J. Wysocki wrote: > On Thu, Mar 14, 2019 at 7:43 AM Viresh Kumar <viresh.kumar@linaro.org> wrote: > > > > Currently we call the cpufreq transition notifiers once for each CPU of > > the policy->cpus cpumask, which isn't that efficient. > > Why isn't it efficient? > > Transitions are per-policy anyway, so if something needs to be done > for each CPU in the policy, it doesn't matter too much which part of > the code carries out the iteration.
Even if per-cpu iteration has to be done at some place, we are avoiding function calls here and the code/locking in the notifier layer as well. Will get more such info into changelog.
> I guess some notifiers need to know what other CPUs there are in the > policy? If so, then why?
You mean about the offline CPUs? I mentioned the rationale in 1/7. It is to avoid bugs where we may end up using a stale value if the CPUs are offlined/onlined regularly.
> > This patchset tries to simplify that by adding another field in struct cpufreq_freqs, > > cpus, so the callback has all the information available with a single > > call for each policy. > > Well, you can argue that the core is simplified by it somewhat, but > the notifiers aren't. They actually get more complex, conceptually > too, because they now need to worry about offline vs online CPUs etc.
24 different parts of the kernel register for transition notifiers and only 5 required update here, the other 19 don't need to do per-cpu stuff and they also get benefited by this work. Those routines will get called only once now, instead of once per every CPU of the policy.
> Also I wonder why you decided to pass a cpumask in freqs instead of > just passing a policy pointer. If you change things from per-CPU to > per-policy, passing the whole policy seems more natural.
I did that because they don't need to use the other fields of the policy today and that doesn't look likely in near future as well.
I can do that if you want, but not sure why more information should be provided than required.
-- viresh
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