On Thu, Mar 14, 2019 at 11:16 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote: > > On 14-03-19, 10:28, Rafael J. Wysocki wrote: > > On Thu, Mar 14, 2019 at 7:43 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> 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. I'm not really convinced about this. CPU online really should take care of updating everything anyway. > > > 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. This is a much better rationale for the change than the one given originally IMO. :-) > > 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. But some of them need to combine the new cpumask with cpu_online_mask() to get what would be policy->cpus effectively. That would be avoidable if you passed the policy pointer to them.
