Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Tue, 22 May 2018 23:52:46 +0200 | Subject | Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked |
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On Tue, May 22, 2018 at 11:41 PM, Joel Fernandes <joel@joelfernandes.org> wrote: > On Tue, May 22, 2018 at 05:27:11PM +0200, Rafael J. Wysocki wrote: >> On Tue, May 22, 2018 at 2:22 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote: >> > On Tuesday, May 22, 2018 1:42:05 PM CEST Rafael J. Wysocki wrote: >> >> On Tue, May 22, 2018 at 1:38 PM, Viresh Kumar <viresh.kumar@linaro.org> wrote: >> >> > On 22-05-18, 13:31, Rafael J. Wysocki wrote: >> >> >> So below is my (compiled-only) version of the $subject patch, obviously based >> >> >> on the Joel's work. >> >> >> >> >> >> Roughly, what it does is to move the fast_switch_enabled path entirely to >> >> >> sugov_update_single() and take the spinlock around sugov_update_commit() >> >> >> in the one-CPU case too. >> > >> > [cut] >> > >> >> > >> >> > Why do you assume that fast switch isn't possible in shared policy >> >> > cases ? It infact is already enabled for few drivers. >> > >> > I hope that fast_switch is not used with devfs_possible_from_any_cpu set in the >> > one-CPU policy case, as that looks racy even without any patching. >> >> Which would be the only case in which sugov_update_single() would run >> on a CPU that is not the target. >> >> And running sugov_update_single() concurrently on two different CPUs >> for the same target is a no-no, as we don't prevent concurrent updates >> from occurring in that path. >> >> Which means that the original patch from Joel will be sufficient as >> long as we ensure that sugov_update_single() can only run on one CPU >> at a time. > > Since target CPU's runqueue lock is held, I don't see how we can run > sugov_update_single concurrently with any other CPU for single policy, so > protecting such race shouldn't be necessary.
If dvfs_possible_from_any_cpu is set, any CPU can run sugov_update_single(), but the kthread will only run on the target itself. So another CPU running sugov_update_single() for the target may be racing with the target's kthread.
> Also the "if (work_in_progress)" check I added to the sugov_update_single > doesn't change the behavior of single policy from what it is in mainline > since we were doing the same thing in already sugov_should_update_freq.
No, it doesn't, which doesn't mean that this is all OK. :-)
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