Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

[Rafael J. Wysocki]

On Tue, May 22, 2018 at 11:41 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx> 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. :-)