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

[Rafael J. Wysocki]

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.