Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()

Rafael J. Wysocki
2016-04-13T16:07:13Z

On Wed, Apr 13, 2016 at 6:05 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Wed, Apr 13, 2016 at 6:48 AM, Rafael J. Wysocki <rafael@kernel.org> wrote:
>> On Wed, Apr 13, 2016 at 2:08 AM, Steve Muckle <steve.muckle@linaro.org> wrote:
>>> On Tue, Apr 12, 2016 at 04:29:06PM +0200, Rafael J. Wysocki wrote:
>>>> This is rather fundamental.
>>>>
>>>> For example, if you look at cpufreq_update_util(), it does this:
>>>>
>>>> data = rcu_dereference_sched(*this_cpu_ptr(&cpufreq_update_util_data));
>>>>
>>>> meaning that it will run the current CPU's utilization update
>>>> callback. Of course, that won't work cross-CPU, because in principle
>>>> different CPUs may use different governors and therefore different
>>>> util update callbacks.
>>>
>>> Will something like the attached (unfinished patches) work? It seems
>>> to for me, but I haven't tested it much beyond confirming the hook is
>>> working on remote wakeups.
>>
>> No, they are not sufficient.
>>
>> First of all, you need to take all of the governors into account and
>> they all make assumptions about updates being run on the CPU being
>> updated.
>>
>> That should be easy to take into account for ondemand/conservative,
>> but intel_pstate is a different story.
>>
>>> I'm relying on the previous comment that it's up to cpufreq drivers to
>>> run stuff on the target policy's CPUs if the driver needs that.
>>
>> That's not the case for the fast frequency switching though, which has
>> to happen on the CPU running the code.
>>
>>> There's still some more work, fixing up some more smp_processor_id()
>>> usage in schedutil, but it should be easy (trace, slow path irq_work
>>> target).
>>>
>>>> If you want to do remote updates, I guess that will require an
>>>> irq_work to run the update on the target CPU, but then you'll probably
>>>> want to neglect the rate limit on it as well, so it looks like a
>>>> "need_update" flag in struct update_util_data will be useful for that.
>>>
>>> Why is it required to run the update on the target CPU?
>>
>> The fast switching and intel_pstate are the main reason.
>>
>> They both have to write to registers of the target CPU and the code to
>> do that needs to run on that CPU.
>
> And these two seem to be the only interesting cases for you, because
> if you need to work for the worker thread to schedule to eventually

s/work/wait/ (sorry)

> change the CPU frequency for you, that will defeat the whole purpose
> here.