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

[Rafael J. Wysocki]

On Mon, Apr 11, 2016 at 11:20 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> On Mon, Apr 11, 2016 at 9:28 PM, Steve Muckle <steve.muckle@xxxxxxxxxx> wrote:
>> Hi Rafael,
>>
>> On 04/01/2016 02:20 AM, Peter Zijlstra wrote:
>>>> > My thinking was in CFS we get rid of the (cpu == smp_processor_id())
>>>> > condition for calling the cpufreq hook.
>>>> >
>>>> > The sched governor can then calculate utilization and frequency required
>>>> > for cpu. If (cpu == smp_processor_id()), the update is processed
>>>> > normally. If (cpu != smp_processor_id()) and the new frequency is higher
>>>> > than cpu's Fcur, the sched gov IPIs cpu to continue running the update
>>>> > operation. Otherwise, the update is dropped.
>>>> >
>>>> > Does that sound plausible?
>>>
>>> Can be done I suppose..
>>
>> Currently we drop schedutil updates for a target CPU which do not occur
>> on that CPU.
>>
>> Is this solely due to platforms which must run the cpufreq driver on the
>> target CPU?
>
> The current code assumes that the CPU running the update will always
> be the one that gets updated.  Anything else would require extra
> synchronization.


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.

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.

I think I can prototype something along these lines, but can you
please tell me more about the case you have in mind?

Thanks,
Rafael