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

[Rafael J. Wysocki]

On Wed, Apr 13, 2016 at 7:53 PM, Steve Muckle <steve.muckle@xxxxxxxxxx> wrote:
> On 04/13/2016 07:45 AM, Rafael J. Wysocki wrote:
>>> I'm concerned generally with the latency to react to changes in
>>> > required capacity due to remote wakeups, which are quite common on SMP
>>> > platforms with shared cache. Unless the hook is called it could take
>>> > up to a tick to react AFAICS if the target CPU is running some other
>>> > task that does not get preempted by the wakeup.
>>
>> So the scenario seems to be that CPU A is running task X and CPU B
>> wakes up task Y on it remotely, but that task has to wait for CPU A to
>> get to it, so you want to increase the frequency of CPU A at the
>> wakeup time so as to reduce the time the woken up task has to wait.
>>
>> In that case task X would not be giving the CPU away (ie. no
>> invocations of schedule()) for the whole tick, so it would be
>> CPU/memory bound.  In that case I would expect CPU A to be running at
>> full capacity already unless this is the first tick period in which
>> task X behaves this way which looks like a corner case to me.
>
> This situation is fairly common in bursty workloads (such as UI driven
> ones).
>
>> Moreover, sending an IPI to CPU A in that case looks like the right
>> thing to do to me anyway.
>
> Sorry I didn't follow - sending an IPI to do what exactly? Perform the
> wakeup operation on the target CPU?

Basically, to run a frequency update.  You can combine that with the
wakeup itself, though, I suppose.