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

[Vincent Guittot]

On 31 March 2016 at 14:34, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Mar 31, 2016 at 02:14:50PM +0200, Vincent Guittot wrote:
>> In fact, I looks for the sequence where the utilization of a rq is not
>> updated until the next tick but i can't find it.
>
> No, util it always updated, however..
>
>> If cpu doesn't share cache, task is added to wake list and an ipi is
>> sent and the utilization.
>
> Here we run:
>
>  ttwu_do_activate()
>    ttwu_activate()
>      activate_task()
>        enqueue_task()
>          p->sched_class->enqueue_task() := enqueue_task_fair()
>            update_load_avg()
>              update_cfs_rq_load_avg()
>                cfs_rq_util_change()
>
> On the local cpu, and we can indeed call out to have the frequency
> changed.
>
>> Otherwise, we directly enqueue the task on
>> the rq and the utilization is updated
>
> But here we run it on a remote cpu, so we cannot call out and the
> frequency remains the same.
>
> So if a remote wakeup on the same LLC domain happens, utilization will
> increase but we will not observe until the next tick.

ok. I forgot that we have the condition cpu == smp_processor_id() in
cfs_rq_util_change.