Re: [PATCH 1/7] sched: Update rq clock on nohz CPU before migrating tasks

[Ingo Molnar <mingo@kernel.org>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> @@ -1115,6 +1116,12 @@ static inline void dec_nr_running(struct rq *rq)
>  
>  extern void update_rq_clock(struct rq *rq);
>  
> +static inline void update_nohz_rq_clock(struct rq *rq)
> +{
> +	if (tick_nohz_extended_cpu(cpu_of(rq)))
> +		update_rq_clock(rq);
> +}

A minor comment: instead of implicitly knowing that full nohz CPUs mean a stale 
rq_clock, how about adding this information to the rq-> itself?

Something like introducing rq->clock_valid, initializing it to 1, and setting it 
to 0 when a CPU stops the tick.

(This would also allow the debug detection of sched_clock() use of stale values.)

We already have a similar flag: rq->skip_clock_update. I'd suggest to introduce a 
'struct sched_clock' helper structure and add the flags and scheduler clock fields 
as:

   rq->clock                 ->       rq->clock.cpu
   rq->clock_task            ->       rq->clock.task
   rq->clock_valid           ->       rq->clock.valid
   rq->clock_skip_uipdate    ->       rq->clock.skip_update
   rq->hrtick_timer          ->       rq->clock.hrtick_timer

   rq->prev_irq_time         ->       rq->clock.prev_irq_time
   rq->prev_steal_time       ->       rq->clock.prev_steal_time
   rq->prev_steal_time_rq    ->       rq->clock.prev_steal_time_rq

Thanks,

	ngo