On Thu 2022-02-24 09:27 -0300, Marcelo Tosatti wrote:
> But Aaron, vmstat_shepherd should be ensuring that per-CPU vmstat_update
> work are queued, if the per-CPU vmstat are out of sync.
Hi Marcelo,
Yes, I agree; albeit, as far as I understand, in the context of a nohz_full
CPU that has its scheduling-clock tick stopped, we cannot rely on any
deferred work.
The purpose of my patch was to prevent a nohz_full CPU from entering idle
state when CPU-specific vmstat data is non-zero.
> And:
>
> static void
> trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
> {
> if (!is_timers_nohz_active())
> return;
>
> /*
> * TODO: This wants some optimizing similar to the code below, but we
> * will do that when we switch from push to pull for deferrable timers.
> */
> if (timer->flags & TIMER_DEFERRABLE) {
> if (tick_nohz_full_cpu(base->cpu))
> wake_up_nohz_cpu(base->cpu);
> return;
> }
>
> * @TIMER_DEFERRABLE: A deferrable timer will work normally when the
> * system is busy, but will not cause a CPU to come out of idle just
> * to service it; instead, the timer will be serviced when the CPU
> * eventually wakes up with a subsequent non-deferrable timer.
>
> You'd want that vmstat_update to execute regardless of whether there are
> armed non-deferrable timers.
>
> Should fix both 1 and 2 AFAICS.
>
If I understand correctly, you are suggesting to switch to a non-deferred
timer for such work when the scheduling-clock tick is stopped? Indeed, it
would address both scenarios yet I'm not sure we'd want that due to the
performance impact which might be more than negligible.
Kind regards,
--
Aaron Tomlin
