On Wed, Feb 16, 2022 at 02:34:12PM +0000 Aaron Tomlin wrote:
> On Thu 2022-02-03 17:22 -0500, Phil Auld wrote:
> > As I said earlier, I don't think you want to call quiet_vmstat()
> > unconditionally. And I don't think this will catch the cases you are
> > trying to fix. Once the tick is stopped tick_nohz_stop_tick should not be
> > getting called again until it's been restarted.
>
> Phil,
>
> Sorry about the delay. If I understand correctly, I see a scenario by which
> tick_nohz_stop_tick() can be called on transition/or exit from idle (e.g.
> default_idle_call()):
No worries. It's possible that I am misunderstanding the issue still... :)
>
> 1. The idle/or scheduling-clock was previously
> stopped
>
> 2. It is considered safe for the scheduling-clock
> tick to remain "stopped"/or omitted; no need to
> reprogram and enable a periodic tick
> (e.g. no queued/or expired pending timer)
>
> ...
> do_idle
> cpuidle_idle_call
> {
>
> ...
>
> .-- default_idle_call
> | arch_cpu_idle
> | goto exit_idle
> |
> | exit_idle:
> | __current_set_polling()
> |
> | }
> | tick_nohz_idle_exit()
> | {
> |
> | tick_stopped = ts->tick_stopped
> |
> | if (tick_stopped)
> | tick_nohz_idle_update_tick(ts, now)
> | if (tick_nohz_full_cpu(smp_processor_id()))
> | __tick_nohz_full_update_tick(ts, now)
> | {
> | int cpu = smp_processor_id()
> |
> | if (can_stop_full_tick(cpu, ts))
> | tick_nohz_stop_sched_tick(ts, cpu)
> | if (tick_nohz_next_event(ts, cpu))
> '-- tick_nohz_stop_tick(ts, cpu)
> }
> }
>
> If I understand correctly, __tick_nohz_full_update_tick() can return with
> no changes to the current tick (e.g. expire time == KTIME_MAX), no?
>
Yeah, I missed that path, so tick_nohz_stop_tick() can get called with the tick
already stopped. My concern was about calling quiet_vmstat() if the tick was not
stopped as per the comment on that function. But looking more closely at
tick_nohz_stop_tick() it won't be doing that with your patch.
If this is fixing the issue you are seeing (I don't remember if you had a
reproducible case or not) then I think this could be a good way to do it.
It does seem to rely on a few things lining up right to get to the call to
tick_nohz_stop_tick().
Cheers,
Phil
>
> Kind regards,
>
> --
> Aaron Tomlin
>
--
