Re: [PATCH 3/5] tick/nohz: Don't shutdown the lowres tick from itself

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Sep 13, 2023 at 09:17:21PM -0400, Joel Fernandes wrote:
> On Tue, Sep 12, 2023 at 6:44 AM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > In lowres dynticks mode, just like in highres dynticks mode, when there
> > is no tick to program in the future, the tick eventually gets
> > deactivated either:
> >
> > * From the idle loop if in idle mode.
> > * From the IRQ exit if in full dynticks mode.
> >
> > Therefore there is no need to deactivate it from the tick itself. This
> > just just brings more overhead in the idle tick path for no reason.
> >
> > Fixes: 62c1256d5447 ("timers/nohz: Switch to ONESHOT_STOPPED in the low-res handler when the tick is stopped")
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> 
> If on some weird hardware, say  ts->next_tick = KTIME_MAX but a
> spurious timer interrupt went off and tick_nohz_handler() did get
> called (yeah weird hypothetical situation), then in
> tick_nohz_stop_tick() we might early return from:
> 
> /* Skip reprogram of event if its not changed */
> if (ts->tick_stopped && (expires == ts->next_tick))
> 
> without no "eventual" reprogramming.
> 
> Maybe we should also reprogram with KTIME_MAX in such a situation?
> Then we can get rid of it from tick_nohz_handler() for the common case
> as you are doing.
> 
> So for weird hardware, with this patch we are not doing an extra
> tick_program_event(KTIME_MAX, 1); like Nick was doing. That makes me a
> tad bit nervous.

So when a tick happens, ts->next_tick is reset to 0 (in tick_sched_handle()).
This way if a timer interrupt fires too early, and that includes also timer
interrupts when next_tick is KTIME_MAX, the timer is always reprogrammed upon
the next idle loop iteration. So this shouldn't happen.

Thanks.