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

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Date:   Mon, 17 Jul 2023 17:30:49 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, vineethrp@...il.com
Subject: Re: [PATCH 3/3] tick/nohz: Don't shutdown the lowres tick from itself

On Sat, Jul 15, 2023 at 08:18:57PM +0200, Frederic Weisbecker wrote:
> On Fri, Jul 14, 2023 at 09:02:43PM -0400, Joel Fernandes wrote:
> > On Fri, Jul 14, 2023 at 8:01 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> > >
> > > On Fri, Jul 14, 2023 at 02:44:49PM -0400, Joel Fernandes wrote:
> > > > On 7/14/23 08:08, Frederic Weisbecker wrote:
> > > > One slight concern here though, where in the idle loop is the removed
> > > > statement "tick_program_event(KTIME_MAX, 1);" happening if the tick was
> > > > already stopped before? If it is happening in tick_nohz_stop_tick(), don't
> > > > we early return from there and avoid doing that
> > > > "tick_program_event(KTIME_MAX, 1);" altogether, if the tick was already
> > > > stopped and the next event has not changed?
> > > >
> > > >         /* Skip reprogram of event if its not changed */
> > > >         if (ts->tick_stopped && (expires == ts->next_tick)) {
> > > >                 /* Sanity check: make sure clockevent is actually programmed */
> > > >                 if (tick == KTIME_MAX || ts->next_tick ==  [...]
> > > >                         return;
> > > >               [...]
> > > >       }
> > >
> > > Sure, if tick_program_event(KTIME_MAX, 1) was already called in the
> > > previous idle loop iteration, then there is no need to call that again.
> > >
> > > Or am I missing something else?
> > 
> > Just take it with a grain of salt but I think you need to still call
> > tick_program_event(KTIME_MAX, 1) here for the case where the tick was
> > previously stopped, and then when the next tick fires (say after a
> > long time T), but that tick is a one-off and does not result in
> > restarting the tick -- then there is no one to call
> > "tick_program_event(KTIME_MAX, 1)".
> 
> I'm a bit confused about that one-off thing. What can trigger that timer
> interrupt if it has been stopped?
> 
> One thing can happen though: a pending timer IRQ while we are stopping the
> tick (IRQs are disabled in that idle loop portion). But then that pending timer
> interrupt is not going to reprogram another one. So it remains stopped.

I think I see what you mean now. Maybe I wrongly assumed the above 'Skip
reprogram of event' code could early return and skip over
"tick_program_event(KTIME_MAX, 1);", but I think it cannot because of the
"expires != ts->next_tick" check.

 Maybe the "tick_program_event(KTIME_MAX, 1)" bit in tick_nohz_handler() is
 supposed to handle buggy hardware where an unexpected timer event came
 through? In such a situation, the idle loop will not write
 "tick_program_event(KTIME_MAX, 1);" again because it already did so the
 previous time, as you pointed.

Adding Vineeth who is also looking into this code.

thanks,

 - Joel