linux-kernel - Re: [PATCH v5 11/18] timer: Split out "get next timer interrupt" functionality

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Date:   Thu, 9 Mar 2023 18:45:46 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v5 11/18] timer: Split out "get next timer interrupt"
 functionality

On Thu, Mar 09, 2023 at 05:30:12PM +0100, Frederic Weisbecker wrote:
> Le Wed, Mar 01, 2023 at 03:17:37PM +0100, Anna-Maria Behnsen a écrit :
> > The functionallity for getting the next timer interrupt in
> > get_next_timer_interrupt() is splitted into a separate function
> > fetch_next_timer_interrupt() to be usable by other callsides.
> > 
> > This is preparatory work for the conversion of the NOHZ timer
> > placement to a pull at expiry time model. No functional change.
> > 
> > Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> [...]
> > diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> > index ff41d978cb22..dfc744545159 100644
> > --- a/kernel/time/timer.c
> > +++ b/kernel/time/timer.c
> > @@ -2040,31 +2071,9 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
> >  		if (time_before(nextevt, basej))
> >  			nextevt = basej;
> >  		tevt.local = basem + (u64)(nextevt - basej) * TICK_NSEC;
> > -		goto unlock;
> > +		tevt.global = KTIME_MAX;
> >  	}
> >  
> > -	/*
> > -	 * If the bases are marked idle, i.e. the next event on both the
> > -	 * local and the global queue are farther away than a tick,
> > -	 * evaluate both bases. No need to check whether one of the bases
> > -	 * has an already expired timer as this is caught by the !is_idle
> > -	 * condition above.
> > -	 */
> > -	if (base_local->timers_pending)
> > -		tevt.local = basem + (u64)(nextevt_local - basej) * TICK_NSEC;
> > -
> > -	/*
> > -	 * If the local queue expires first, then the global event can be
> > -	 * ignored. The CPU wakes up before that. If the global queue is
> > -	 * empty, nothing to do either.
> > -	 */
> > -	if (!local_first && base_global->timers_pending)
> > -		tevt.global = basem + (u64)(nextevt_global - basej) * TICK_NSEC;
> > -
> > -unlock:
> > -	raw_spin_unlock(&base_global->lock);
> > -	raw_spin_unlock(&base_local->lock);
> > -
> >  	tevt.local = min_t(u64, tevt.local, tevt.global);
> 
> So if you leave that last line, it means that the CPU will eventually
> and unconditionally wake up for the next global timer if it's before the
> next local timer. Am I understanding this right and, if so, is that intended?

Nevermind, that's removed on the main patch.

Sorry for the noise.