linux-kernel - Re: [PATCH v4 14/16] timer: Implement the hierarchical pull model

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Date:   Wed, 9 Nov 2022 18:13:12 +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 v4 14/16] timer: Implement the hierarchical pull model

On Tue, Nov 08, 2022 at 06:02:11PM +0100, Anna-Maria Behnsen wrote:
> On Tue, 8 Nov 2022, Frederic Weisbecker wrote:
> 
> > On Fri, Nov 04, 2022 at 03:57:35PM +0100, Anna-Maria Behnsen wrote:
> > > @@ -1859,6 +1863,36 @@ void forward_and_idle_timer_bases(unsigned long basej, u64 basem,
> > >  	 */
> > >  	is_idle = time_after(nextevt, basej + 1);
> > >  
> > > +	if (is_idle) {
> > > +		u64 next_tmigr;
> > > +
> > > +		next_tmigr = tmigr_cpu_deactivate(tevt->global);
> > > +
> > > +		tevt->global = KTIME_MAX;
> > > +
> > > +		/*
> > > +		 * If CPU is the last going idle in timer migration
> > > +		 * hierarchy, make sure CPU will wake up in time to handle
> > > +		 * remote timers. next_tmigr == KTIME_MAX if other CPUs are
> > > +		 * still active.
> > > +		 */
> > > +		if (next_tmigr < tevt->local) {
> > > +			u64 tmp;
> > > +
> > > +			/* If we missed a tick already, force 0 delta */
> > > +			if (next_tmigr < basem)
> > > +				next_tmigr = basem;
> > > +
> > > +			tmp = div_u64(next_tmigr - basem, TICK_NSEC);
> > > +
> > > +			nextevt = basej + (unsigned long)tmp;
> > > +			tevt->local = next_tmigr;
> > > +			is_idle = time_after(nextevt, basej + 1);
> > 
> > So after that, tevt->global shouldn't matter anymore for tick_nohz_next_event(),
> > right? If so then probably that line can go away (with a comment specifying why we can
> > ignore the global part)?:
> > 
> >        tevt.local = min_t(u64, tevt.local, tevt.global);
> > 
> 
> tevt->global is set to KTIME_MAX anyway. So the whole tevt information is
> also no longer required in tick_nohz_next_event(). I need to rework the
> patch where this was introduced. Then the forward_and_idle_timer_bases()
> could still simply return the next timer and then there is no longer a
> point against using your idea with naming of the functions.

You got it! ;-)