linux-kernel - Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

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Message-ID: <ZcFE9vVTqyVZ_0SC@pavilion.home>
Date: Mon, 5 Feb 2024 21:28:38 +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>,
	Rik van Riel <riel@...riel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Lukasz Luba <lukasz.luba@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Le Mon, Feb 05, 2024 at 04:59:45PM +0100, Anna-Maria Behnsen a écrit :
> Frederic Weisbecker <frederic@...nel.org> writes:
> 
> > Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> >> +static bool tmigr_handle_remote_up(struct tmigr_group *group,
> >> +				   struct tmigr_group *child,
> >> +				   void *ptr)
> >> +{
> >> +	struct tmigr_remote_data *data = ptr;
> >> +	u64 now, next = KTIME_MAX;
> >> +	struct tmigr_event *evt;
> >> +	unsigned long jif;
> >> +	u8 childmask;
> >> +
> >> +	jif = data->basej;
> >> +	now = data->now;
> >> +
> >> +	childmask = data->childmask;
> >> +
> >> +again:
> >> +	/*
> >> +	 * Handle the group only if @childmask is the migrator or if the
> >> +	 * group has no migrator. Otherwise the group is active and is
> >> +	 * handled by its own migrator.
> >> +	 */
> >> +	if (!tmigr_check_migrator(group, childmask))
> >> +		return true;
> >> +
> >> +	raw_spin_lock_irq(&group->lock);
> >> +
> >> +	evt = tmigr_next_expired_groupevt(group, now);
> >> +
> >> +	if (evt) {
> >> +		unsigned int remote_cpu = evt->cpu;
> >> +
> >> +		raw_spin_unlock_irq(&group->lock);
> >> +
> >> +		next = tmigr_handle_remote_cpu(remote_cpu, now, jif);
> >> +
> >> +		/* check if there is another event, that needs to be handled */
> >> +		goto again;
> >> +	} else {
> >> +		raw_spin_unlock_irq(&group->lock);
> >> +	}
> >> +
> >> +	/*
> >> +	 * Update of childmask for the next level and keep track of the expiry
> >> +	 * of the first event that needs to be handled
> >> +	 */
> >> +	data->childmask = group->childmask;
> >> +	data->firstexp = next;
> >
> > So assume we have:
> >
> >             [GRP1:0]
> >         migrator = [GRP0:0]
> >         active   = [GRP0:0]
> >         nextevt  = TIMER3
> >         /                    \
> >     [GRP0:0]                  [GRP0:1]
> >  migrator = CPU0           migrator = NONE
> >  active   = CPU0           active   = NONE
> >  nextevt  = KTIME_MAX      nextevt  = TIMER3
> >     /         \                /         \
> >    0           1              2           3
> >   idle       idle           idle         idle (TIMER3)
> >
> > Then CPU 0 goes idle:
> >
> >             [GRP1:0]
> >         migrator = NONE
> >         active   = NONE
> >         nextevt  = TIMER3
> >         /                    \
> >     [GRP0:0]                  [GRP0:1]
> >  migrator = NONE           migrator = NONE
> >  active   = NONE           active   = NONE
> >  nextevt  = KTIME_MAX      nextevt  = TIMER3
> >     /         \                /         \
> >    0           1              2           3
> >   idle       idle           idle         idle (TIMER3)
> >
> > CPU 0 is the idle migrator and its tmc->wakeup is TIMER3.
> > But CPU 0 has a local timer that expires before TIMER3.
> >
> > When that timer interrupt fires, it raises the softirq, which
> > executes on IRQ tail. So CPU0 eventually calls tmigr_handle_remote()
> > before TIMER3 has expired.
> >
> > This leads to tmigr_next_expired_groupevt() to return NULL and then
> > data->firstexp = KTIME_MAX and then tmc->wakeup = KTIME_MAX.
> >
> > Later on, tmigr_new_timer() is called with a KTIME_MAX global
> > event and so tmc->wakeup stays with KTIME_MAX, ignoring TIMER3.
> >
> > It looks like you need to handle the tmigr_next_expired_groupevt()
> > case returning NULL.
> 
> Yes. You are right. group->next_expiry is updated via
> tmigr_next_expired_groupevt(). So I should take this value to rely on
> for the firstevt.
> 
> But with this, we do not need the return value of
> tmigr_handle_remote_cpu(). When the upperst level is inactive and there
> is a timer (e.g. propagated there by tmigr_handle_remote_cpu()), the
> return value of tmigr_handle_remote_cpu() is the next_expiry of the top
> level group. But this value is also noticed by walking the hierarchy up
> to the top level with tmigr_handle_remote_up().

Indeed that's a nice simplification!

This also cure an issue with KTIME_MAX overwriting data->firstexp in
tmigr_handle_remote_up() while walking upper level after an expiration
on below level.

Thanks.