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

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Date: Thu, 01 Feb 2024 21:52:59 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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

Frederic Weisbecker <frederic@...nel.org> writes:

> Le Thu, Feb 01, 2024 at 05:15:37PM +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 void tmigr_connect_child_parent(struct tmigr_group *child,
>> >> +				       struct tmigr_group *parent)
>> >> +{
>> >> +	union tmigr_state childstate;
>> >> +
>> >> +	raw_spin_lock_irq(&child->lock);
>> >> +	raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);
>> >> +
>> >> +	child->parent = parent;
>> >> +	child->childmask = BIT(parent->num_children++);
>> >> +
>> >> +	raw_spin_unlock(&parent->lock);
>> >> +	raw_spin_unlock_irq(&child->lock);
>> >> +
>> >> +	/*
>> >> +	 * To prevent inconsistent states, active children need to be active in
>> >> +	 * the new parent as well. Inactive children are already marked inactive
>> >> +	 * in the parent group.
>> >> +	 */
>> >> +	childstate.state = atomic_read(&child->migr_state);
>> >> +	if (childstate.migrator != TMIGR_NONE) {
>> >
>> > Is it possible here to connect a running online child (not one that we just
>> > created) to a new parent?
>> 
>> connect_child_parent() is only executed for the just created ones. So,
>> yes in theory this would be possible, but it doesn't happen as
>> tmigr_setup_groups() takes care to make it right (hopefully :)). When a
>> LVL0 group has some space left, only the connection between tmc and the
>> LVL0 group is done in tmigr_setup_groups(). If there is no space left in
>> LVL0 group, then a new group is created and depending on the levels
>> which has to be created only executed for the new ones.
>> 
>> > If not, is it possible that a newly created child is
>> > not TMIGR_NONE?
>> 
>> Yes. See tmigr_cpu_online(). When new groups have to be created starting
>> from LVL0, then they are not active - so TMIGR_NONE is set. Activating
>> the new online CPU is done afterwards.
>> 
>> But if it is required to add also a new level at the top, then it is
>> mandatory to propagate the active state of the already existing child to
>> the new parent. The connect_child_parent() is then also executed for the
>> formerly top level group (child) to the newly created group (parent).
>
> Ah and this is why we have the "if (childstate.migrator != TMIGR_NONE)"
> branch, right?

yes - I see, comments would be helpful here :)

>> > Heh, I was about to say that it's impossible that timer_base_is_idle()
>> > at this stage but actually if we run in nohz_full...
>> >
>> > It happens so that nohz_full is deactivated until rcutree_online_cpu()
>> > which calls tick_dep_clear() but it's a pure coincidence that might
>> > disappear one day. So yes, let's keep it that way.
>> 
>> I instrumented the code (with NOHZ FULL and NOHZ_IDLE) to make sure the
>> timer migration hierarchy state 'idle' is in sync with the timer base
>> 'idle'. And this was one part where it was possible that it runs out of
>> sync as I remember correctly. But if I understood you correctly, this
>> shouldn't happen at the moment?
>
> Well, it's not supposed to :-)

Hmm, let me double check this and run the tests on the instrumented
version...

Thanks,

	Anna-Maria