linux-kernel - Re: [PATCH v11 18/20] timers: Implement the hierarchical pull model

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Date: Thu, 22 Feb 2024 11:25:33 +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>,
	Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCH v11 18/20] timers: Implement the hierarchical pull model

On Thu, Feb 22, 2024 at 09:17:29AM +0100, Anna-Maria Behnsen wrote:
> Frederic Weisbecker <frederic@...nel.org> writes:
> 
> > Le Wed, Feb 21, 2024 at 10:05:46AM +0100, Anna-Maria Behnsen a écrit :
> >> Placing timers at enqueue time on a target CPU based on dubious heuristics
> >> does not make any sense:
> >> 
> >>  1) Most timer wheel timers are canceled or rearmed before they expire.
> >> 
> >>  2) The heuristics to predict which CPU will be busy when the timer expires
> >>     are wrong by definition.
> >> 
> >> So placing the timers at enqueue wastes precious cycles.
> >> 
> >> The proper solution to this problem is to always queue the timers on the
> >> local CPU and allow the non pinned timers to be pulled onto a busy CPU at
> >> expiry time.
> >> 
> >> Therefore split the timer storage into local pinned and global timers:
> >> Local pinned timers are always expired on the CPU on which they have been
> >> queued. Global timers can be expired on any CPU.
> >> 
> >> As long as a CPU is busy it expires both local and global timers. When a
> >> CPU goes idle it arms for the first expiring local timer. If the first
> >> expiring pinned (local) timer is before the first expiring movable timer,
> >> then no action is required because the CPU will wake up before the first
> >> movable timer expires. If the first expiring movable timer is before the
> >> first expiring pinned (local) timer, then this timer is queued into an idle
> >> timerqueue and eventually expired by another active CPU.
> >> 
> >> To avoid global locking the timerqueues are implemented as a hierarchy. The
> >> lowest level of the hierarchy holds the CPUs. The CPUs are associated to
> >> groups of 8, which are separated per node. If more than one CPU group
> >> exist, then a second level in the hierarchy collects the groups. Depending
> >> on the size of the system more than 2 levels are required. Each group has a
> >> "migrator" which checks the timerqueue during the tick for remote expirable
> >> timers.
> >> 
> >> If the last CPU in a group goes idle it reports the first expiring event in
> >> the group up to the next group(s) in the hierarchy. If the last CPU goes
> >> idle it arms its timer for the first system wide expiring timer to ensure
> >> that no timer event is missed.
> >> 
> >> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> >
> > Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> >
> > Though I must confess I only had a shallow look at group creation (init_group,
> > get_group, connect_child_parent, setup_groups, add_cpu, and tmigr_init). But for
> > the rest, I'm running out of bad scenarios. Time for the rest of the world to
> > find them!
> >
> > Thanks for the hard work!
> 
> Thank you for your support!
> 
> Did you saw the v11a for this patch? It is only a cleanup - it removes
> an unused variable. Just asking, because Review was for plain v11 patch.

Yep, I reviewed the 11a one, with the "leftmost" variable off.

Thanks.