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Date: Wed, 21 Feb 2024 23:45:03 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	John Stultz <jstultz@google.com>,
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	Eric Dumazet <edumazet@google.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Arjan van de Ven <arjan@infradead.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Rik van Riel <riel@surriel.com>,
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	Giovanni Gherdovich <ggherdovich@suse.cz>,
	Lukasz Luba <lukasz.luba@arm.com>,
	"Gautham R . Shenoy" <gautham.shenoy@amd.com>,
	Srinivas Pandruvada <srinivas.pandruvada@intel.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Christian Loehle <christian.loehle@arm.com>
Subject: Re: [PATCH v11 18/20] timers: Implement the hierarchical pull model
Message-ID: <ZdZ87whmpul8xIBI@pavilion.home>
References: <20240221090548.36600-1-anna-maria@linutronix.de>
 <20240221090548.36600-19-anna-maria@linutronix.de>
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Le Wed, Feb 21, 2024 at 10:05:46AM +0100, Anna-Maria Behnsen a ?crit :
> 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@linutronix.de>

Reviewed-by: Frederic Weisbecker <frederic@kernel.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!