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Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 30 Nov 2022 09:43:13 -1000
From:   Tejun Heo <tj@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org,
        Uladzislau Rezki <urezki@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH rcu 13/16] workqueue: Make queue_rcu_work() use
 call_rcu_hurry()
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References: <20221130181316.GA1012431@paulmck-ThinkPad-P17-Gen-1>
 <20221130181325.1012760-13-paulmck@kernel.org>
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On Wed, Nov 30, 2022 at 10:13:22AM -0800, Paul E. McKenney wrote:
> From: Uladzislau Rezki <urezki@gmail.com>
> 
> Earlier commits in this series allow battery-powered systems to build
> their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option.
> This Kconfig option causes call_rcu() to delay its callbacks in order
> to batch them.  This means that a given RCU grace period covers more
> callbacks, thus reducing the number of grace periods, in turn reducing
> the amount of energy consumed, which increases battery lifetime which
> can be a very good thing.  This is not a subtle effect: In some important
> use cases, the battery lifetime is increased by more than 10%.
> 
> This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
> callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
> parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.
> 
> Delaying callbacks is normally not a problem because most callbacks do
> nothing but free memory.  If the system is short on memory, a shrinker
> will kick all currently queued lazy callbacks out of their laziness,
> thus freeing their memory in short order.  Similarly, the rcu_barrier()
> function, which blocks until all currently queued callbacks are invoked,
> will also kick lazy callbacks, thus enabling rcu_barrier() to complete
> in a timely manner.
> 
> However, there are some cases where laziness is not a good option.
> For example, synchronize_rcu() invokes call_rcu(), and blocks until
> the newly queued callback is invoked.  It would not be a good for
> synchronize_rcu() to block for ten seconds, even on an idle system.
> Therefore, synchronize_rcu() invokes call_rcu_hurry() instead of
> call_rcu().  The arrival of a non-lazy call_rcu_hurry() callback on a
> given CPU kicks any lazy callbacks that might be already queued on that
> CPU.  After all, if there is going to be a grace period, all callbacks
> might as well get full benefit from it.
> 
> Yes, this could be done the other way around by creating a
> call_rcu_lazy(), but earlier experience with this approach and
> feedback at the 2022 Linux Plumbers Conference shifted the approach
> to call_rcu() being lazy with call_rcu_hurry() for the few places
> where laziness is inappropriate.
> 
> And another call_rcu() instance that cannot be lazy is the one
> in queue_rcu_work(), given that callers to queue_rcu_work() are
> not necessarily OK with long delays.
> 
> Therefore, make queue_rcu_work() use call_rcu_hurry() in order to revert
> to the old behavior.
> 
> [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
> 
> Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun