linux-kernel - Re: [PATCH v4 0/4] Reduce synchronize

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Date: Fri, 26 Jan 2024 23:07:18 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: RCU <rcu@...r.kernel.org>, Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v4 0/4] Reduce synchronize_rcu() latency(v4)

On Thu, Jan 04, 2024 at 05:25:06PM +0100, Uladzislau Rezki (Sony) wrote:
> This is a v4 that tends to improve synchronize_rcu() call. To be more
> specific it is about reducing a waiting time(especially worst cases)
> of caller that blocks until a grace period is elapsed. 
> 
> In general, this series separates synchronize_rcu() callers from other
> callbacks. We keep a dedicated an independent queue, thus the processing
> of it starts as soon as grace period is over, so there is no need to wait
> until other callbacks are processed one by one. Please note, a number of
> callbacks can be 10K, 20K, 60K and so on. That is why this series maintain
> a separate track for this call that blocks a context.

And before I forget (again), a possible follow-on to this work is to
reduce cond_synchronize_rcu() and cond_synchronize_rcu_full() latency.
Right now, these wait for a full additional grace period (and maybe
more) when the required grace period has not elapsed.  In contrast,
this work might enable waiting only for the needed portion of a grace
period to elapse.

							Thanx, Paul

> v3 -> v4:
>  - Squash patches;
>  - Add more description;
>  - Fix comments based on v3 feedback.
> 
> v3: https://lore.kernel.org/lkml/cd45b0b5-f86b-43fb-a5f3-47d340cd4f9f@paulmck-laptop/T/
> v2: https://lore.kernel.org/all/20231030131254.488186-1-urezki@gmail.com/T/
> v1: https://lore.kernel.org/lkml/20231025140915.590390-1-urezki@gmail.com/T/
> 
> Neeraj Upadhyay (1):
>   rcu: Improve handling of synchronize_rcu() users
> 
> Uladzislau Rezki (Sony) (3):
>   rcu: Reduce synchronize_rcu() latency
>   rcu: Add a trace event for synchronize_rcu_normal()
>   rcu: Support direct wake-up of synchronize_rcu() users
> 
>  .../admin-guide/kernel-parameters.txt         |  14 +
>  include/trace/events/rcu.h                    |  27 ++
>  kernel/rcu/Kconfig.debug                      |  12 +
>  kernel/rcu/tree.c                             | 361 +++++++++++++++++-
>  kernel/rcu/tree.h                             |  19 +
>  kernel/rcu/tree_exp.h                         |   2 +-
>  6 files changed, 433 insertions(+), 2 deletions(-)
> 
> -- 
> 2.39.2
>