linux-kernel - Re: [PATCH] rcu: Use system_unbound_wq to avoid disturbing isolated CPUs

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Message-ID: <ZqEB9Yz0womtmDRg@gmail.com>
Date: Wed, 24 Jul 2024 06:30:29 -0700
From: Breno Leitao <leitao@...ian.org>
To: Waiman Long <longman@...hat.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, Vratislav Bendel <vbendel@...hat.com>
Subject: Re: [PATCH] rcu: Use system_unbound_wq to avoid disturbing isolated
 CPUs

On Tue, Jul 23, 2024 at 02:10:25PM -0400, Waiman Long wrote:
> It was discovered that isolated CPUs could sometimes be disturbed by
> kworkers processing kfree_rcu() works causing higher than expected
> latency. It is because the RCU core uses "system_wq" which doesn't have
> the WQ_UNBOUND flag to handle all its work items. Fix this violation of
> latency limits by using "system_unbound_wq" in the RCU core instead.
> This will ensure that those work items will not be run on CPUs marked
> as isolated.
> 
> Beside the WQ_UNBOUND flag, the other major difference between system_wq
> and system_unbound_wq is their max_active count. The system_unbound_wq
> has a max_active of WQ_MAX_ACTIVE (512) while system_wq's max_active
> is WQ_DFL_ACTIVE (256) which is half of WQ_MAX_ACTIVE.
> 
> Reported-by: Vratislav Bendel <vbendel@...hat.com>

I've seen this problem a while ago and reported to the list:

	https://lore.kernel.org/all/Zp906X7VJGNKl5fW@gmail.com/

I've just applied this test, and run my workload for 2 hours without
hitting this issue. Thanks for solving it.

Tested-by: Breno Leitao <leitao@...ian.org>