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

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Message-ID: <1d011339-364b-42ce-9c7a-fc99f7be0e1c@redhat.com>
Date: Sun, 28 Jul 2024 23:06:44 -0400
From: Waiman Long <longman@...hat.com>
To: Breno Leitao <leitao@...ian.org>
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 7/24/24 09:30, Breno Leitao wrote:
> 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>

Thank for testing this patch. So it is just us that saw this problem.

Cheers,
Longman