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

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Message-ID: <96aec91c-aa5c-4352-b93c-323b22011370@redhat.com>
Date: Thu, 25 Jul 2024 13:02:01 -0400
From: Waiman Long <longman@...hat.com>
To: Neeraj Upadhyay <Neeraj.Upadhyay@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
 Frederic Weisbecker <frederic@...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 7/25/24 11:35, Neeraj Upadhyay 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.
>>
> Alternative approach here could be, in case we want to keep per CPU worker
> pools, define a wq with WQ_CPU_INTENSIVE flag. Are there cases where
> WQ_CPU_INTENSIVE wq won't be sufficient for the problem this patch
> is fixing?

What exactly will we gain by defining a WQ_CPU_INTENSIVE workqueue? Or 
what will we lose by using system_unbound_wq? All the calls that are 
modified to use system_unbound_wq are using WORK_CPU_UNBOUND as their 
cpu. IOW, they doesn't care which CPUs are used to run the work items. 
The only downside I can see is the possible loss of some cache locality.

In fact, WQ_CPU_INTENSIVE can be considered a subset of WQ_UNBOUND. An 
WQ_UNBOUND workqueue will avoid using isolated CPUs, but not a 
WQ_CPU_INTENSIVE workqueue.

Cheers,
Longman