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

[Waiman Long <longman@redhat.com>]

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