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

[Breno Leitao]

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@xxxxxxxxxx>

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

	https://lore.kernel.org/all/Zp906X7VJGNKl5fW@xxxxxxxxx/

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@xxxxxxxxxx>