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

[Waiman Long]

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@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>

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

Cheers,
Longman