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

[Waiman Long <longman@redhat.com>]

On 7/25/24 15:33, Neeraj Upadhyay wrote:
> On Thu, Jul 25, 2024 at 01:02:01PM -0400, Waiman Long wrote:
>> 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.
>>
> For the nohz_full case, where unbounded pool workers run only on housekeeping CPU
> (cpu0), if multiple other CPUs are queuing work, the execution of those
> works could get delayed. However, this should not generally happen as
> other CPUs would be mostly running in user mode.
Well, it there is only one housekeeping CPU, a lot of background kernel 
tasks will be slowed down. Users should be careful about the proper 
balance between the number of housekeeping and nohz-full CPUs.
>
>
>> 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.
> Got it, thanks!
>
> I have picked the patch for further review and testing [1]
>
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/neeraj.upadhyay/linux-rcu.git/log/?h=next

Thanks, let me know if you see any problem.

Cheers,
Longman