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

[Neeraj Upadhyay <Neeraj.Upadhyay@kernel.org>]

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?


- Neeraj

> 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@redhat.com>
> Closes: https://issues.redhat.com/browse/RHEL-50220
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/rcu/tasks.h | 4 ++--
>  kernel/rcu/tree.c  | 8 ++++----
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e641cc681901..494aa9513d0b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3539,10 +3539,10 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
>  	if (delayed_work_pending(&krcp->monitor_work)) {
>  		delay_left = krcp->monitor_work.timer.expires - jiffies;
>  		if (delay < delay_left)
> -			mod_delayed_work(system_wq, &krcp->monitor_work, delay);
> +			mod_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
>  		return;
>  	}
> -	queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> +	queue_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
>  }
>  
>  static void
> @@ -3634,7 +3634,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  			// be that the work is in the pending state when
>  			// channels have been detached following by each
>  			// other.
> -			queue_rcu_work(system_wq, &krwp->rcu_work);
> +			queue_rcu_work(system_unbound_wq, &krwp->rcu_work);
>  		}
>  	}
>  
> @@ -3704,7 +3704,7 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
>  			!atomic_xchg(&krcp->work_in_progress, 1)) {
>  		if (atomic_read(&krcp->backoff_page_cache_fill)) {
> -			queue_delayed_work(system_wq,
> +			queue_delayed_work(system_unbound_wq,
>  				&krcp->page_cache_work,
>  					msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
>  		} else {
> -- 
> 2.43.5
>