rcu: Use system_unbound_wq to avoid disturbing isolated CPUs

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 <[email protected]>
Closes: https://issues.redhat.com/browse/RHEL-50220
Signed-off-by: Waiman Long <[email protected]>
Reviewed-by: "Uladzislau Rezki (Sony)" <[email protected]>
Tested-by: Breno Leitao <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e641cc681901a5e5c7a7e1791a836ba0db89d7d3..494aa9513d0b479725b05a77463b316f682f2f41 100644 (file)
--- 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 {