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rcu: Use system_unbound_wq to avoid disturbing isolated CPUs

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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@redhat.com>
Closes: https://issues.redhat.com/browse/RHEL-50220
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Tested-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit is contained in:
Waiman Long 2024-07-23 14:10:25 -04:00 committed by Neeraj Upadhyay
parent 8400291e28
commit 0aac9daef6
1 changed files with 4 additions and 4 deletions
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8
kernel/rcu/tree.c
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@ -3539,10 +3539,10 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
if (delayed_work_pending(&krcp->monitor_work)) { if (delayed_work_pending(&krcp->monitor_work)) {
delay_left = krcp->monitor_work.timer.expires - jiffies; delay_left = krcp->monitor_work.timer.expires - jiffies;
if (delay < delay_left) if (delay < delay_left)
mod_delayed_work(system_wq, &krcp->monitor_work, delay); mod_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
return; return;
} }
queue_delayed_work(system_wq, &krcp->monitor_work, delay); queue_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
} }
static void 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 // be that the work is in the pending state when
// channels have been detached following by each // channels have been detached following by each
// other. // 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 && if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
!atomic_xchg(&krcp->work_in_progress, 1)) { !atomic_xchg(&krcp->work_in_progress, 1)) {
if (atomic_read(&krcp->backoff_page_cache_fill)) { if (atomic_read(&krcp->backoff_page_cache_fill)) {
queue_delayed_work(system_wq, queue_delayed_work(system_unbound_wq,
&krcp->page_cache_work, &krcp->page_cache_work,
msecs_to_jiffies(rcu_delay_page_cache_fill_msec)); msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
} else { } else {