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@xxxxxxxxxx> > Closes: https://issues.redhat.com/browse/RHEL-50220 > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > --- > 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 >
