[PATCH 2/3] sched/fair: Prevent active LB from preempting higher sched classes

[Valentin Schneider <valentin.schneider@arm.com>]

The CFS load balancer can cause the cpu_stopper to run a function to
try and steal a rq's currently running task. However, it so happens
that while only CFS tasks will ever be migrated by that function, we
can end up preempting higher sched class tasks, since it is executed
by the cpu_stopper.

I don't expect this to be exceedingly common: we still need to have
had a busiest group in the first place, which needs

  busiest->sum_nr_running != 0

which is a cfs.h_nr_running sum, so we should have something to pull,
but if we fail to pull anything and the remote rq is executing
an RT/DL task we can hit this.

Add an extra check to not trigger the cpu_stopper if the remote
rq's running task isn't CFS.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b56b8edee3d3..79bd6ead589c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8834,6 +8834,10 @@ static inline enum alb_status active_load_balance(struct lb_env *env)
 
 	raw_spin_lock_irqsave(&busiest->lock, flags);
 
+	/* Make sure we're not about to stop a task from a higher sched class */
+	if (busiest->curr->sched_class != &fair_sched_class)
+		goto unlock;
+
 	/*
 	 * Don't kick the active_load_balance_cpu_stop, if the curr task on
 	 * busiest CPU can't be moved to dst_cpu:
--
2.22.0