[RFC PATCH] sched/topology: clear freecpu bit on detach

[Doug Berger]

There is a hazard in the deadline scheduler where an offlined CPU
can have its free_cpus bit left set in the def_root_domain when
the schedutil cpufreq governor is used. This can allow a deadline
thread to be pushed to the runqueue of a powered down CPU which
breaks scheduling. The details can be found here:
https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@xxxxxxxxx

The free_cpus mask is expected to be cleared by set_rq_offline();
however, the hazard occurs before the root domain is made online
during CPU hotplug so that function is not invoked for the CPU
that is being made active.

This commit works around the issue by ensuring the free_cpus bit
for a CPU is always cleared when the CPU is removed from a
root_domain. This likely makes the call of cpudl_clear_freecpu()
in rq_offline_dl() fully redundant, but I have not removed it
here because I am not certain of all flows.

It seems likely that a better solution is possible from someone
more familiar with the scheduler implementation, but this
approach is minimally invasive from someone who is not.

Fixes: 120455c514f7 ("sched: Fix hotplug vs CPU bandwidth control")
Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
---
 kernel/sched/topology.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index da33ec9e94ab..3cbc14953c36 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -499,6 +499,7 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
 			set_rq_offline(rq);
 
 		cpumask_clear_cpu(rq->cpu, old_rd->span);
+		cpudl_clear_freecpu(&old_rd->cpudl, rq->cpu);
 
 		/*
 		 * If we don't want to free the old_rd yet then
-- 
2.34.1