[PATCH v2] sched/fair: Fix CPU bandwidth limit bypass during CPU hotplug

[Vishal Chourasia]

CPU controller limits are not properly enforced during CPU hotplug
operations, particularly during CPU offline. When a CPU goes offline,
throttled processes are unintentionally being unthrottled across all CPUs
in the system, allowing them to exceed their assigned quota limits.

Consider below for an example,

Assigning 6.25% bandwidth limit to a cgroup
in a 8 CPU system, where, workload is running 8 threads for 20 seconds at
100% CPU utilization, expected (user+sys) time = 10 seconds.

$ cat /sys/fs/cgroup/test/cpu.max
50000 100000

$ ./ebizzy -t 8 -S 20        // non-hotplug case
real 20.00 s
user 10.81 s                 // intended behaviour
sys   0.00 s

$ ./ebizzy -t 8 -S 20        // hotplug case
real 20.00 s
user 14.43 s                 // Workload is able to run for 14 secs
sys   0.00 s                 // when it should have only run for 10 secs

During CPU hotplug, scheduler domains are rebuilt and cpu_attach_domain
is called for every active CPU to update the root domain. That ends up
calling rq_offline_fair which un-throttles any throttled hierarchies.

Unthrottling should only occur for the CPU being hotplugged to allow its
throttled processes to become runnable and get migrated to other CPUs.

With current patch applied,
$ ./ebizzy -t 8 -S 20        // hotplug case
real 21.00 s
user 10.16 s                 // intended behaviour
sys   0.00 s

Note: hotplug operation (online, offline) was performed in while(1) loop
Signed-off-by: Vishal Chourasia <vishalc@xxxxxxxxxxxxx>
Tested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>

v1: https://lore.kernel.org/all/20241126064812.809903-2-vishalc@xxxxxxxxxxxxx

---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fbdca89c677f..e28a8e056ebf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6684,7 +6684,8 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 	list_for_each_entry_rcu(tg, &task_groups, list) {
 		struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
 
-		if (!cfs_rq->runtime_enabled)
+		/* Only unthrottle the CPU being hotplugged */
+		if (!cfs_rq->runtime_enabled || cpumask_test_cpu(cpu_of(rq), cpu_active_mask))
 			continue;
 
 		/*
-- 
2.47.0