Re: [PATCH v3] sched/fair: Fix CPU bandwidth limit bypass during CPU hotplug

Peter Zijlstra
2024-12-10T14:43:18Z

On Tue, Dec 10, 2024 at 03:53:47PM +0530, Vishal Chourasia wrote:
> 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@linux.ibm.com>
> Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>

Did you mean this?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2c4ebfc82917..b6afb8337e73 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6696,6 +6696,9 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)

lockdep_assert_rq_held(rq);

+ if (cpumask_test_cpu(cpu_of(rq), cpu_active_mask))
+ return;
+
/*
* The rq clock has already been updated in the
* set_rq_offline(), so we should skip updating