The following commit has been merged into the sched/core branch of tip: Commit-ID: af98d8a36a963e758e84266d152b92c7b51d4ecb Gitweb: https://git.kernel.org/tip/af98d8a36a963e758e84266d152b92c7b51d4ecb Author: Vishal Chourasia <vishalc@xxxxxxxxxxxxx> AuthorDate: Thu, 12 Dec 2024 10:01:03 +05:30 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Tue, 17 Dec 2024 17:47:22 +01:00 sched/fair: Fix CPU bandwidth limit bypass during CPU hotplug 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 This also has another symptom, when a CPU goes offline, and if the cfs_rq is not in throttled state and the runtime_remaining still had plenty remaining, it gets reset to 1 here, causing the runtime_remaining of cfs_rq to be quickly depleted. Note: hotplug operation (online, offline) was performed in while(1) loop v3: https://lore.kernel.org/all/20241210102346.228663-2-vishalc@xxxxxxxxxxxxx v2: https://lore.kernel.org/all/20241207052730.1746380-2-vishalc@xxxxxxxxxxxxx v1: https://lore.kernel.org/all/20241126064812.809903-2-vishalc@xxxxxxxxxxxxx Suggested-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx> Signed-off-by: Vishal Chourasia <vishalc@xxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx> Tested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx> Tested-by: Samir Mulani <samir@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20241212043102.584863-2-vishalc@xxxxxxxxxxxxx --- kernel/sched/fair.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 2c4ebfc..8f641c9 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6696,6 +6696,10 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq) lockdep_assert_rq_held(rq); + // Do not unthrottle for an active CPU + 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 @@ -6711,18 +6715,20 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq) continue; /* - * clock_task is not advancing so we just need to make sure - * there's some valid quota amount - */ - cfs_rq->runtime_remaining = 1; - /* * Offline rq is schedulable till CPU is completely disabled * in take_cpu_down(), so we prevent new cfs throttling here. */ cfs_rq->runtime_enabled = 0; - if (cfs_rq_throttled(cfs_rq)) - unthrottle_cfs_rq(cfs_rq); + if (!cfs_rq_throttled(cfs_rq)) + continue; + + /* + * clock_task is not advancing so we just need to make sure + * there's some valid quota amount + */ + cfs_rq->runtime_remaining = 1; + unthrottle_cfs_rq(cfs_rq); } rcu_read_unlock();
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