6.12-stable review patch. If anyone has any objections, please let me know.
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From: K Prateek Nayak kprateek.nayak@amd.com
[ Upstream commit 3429dd57f0deb1a602c2624a1dd7c4c11b6c4734 ]
set_delayed() adjusts cfs_rq->h_nr_runnable for the hierarchy when an entity is delayed irrespective of whether the entity corresponds to a task or a cfs_rq.
Consider the following scenario:
root / \ A B (*) delayed since B is no longer eligible on root | | Task0 Task1 <--- dequeue_task_fair() - task blocks
When Task1 blocks (dequeue_entity() for task's se returns true), dequeue_entities() will continue adjusting cfs_rq->h_nr_* for the hierarchy of Task1. However, when the sched_entity corresponding to cfs_rq B is delayed, set_delayed() will adjust the h_nr_runnable for the hierarchy too leading to both dequeue_entity() and set_delayed() decrementing h_nr_runnable for the dequeue of the same task.
A SCHED_WARN_ON() to inspect h_nr_runnable post its update in dequeue_entities() like below:
cfs_rq->h_nr_runnable -= h_nr_runnable; SCHED_WARN_ON(((int) cfs_rq->h_nr_runnable) < 0);
is consistently tripped when running wakeup intensive workloads like hackbench in a cgroup.
This error is self correcting since cfs_rq are per-cpu and cannot migrate. The entitiy is either picked for full dequeue or is requeued when a task wakes up below it. Both those paths call clear_delayed() which again increments h_nr_runnable of the hierarchy without considering if the entity corresponds to a task or not.
h_nr_runnable will eventually reflect the correct value however in the interim, the incorrect values can still influence PELT calculation which uses se->runnable_weight or cfs_rq->h_nr_runnable.
Since only delayed tasks take the early return path in dequeue_entities() and enqueue_task_fair(), adjust the h_nr_runnable in {set,clear}_delayed() only when a task is delayed as this path skips the h_nr_* update loops and returns early.
For entities corresponding to cfs_rq, the h_nr_* update loop in the caller will do the right thing.
Fixes: 76f2f783294d ("sched/eevdf: More PELT vs DELAYED_DEQUEUE") Signed-off-by: K Prateek Nayak kprateek.nayak@amd.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Gautham R. Shenoy gautham.shenoy@amd.com Tested-by: Swapnil Sapkal swapnil.sapkal@amd.com Link: https://lkml.kernel.org/r/20250117105852.23908-1-kprateek.nayak@amd.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 65e7be6448720..ddc096d6b0c20 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5481,6 +5481,15 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq); static void set_delayed(struct sched_entity *se) { se->sched_delayed = 1; + + /* + * Delayed se of cfs_rq have no tasks queued on them. + * Do not adjust h_nr_runnable since dequeue_entities() + * will account it for blocked tasks. + */ + if (!entity_is_task(se)) + return; + for_each_sched_entity(se) { struct cfs_rq *cfs_rq = cfs_rq_of(se);
@@ -5493,6 +5502,16 @@ static void set_delayed(struct sched_entity *se) static void clear_delayed(struct sched_entity *se) { se->sched_delayed = 0; + + /* + * Delayed se of cfs_rq have no tasks queued on them. + * Do not adjust h_nr_runnable since a dequeue has + * already accounted for it or an enqueue of a task + * below it will account for it in enqueue_task_fair(). + */ + if (!entity_is_task(se)) + return; + for_each_sched_entity(se) { struct cfs_rq *cfs_rq = cfs_rq_of(se);