The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: 3429dd57f0deb1a602c2624a1dd7c4c11b6c4734
Gitweb: https://git.kernel.org/tip/3429dd57f0deb1a602c2624a1dd7c4c11b6c4734
Author: K Prateek Nayak <kprateek.nayak@xxxxxxx>
AuthorDate: Fri, 17 Jan 2025 10:58:52
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 21 Jan 2025 13:13:36 +01:00
sched/fair: Fix inaccurate h_nr_runnable accounting with delayed dequeue
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@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
Tested-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
Link: https://lkml.kernel.org/r/20250117105852.23908-1-kprateek.nayak@xxxxxxx
---
kernel/sched/fair.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2695843..f4e4d3e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5372,6 +5372,15 @@ 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);
@@ -5384,6 +5393,16 @@ 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);
|