[RFC PATCH] sched/ext: add sched_info_enqueue in dispatch path for schedstat

[Fernand Sieber <sieberf@amazon.com>]

SCX tasks re-dispatched from BPF (e.g., after bandwidth throttling)
bypass enqueue_task() and go directly through dispatch_enqueue().
This skips sched_info_enqueue(), leaving last_queued at 0, which
prevents run_delay from accumulating in /proc/<pid>/schedstat.

Add sched_info_enqueue() in dispatch_enqueue() when last_queued is
not already set. This ensures run_delay correctly reflects the time
a task spends waiting for a CPU after being dispatched, including
time spent in BPF-managed throttle queues.

Without this fix, schedstat shows frozen run_delay values for SCX
tasks that go through throttle/unthrottle cycles.

Signed-off-by: Fernand Sieber <sieberf@amazon.com>
---
 kernel/sched/ext.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7ebdaf75d..827a96e39 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1525,6 +1525,10 @@ static void dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
 	WARN_ON_ONCE((p->scx.dsq_flags & SCX_TASK_DSQ_ON_PRIQ) ||
 		     !RB_EMPTY_NODE(&p->scx.dsq_priq));
 
+	/* Track queue time for schedstat run_delay accounting */
+	if (!p->sched_info.last_queued)
+		sched_info_enqueue(task_rq(p), p);
+
 	if (!is_local) {
 		raw_spin_lock_nested(&dsq->lock,
 			(enq_flags & SCX_ENQ_NESTED) ? SINGLE_DEPTH_NESTING : 0);
-- 
2.47.3




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