[PATCH sched_ext/for-7.0-fixes] sched_ext: Fix enqueue_task_scx() truncation of upper enqueue flags

[Tejun Heo]

enqueue_task_scx() takes int enq_flags from the sched_class interface.
SCX enqueue flags starting at bit 32 (SCX_ENQ_PREEMPT and above) are
silently truncated when passed through activate_task(). extra_enq_flags
was added as a workaround - storing high bits in rq->scx.extra_enq_flags
and OR-ing them back in enqueue_task_scx(). However, the OR target is
still the int parameter, so the high bits are lost anyway.

The current impact is limited as the only affected flag is SCX_ENQ_PREEMPT
which is informational to the BPF scheduler - its loss means the scheduler
doesn't know about preemption but doesn't cause incorrect behavior.

Fix by renaming the int parameter to core_enq_flags and introducing a
u64 enq_flags local that merges both sources. All downstream functions
already take u64 enq_flags.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@xxxxxxxxxxxxxxx # v6.12+
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index XXXXXXX..XXXXXXX 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1728,16 +1728,15 @@ static void clr_task_runnable(struct task_struct *p)
 		p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
 }

-static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags)
+static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_flags)
 {
 	struct scx_sched *sch = scx_task_sched(p);
 	int sticky_cpu = p->scx.sticky_cpu;
+	u64 enq_flags = core_enq_flags | rq->scx.extra_enq_flags;

 	if (enq_flags & ENQUEUE_WAKEUP)
 		rq->scx.flags |= SCX_RQ_IN_WAKEUP;

-	enq_flags |= rq->scx.extra_enq_flags;
-
 	/*
 	 * Restoring a running task will be immediately followed by
 	 * set_next_task_scx() which expects the task to not be on the BPF
--
tejun