tjh.dev / kernel
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sched_ext: Fix enqueue_task_scx() truncation of upper enqueue flags

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@vger.kernel.org # v6.12+
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Tejun Heo 57ccf5cc 2a0596d5

+2 -3
+2 -3
kernel/sched/ext.c
··· 1470 1470 p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT; 1471 1471 } 1472 1472 1473 - static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags) 1473 + static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_flags) 1474 1474 { 1475 1475 struct scx_sched *sch = scx_root; 1476 1476 int sticky_cpu = p->scx.sticky_cpu; 1477 + u64 enq_flags = core_enq_flags | rq->scx.extra_enq_flags; 1477 1478 1478 1479 if (enq_flags & ENQUEUE_WAKEUP) 1479 1480 rq->scx.flags |= SCX_RQ_IN_WAKEUP; 1480 - 1481 - enq_flags |= rq->scx.extra_enq_flags; 1482 1481 1483 1482 if (sticky_cpu >= 0) 1484 1483 p->scx.sticky_cpu = -1;