[PATCH v2 01/14] sched_ext: Don't set ddsp_dsq_id during select_cpu in bypass mode

Tejun Heo
2025-11-10T20:56:38Z

In the default CPU selection path used during bypass mode, select_task_rq_scx()
sets p->scx.ddsp_dsq_id to SCX_DSQ_LOCAL to emulate direct dispatch. However,
do_enqueue_task() ignores ddsp_dsq_id in bypass mode and queues to the global
DSQ instead, rendering the setting unnecessary.

Don't set ddsp_dsq_id from bypass. Just return the selected CPU, which has the
effect of waking up the picked idle CPU. Later patches will implement per-CPU
bypass DSQs to resolve this issue in a more proper way.

v2: Removed incorrect bug fix claim about dangling ddsp_dsq_id triggering
WARN_ON_ONCE(). dispatch_enqueue() always clears ddsp_dsq_id even in bypass
mode (Andrea).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrea Righi <andrea.righi@linux.dev>
---
kernel/sched/ext.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 652a364e9e4c..cf8d86a2585c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2521,12 +2521,8 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
s32 cpu;

cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, NULL, 0);
- if (cpu >= 0) {
- refill_task_slice_dfl(sch, p);
- p->scx.ddsp_dsq_id = SCX_DSQ_LOCAL;
- } else {
+ if (cpu < 0)
cpu = prev_cpu;
- }
p->scx.selected_cpu = cpu;

if (rq_bypass)
--
2.51.2