[PATCH] sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()

[PATCH] sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()

[Andrea Righi]

scx_bpf_reenqueue_local() can be invoked from ops.cpu_release() to give
tasks that are queued to the local DSQ a chance to migrate to other
CPUs, when a CPU is taken by a higher scheduling class.

However, there is no point re-enqueuing tasks that can only run on that
particular CPU, as they would simply be re-added to the same local DSQ
without any benefit.

Therefore, skip per-CPU tasks in scx_bpf_reenqueue_local().

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7b9dfee858e79..22f979f9acde2 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6972,8 +6972,12 @@ __bpf_kfunc u32 scx_bpf_reenqueue_local(void)
 		 * CPUs disagree, they use %ENQUEUE_RESTORE which is bypassed to
 		 * the current local DSQ for running tasks and thus are not
 		 * visible to the BPF scheduler.
+		 *
+		 * Also skip re-enqueueing tasks that can only run on this
+		 * CPU, as they would just be re-added to the same local
+		 * DSQ without any benefit.
 		 */
-		if (p->migration_pending)
+		if (p->migration_pending || is_migration_disabled(p) || p->nr_cpus_allowed == 1)
 			continue;
 
 		dispatch_dequeue(rq, p);
-- 
2.48.1

Re: [PATCH] sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()

[Tejun Heo]

On Thu, Mar 13, 2025 at 06:03:21PM +0100, Andrea Righi wrote:
> scx_bpf_reenqueue_local() can be invoked from ops.cpu_release() to give
> tasks that are queued to the local DSQ a chance to migrate to other
> CPUs, when a CPU is taken by a higher scheduling class.
> 
> However, there is no point re-enqueuing tasks that can only run on that
> particular CPU, as they would simply be re-added to the same local DSQ
> without any benefit.
> 
> Therefore, skip per-CPU tasks in scx_bpf_reenqueue_local().
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

Applied to sched_ext/for-6.15.

Thanks.

-- 
tejun