linux-kernel - [PATCH] sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue

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Message-ID: <20250313170321.99510-1-arighi@nvidia.com>
Date: Thu, 13 Mar 2025 18:03:21 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()

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@...dia.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