linux-kernel - [PATCH] sched_ext: idle: Handle migration-disabled tasks in idle selection

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Message-ID: <20250705054351.332038-1-arighi@nvidia.com>
Date: Sat,  5 Jul 2025 07:43:51 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>
Cc: sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH] sched_ext: idle: Handle migration-disabled tasks in idle selection

When SCX_OPS_ENQ_MIGRATION_DISABLED is enabled, migration-disabled tasks
are also routed to ops.enqueue(). A scheduler may attempt to dispatch
such tasks directly to an idle CPU using the default idle selection
policy via scx_bpf_select_cpu_and() or scx_bpf_select_cpu_dfl().

This scenario must be properly handled by the built-in idle policy to
avoid returning an idle CPU where the target task isn't allowed to run.
Otherwise, it can lead to errors such as:

 EXIT: runtime error (SCX_DSQ_LOCAL[_ON] cannot move migration disabled Chrome_ChildIOT[291646] from CPU 3 to 14)

Prevent this by explicitly handling migration-disabled tasks in the
built-in idle selection logic, maintaining their CPU affinity.

Fixes: a730e3f7a48bc ("sched_ext: idle: Consolidate default idle CPU selection kfuncs")
Signed-off-by: Andrea Righi <arighi@...dia.com>
---
 kernel/sched/ext_idle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 6d29d3cbc6707..001fb88a8481d 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -903,7 +903,7 @@ s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 	 * selection optimizations and simply check whether the previously
 	 * used CPU is idle and within the allowed cpumask.
 	 */
-	if (p->nr_cpus_allowed == 1) {
+	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
 		if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
 		    scx_idle_test_and_clear_cpu(prev_cpu))
 			cpu = prev_cpu;
-- 
2.50.0