FAILED: patch "[PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF code" failed to apply to 6.16-stable tree

The patch below does not apply to the 6.16-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.16.y
git checkout FETCH_HEAD
git cherry-pick -x 55ed11b181c43d81ce03b50209e4e7c4a14ba099
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025092952-wooing-result-72e9@gregkh' --subject-prefix 'PATCH 6.16.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 55ed11b181c43d81ce03b50209e4e7c4a14ba099 Mon Sep 17 00:00:00 2001
From: Andrea Righi <[email protected]>
Date: Sat, 20 Sep 2025 15:26:21 +0200
Subject: [PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF code

When scx_bpf_select_cpu_dfl()/and() kfuncs are invoked outside of ops.select_cpu() we can't rely on @p->migration_disabled to determine if migration is disabled for the task @p.

In fact, migration is always disabled for the current task while running BPF code: __bpf_prog_enter() disables migration and __bpf_prog_exit() re-enables it.

To handle this, when @p->migration_disabled == 1, check whether @p is the current task. If so, migration was not disabled before entering the callback, otherwise migration was disabled.

This ensures correct idle CPU selection in all cases. The behavior of ops.select_cpu() remains unchanged, because this callback is never invoked for the current task and migration-disabled tasks are always excluded.

Example: without this change scx_bpf_select_cpu_and() called from ops.enqueue() always returns -EBUSY; with this change applied, it correctly returns idle CPUs.

Fixes 06efc9fe0b8de ("sched_ext: idle: Handle migration-disabled tasks in idle selection")

Cc [email protected] # v6.16+

Signed-off-by Andrea Righi <[email protected]>

Acked-by Changwoo Min <[email protected]>

Signed-off-by Tejun Heo <[email protected]>

kernel/sched/ext_idle.c +27 -1

index 7174e1c1a392..537c6992bb63 100644
@@ -856,6 +856,32 @@ static bool check_builtin_idle_enabled(void)
 	return false;
 }
 
+/*
+ * Determine whether @p is a migration-disabled task in the context of BPF
+ * code.
+ *
+ * We can't simply check whether @p->migration_disabled is set in a
+ * sched_ext callback, because migration is always disabled for the current
+ * task while running BPF code.
+ *
+ * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively
+ * disable and re-enable migration. For this reason, the current task
+ * inside a sched_ext callback is always a migration-disabled task.
+ *
+ * Therefore, when @p->migration_disabled == 1, check whether @p is the
+ * current task or not: if it is, then migration was not disabled before
+ * entering the callback, otherwise migration was disabled.
+ *
+ * Returns true if @p is migration-disabled, false otherwise.
+ */
+static bool is_bpf_migration_disabled(const struct task_struct *p)
+{
+	if (p->migration_disabled == 1)
+		return p != current;
+	else
+		return p->migration_disabled;
+}
+
 static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 				 const struct cpumask *allowed, u64 flags)
 {

@@ -898,7 +924,7 @@ static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_f
 	 * selection optimizations and simply check whether the previously
 	 * used CPU is idle and within the allowed cpumask.
 	 */
-	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
+	if (p->nr_cpus_allowed == 1 || is_bpf_migration_disabled(p)) {
 		if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
 		    scx_idle_test_and_clear_cpu(prev_cpu))
 			cpu = prev_cpu;

Hi Greg,

Quoted 8 lines · On Mon, Sep 29, 2025 at 01:40:52PM +0200, [email protected] wrote:

The patch below does not apply to the 6.16-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

Quoted 8 lines

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.16.y
git checkout FETCH_HEAD
git cherry-pick -x 55ed11b181c43d81ce03b50209e4e7c4a14ba099
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025092952-wooing-result-72e9@gregkh' --subject-prefix 'PATCH 6.16.y' HEAD^..

Possible dependencies:

This patch depends on upstream commit 353656eb84fe ("sched_ext: Make scx_idle_cpu() and related helpers static").

To resolve the conflict I think the best would be to apply commit 353656eb84fef ("sched_ext: idle: Make local functions static in ext_idle.c") to 6.16-stable as well.

This commit only makes some functions static (no functional changes), so it should be safe for stable and it'd keep the code more aligned with upstream.