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: stable@vger.kernel.org # v6.16+
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 7174e1c1a392c734d829438be4876f30ee12cbf0..537c6992bb63f77d31133e28ac9cddf03fd256ca 100644 (file)
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -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;