sched_ext: Fix incorrect assumption about migration disabled tasks in task_can_run_on_remote_rq()

While fixing migration disabled task handling, 3296682157 ("sched_ext: Fix
migration disabled handling in targeted dispatches") assumed that a
migration disabled task's ->cpus_ptr would only have the pinned CPU. While
this is eventually true for migration disabled tasks that are switched out,
->cpus_ptr update is performed by migrate_disable_switch() which is called
right before context_switch() in __scheduler(). However, the task is
enqueued earlier during pick_next_task() via put_prev_task_scx(), so there
is a race window where another CPU can see the task on a DSQ.

If the CPU tries to dispatch the migration disabled task while in that
window, task_allowed_on_cpu() will succeed and task_can_run_on_remote_rq()
will subsequently trigger SCHED_WARN(is_migration_disabled()).

  WARNING: CPU: 8 PID: 1837 at kernel/sched/ext.c:2466 task_can_run_on_remote_rq+0x12e/0x140
  Sched_ext: layered (enabled+all), task: runnable_at=-10ms
  RIP: 0010:task_can_run_on_remote_rq+0x12e/0x140
  ...
   <TASK>
   consume_dispatch_q+0xab/0x220
   scx_bpf_dsq_move_to_local+0x58/0xd0
   bpf_prog_84dd17b0654b6cf0_layered_dispatch+0x290/0x1cfa
   bpf__sched_ext_ops_dispatch+0x4b/0xab
   balance_one+0x1fe/0x3b0
   balance_scx+0x61/0x1d0
   prev_balance+0x46/0xc0
   __pick_next_task+0x73/0x1c0
   __schedule+0x206/0x1730
   schedule+0x3a/0x160
   __do_sys_sched_yield+0xe/0x20
   do_syscall_64+0xbb/0x1e0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fix it by converting the SCHED_WARN() back to a regular failure path. Also,
perform the migration disabled test before task_allowed_on_cpu() test so
that BPF schedulers which fail to handle migration disabled tasks can be
noticed easily.

While at it, adjust scx_ops_error() message for !task_allowed_on_cpu() case
for brevity and consistency.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 3296682157 ("sched_ext: Fix migration disabled handling in targeted dispatches")
Acked-by: Andrea Righi <arighi@nvidia.com>
Reported-by: Jake Hillion <jakehillion@meta.com>

kernel/sched/ext.c

@@ -2343,6 +2343,25 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,
 
 	SCHED_WARN_ON(task_cpu(p) == cpu);
 
+	/*
+	 * If @p has migration disabled, @p->cpus_ptr is updated to contain only
+	 * the pinned CPU in migrate_disable_switch() while @p is being switched
+	 * out. However, put_prev_task_scx() is called before @p->cpus_ptr is
+	 * updated and thus another CPU may see @p on a DSQ inbetween leading to
+	 * @p passing the below task_allowed_on_cpu() check while migration is
+	 * disabled.
+	 *
+	 * Test the migration disabled state first as the race window is narrow
+	 * and the BPF scheduler failing to check migration disabled state can
+	 * easily be masked if task_allowed_on_cpu() is done first.
+	 */
+	if (unlikely(is_migration_disabled(p))) {
+		if (trigger_error)
+			scx_ops_error("SCX_DSQ_LOCAL[_ON] cannot move migration disabled %s[%d] from CPU %d to %d",
+				      p->comm, p->pid, task_cpu(p), cpu);
+		return false;
+	}
+
 	/*
 	 * We don't require the BPF scheduler to avoid dispatching to offline
 	 * CPUs mostly for convenience but also because CPUs can go offline
@@ -2351,17 +2370,11 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,
 	 */
 	if (!task_allowed_on_cpu(p, cpu)) {
 		if (trigger_error)
-			scx_ops_error("SCX_DSQ_LOCAL[_ON] verdict target cpu %d not allowed for %s[%d]",
-				      cpu_of(rq), p->comm, p->pid);
+			scx_ops_error("SCX_DSQ_LOCAL[_ON] target CPU %d not allowed for %s[%d]",
+				      cpu, p->comm, p->pid);
 		return false;
 	}
 
-	/*
-	 * If @p has migration disabled, @p->cpus_ptr only contains its current
-	 * CPU and the above task_allowed_on_cpu() test should have failed.
-	 */
-	SCHED_WARN_ON(is_migration_disabled(p));
-
 	if (!scx_rq_online(rq))
 		return false;