commit f3f08c3acfb8860e07a22814a344e83c99ad7398 upstream.

While fixing migration disabled task handling, 32966821574c ("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: 32966821574c ("sched_ext: Fix migration disabled handling in targeted dispatches")
Acked-by: Andrea Righi <arighi@nvidia.com>
Reported-by: Jake Hillion <jakehillion@meta.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>