Re: [PATCH v2] sched/core: handle affine_move_task failure case gracefully

[Valentin Schneider]

On 14/08/24 17:31, Daniel Vacek wrote:
> CPU hangs were reported while offlining/onlining CPUs on s390.
>
> Analyzing the vmcore data shows `stop_one_cpu_nowait()` in `affine_move_task()`
> can fail when racing with off-/on-lining resulting in a deadlock waiting for
> the pending migration stop work completion which is never done.
>
> Fix this by gracefully handling such condition.
>
> Fixes: 9e81889c7648 ("sched: Fix affine_move_task() self-concurrency")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Bill Peters <wpeters@xxxxxxxxx>
> Tested-by: Bill Peters <wpeters@xxxxxxxxx>
> Signed-off-by: Daniel Vacek <neelx@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f3951e4a55e5b..40a3c9ff74077 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2871,8 +2871,25 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
>               preempt_disable();
>               task_rq_unlock(rq, p, rf);
>               if (!stop_pending) {
> -			stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> -					    &pending->arg, &pending->stop_work);
> +			stop_pending =
> +				stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> +						    &pending->arg, &pending->stop_work);
> +			/*
> +			 * The state resulting in this failure is not expected
> +			 * at this point. At least report a WARNING to be able
> +			 * to panic and further debug if reproduced.
> +			 */
> +			if (WARN_ON(!stop_pending)) {

I don't think it can happen when @p's CPU is being hot-unplugged, because
takedown_cpu()->take_cpu_down() is done via stop_machine_cpuslocked(), so
that task waking/running on that CPU implies the stopper is enabled.

I'm thinking there may be a window somewhere during hotplug, something like
so?

CPU0                       CPU1                       CPU2
----                       ----                       ----
                           secondary_start_kernel()
                             set_cpu_online(true)
wake_up_process(p)
  WRITE_ONCE(p->__state, TASK_WAKING);
  set_task_cpu(p, CPU1) // CPU1 is prev CPU
  ttwu_queue_wakelist();
                                                     affine_move_task(p)
                                                       stop_one_cpu_nowait() // FAILS

                             cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
                               cpuhp_online_idle()
                                 stop_machine_unpark(); // <-- too late