Re: [PATCH 2/2] workqueue: Fix affinity of kworkers attached during late hotplug

Vincent Donnefort
2020-12-11T11:41:22Z

Hi Valentin,

On Thu, Dec 10, 2020 at 04:38:30PM +0000, Valentin Schneider wrote:
> Per-CPU kworkers forcefully migrated away by hotplug via
> workqueue_offline_cpu() can end up spawning more kworkers via
>
> manage_workers() -> maybe_create_worker()
>
> Workers created at this point will be bound using
>
> pool->attrs->cpumask
>
> which in this case is wrong, as the hotplug state machine already migrated
> all pinned kworkers away from this CPU. This ends up triggering the BUG_ON
> condition is sched_cpu_dying() (i.e. there's a kworker enqueued on the
> dying rq).
>
> Special-case workers being attached to DISASSOCIATED pools and bind them to
> cpu_active_mask, mimicking them being present when workqueue_offline_cpu()
> was invoked.
>
> Link: https://lore.kernel.org/r/ff62e3ee994efb3620177bf7b19fab16f4866845.camel@redhat.com
> Fixes: 06249738a41a ("workqueue: Manually break affinity on hotplug")

Isn't the problem introduced by 1cf12e0 ("sched/hotplug: Consolidate
task migration on CPU unplug") ?

Previously we had:

AP_WORKQUEUE_ONLINE -> set POOL_DISASSOCIATED
...
TEARDOWN_CPU -> clear CPU in cpu_online_mask
|
|-AP_SCHED_STARTING -> migrate_tasks()
|
AP_OFFLINE

worker_attach_to_pool(), is "protected" by the cpu_online_mask in
set_cpus_allowed_ptr(). IIUC, now, the tasks being migrated before the
cpu_online_mask is actually flipped, there's a window, between
CPUHP_AP_SCHED_WAIT_EMPTY and CPUHP_TEARDOWN_CPU where a kworker can wake-up
a new one, for the hotunplugged pool that wouldn't be caught by the
hotunplug migration.

> Reported-by: Qian Cai <cai@redhat.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> kernel/workqueue.c | 24 +++++++++++++++++-------
> 1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 9880b6c0e272..fb1418edf85c 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1848,19 +1848,29 @@ static void worker_attach_to_pool(struct worker *worker,
> {
> mutex_lock(&wq_pool_attach_mutex);
>
> - /*
> - * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
> - * online CPUs. It'll be re-applied when any of the CPUs come up.
> - */
> - set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
> -
> /*
> * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
> * stable across this function. See the comments above the flag
> * definition for details.
> + *
> + * Worker might get attached to a pool *after* workqueue_offline_cpu()
> + * was run - e.g. created by manage_workers() from a kworker which was
> + * forcefully moved away by hotplug. Kworkers created from this point on
> + * need to have their affinity changed as if they were present during
> + * workqueue_offline_cpu().
> + *
> + * This will be resolved in rebind_workers().
> */
> - if (pool->flags & POOL_DISASSOCIATED)
> + if (pool->flags & POOL_DISASSOCIATED) {
> worker->flags |= WORKER_UNBOUND;
> + set_cpus_allowed_ptr(worker->task, cpu_active_mask);
> + } else {
> + /*
> + * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
> + * online CPUs. It'll be re-applied when any of the CPUs come up.
> + */

Does this comment still stand ? IIUC, we should always be in the
POOL_DISASSOCIATED case if the CPU from cpumask is offline. Unless a
pool->attrs->cpumask can have several CPUs. In that case maybe we should check
for the cpu_active_mask here too ?

--
Vincent

> + set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
> + }
>
> list_add_tail(&worker->node, &pool->workers);
> worker->pool = pool;
> --
> 2.27.0
>