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.ca...@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 <c...@redhat.com> > Signed-off-by: Valentin Schneider <valentin.schnei...@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 >