Since commit
1cf12e08bc4d ("sched/hotplug: Consolidate task migration on CPU unplug")
tasks are expected to move themselves out of a out-going CPU. For most
tasks this will be done automagically via BALANCE_PUSH, but percpu kthreads
will have to cooperate and move themselves away one way or another.
Currently, some percpu kthreads (workqueues being a notable exemple) do not
cooperate nicely and can end up on an out-going CPU at the time
sched_cpu_dying() is invoked.
Print the dying rq's tasks to shed some light on the stragglers.
Signed-off-by: Valentin Schneider <[email protected]>
---
As Peter pointed out, this should really be caught much earlier than
sched_cpu_dying().
If we go down the route of preventing kthreads from being affined to
!active CPUs in __set_cpus_allowed_ptr() (genuine percpu kthreads sidestep
it via kthread_bind_mask()), then I *think* we could catch this in wakeups,
i.e. select_task_rq(). I've been playing around there, but it's not as
straightforward as I'd have hoped.
---
kernel/sched/core.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a08a39d7cdb..d784dd1ae436 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7564,6 +7564,25 @@ static void calc_load_migrate(struct rq *rq)
atomic_long_add(delta, &calc_load_tasks);
}
+static void dump_rq_tasks(struct rq *rq, const char *loglvl)
+{
+ struct task_struct *g, *p;
+ int cpu = cpu_of(rq);
+
+ lockdep_assert_held(&rq->lock);
+
+ printk("%sCPU%d enqueued tasks (%u total):\n", loglvl, cpu,
rq->nr_running);
+ for_each_process_thread(g, p) {
+ if (task_cpu(p) != cpu)
+ continue;
+
+ if (!task_on_rq_queued(p))
+ continue;
+
+ printk("%s\tpid: %d, name: %s\n", loglvl, p->pid, p->comm);
+ }
+}
+
int sched_cpu_dying(unsigned int cpu)
{
struct rq *rq = cpu_rq(cpu);
@@ -7573,7 +7592,10 @@ int sched_cpu_dying(unsigned int cpu)
sched_tick_stop(cpu);
rq_lock_irqsave(rq, &rf);
- BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
+ if (rq->nr_running != 1 || rq_has_pinned_tasks(rq)) {
+ WARN(true, "Dying CPU not properly vacated!");
+ dump_rq_tasks(rq, KERN_WARNING);
+ }
rq_unlock_irqrestore(rq, &rf);
calc_load_migrate(rq);
--
2.27.0
