Re: [PATCH v2] psi: get poll_work to run when calling poll syscall next time

From: Jason Xing
Date: Wed Aug 14 2019 - 22:03:40 EST


Hello,

It's been delayed for no reason a couple of days. Any comments and suggestions on this patch V2 would be appreciated.

Thanks,
Jason

On 2019/7/30 äå1:16, Jason Xing wrote:
Only when calling the poll syscall the first time can user
receive POLLPRI correctly. After that, user always fails to
acquire the event signal.

Reproduce case:
1. Get the monitor code in Documentation/accounting/psi.txt
2. Run it, and wait for the event triggered.
3. Kill and restart the process.

If the user doesn't kill the monitor process, it seems the
poll_work works fine. After killing and restarting the monitor,
the poll_work in kernel will never run again due to the wrong
value of poll_scheduled. Therefore, we should reset the value
as group_init() does after the last trigger is destroyed.

[PATCH V2]
In the patch v2, I put the atomic_set(&group->poll_scheduled, 0);
into the right place.
Here I quoted from Johannes as the best explaination:
"The question is why we can end up with poll_scheduled = 1 but the work
not running (which would reset it to 0). And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker. The
cancel needs to pair with resetting the poll_scheduled flag."

Signed-off-by: Jason Xing <kerneljasonxing@xxxxxxxxxxxxxxxxx>
Reviewed-by: Caspar Zhang <caspar@xxxxxxxxxxxxxxxxx>
Reviewed-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
kernel/sched/psi.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 7acc632..acdada0 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1131,7 +1131,14 @@ static void psi_trigger_destroy(struct kref *ref)
* deadlock while waiting for psi_poll_work to acquire trigger_lock
*/
if (kworker_to_destroy) {
+ /*
+ * After the RCU grace period has expired, the worker
+ * can no longer be found through group->poll_kworker.
+ * But it might have been already scheduled before
+ * that - deschedule it cleanly before destroying it.
+ */
kthread_cancel_delayed_work_sync(&group->poll_work);
+ atomic_set(&group->poll_scheduled, 0);
kthread_destroy_worker(kworker_to_destroy);
}
kfree(t);