Hello, On Mon, Jul 30, 2018 at 04:46:47PM +0200, Michal Hocko wrote: > On Mon 30-07-18 23:34:23, Tetsuo Handa wrote: > > On 2018/07/30 18:32, Michal Hocko wrote: > [...] > > > This one is waiting for draining and we are in mm_percpu_wq WQ context > > > which has its rescuer so no other activity can block us for ever. So > > > this certainly shouldn't deadlock. It can be dead slow but well, this is > > > what you will get when your shoot your system to death. > > > > We need schedule_timeout_*() to allow such WQ_MEM_RECLAIM workqueues to wake up. (Tejun, > > is my understanding correct?) Lack of schedule_timeout_*() does block WQ_MEM_RECLAIM > > workqueues forever. > > Hmm. This doesn't match my understanding of what WQ_MEM_RECLAIM actually > guarantees. If you are right then the whole thing sounds quite fragile > to me TBH. Workqueue doesn't think the cpu is stalled as long as one of the per-cpu kworkers is running. The assumption is that kernel threads are not supposed to be busy-looping indefinitely (and they really shouldn't). We can add timeout mechanism to workqueue so that it kicks off other kworkers if one of them is in running state for too long, but idk, if there's an indefinite busy loop condition in kernel threads, we really should get rid of them and hung task watchdog is pretty effective at finding these cases (at least with preemption disabled). Thanks. -- tejun
