On 2018/07/30 23:54, Tejun Heo wrote: > 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). WQ_MEM_RECLAIM guarantees that "struct task_struct" is preallocated. But WQ_MEM_RECLAIM does not guarantee that the pending work is started as soon as an item was queued. Same rule applies to both WQ_MEM_RECLAIM workqueues and !WQ_MEM_RECLAIM workqueues regarding when to start a pending work (i.e. when schedule_timeout_*() is called). Is this correct? > 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). Currently the page allocator has a path which can loop forever with only cond_resched(). > > Thanks. >
