On Mon 30-07-18 08:44:24, Tejun Heo wrote: > Hello, > > On Tue, Jul 31, 2018 at 12:25:04AM +0900, Tetsuo Handa wrote: > > 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? > > WQ_MEM_RECLAIM guarantees that there's always gonna exist at least one > kworker running the workqueue. But all per-cpu kworkers are subject > to concurrency limiting execution - ie. if there are any per-cpu > actively running on a cpu, no futher kworkers will be scheduled. Well, in the ideal world we would _use_ that pre-allocated kworker if there are no other available because they are doing something that takes a long time to accomplish. Page allocator can spend a lot of time if we are struggling to death to get some memory. > > > 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(). > > Yeah, workqueue can choke on things like that and kthread indefinitely > busy looping doesn't do anybody any good. Yeah, I do agree. But this is much easier said than done ;) Sure we have that hack that does sleep rather than cond_resched in the page allocator. We can and will "fix" it to be unconditional in the should_reclaim_retry [1] but this whole thing is really subtle. It just take one misbehaving worker and something which is really important to run will get stuck. That being said I will post the patch with updated changelog recording this. [1] http://lkml.kernel.org/r/ca3da8b8-1bb5-c302-b190-fa6cebab58ca@xxxxxxxxxxxxxxxxxxx -- Michal Hocko SUSE Labs
