Hello, Steven, Peter. On Wed, Mar 19, 2014 at 08:34:07PM +0100, Peter Zijlstra wrote: > The way I understand workqueues is that we cannot guarantee concurrency > like this. It tries, but there's no guarantee. So, the guarantee is that if a workqueue has WQ_MEM_RECLAIM, it'll always have at least one worker thread working on it, so workqueues which may be depended upon during memory reclaim should have the flag set and must not require more than single level of concurrency to make forward progress. Workqueues w/o memory reclaim set depend on the fact that eventually memory will be reclaimed and enough number of workers necessary to make forward progress will be made available. > WQ_MAX_ACTIVE seems to be a hard upper limit of concurrent workers. So > given 511 other blocked works, the described problem will always happen. That actually is per-workqueue limit and workqueue core will try to create as many workers as possible to satisfy the demanded concurrency. ie. having two workqueues with the same max_active means that the total number of workers may reach 2 * max_active; however, this is no guarantee. If the system is under memory pressure and the workqueues don't have MEM_RECLAIM set, they may not get any concurrency until more memory is made available. > Creating another workqueue doesn't actually create more threads. It looks like the issue Steven is describing is caused by having a dependency chain longer than 1 through rwsem in a MEM_RECLAIM workqueue. Moving the write work items to a separate workqueue breaks the r-w-r chain and ensures that forward progress can be made with single level of concurrency on both workqueues, so, yeah, it looks like the correct fix to me. It it scarily subtle tho and quite likely to present in other code paths too. :( Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-cifs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
