On Monday, 14 May 2007 23:48, Oleg Nesterov wrote: > On 05/14, Rafael J. Wysocki wrote: > > > > On Monday, 14 May 2007 18:55, Oleg Nesterov wrote: > > > > > > Rafael, I am afraid we are making too much noise, and this may confuse > > > Alex > > > and Andrew. > > > > > > First, we should decide how to fix the bug we have in 2.6.22. I prefer a > > > simple > > > "make freezeable workqueues singlethread" I sent. It was acked by Alex, > > > it is > > > simple, and it is also good because tifm doesn't need multithreaded wq > > > anyway. > > > > Yes, I've already agreed with that. > > Ah, OK, I misunderstood your message as if you propose this fix for 2.6.22.
Never mind. :-) > > > - Do we need freezeable workqueues ? > > > > Well, we have at least one case in which they appear to be useful. > > So, in the long term, should we change this only user, or we think we better > fix > freezeable wqs again? Long term, I'd like to have freezable workqueues, so that people don't have to use "raw" kernel threads only because they need some synchronization with hibertnation/suspend. Plus some cases in which workqueues are used by fs-related code make me worry. OTOH, I have some concerns with that (please see [*] below). > > > WORK2->func() completes. > > > > > > freezer comes. cwq->thread notices TIF_FREEZE and goes to refrigerator > > > before > > > executing that barrier. > > This is not possible. cwq->thread _must_ notice the barrier before it goes to > refrigerator. > > So, given that we have cpu_populated_map we can re-introduce take_over_work() > along with migrate_sequence and thus we can fix freezeable multithreaded wqs. [*] The problem is, though, that freezable workqueus have some potential to fail the freezer. Namely, suppose task A calls flush_workqueue() on a freezable workqueue, finds some work items in there, inserts the barrier and waits for completion (TASK_UNINTERRUPTIBLE). In the meantime, TIF_FREEZE is set on the worker thread, which is then woken up and goes to the refrigerator. Thus if A is not NOFREEZE, the freezing of tasks will fail (A must be a kernel thread for this to happen, but still). Worse yet, if A is NOFREEZE, it will be blocked until the worker thread is woken up. To avoid this, I think, we may need to redesign the freezer, so that freezable worker threads are frozen after all of the other kernel threads. Additionally, we'd need to make a rule that NOFREEZE kernel threads must not call flush_workqueue() or cancel_work_sync() on freezable workqueues. > > > If we have take_over_work() we should use it for any workqueue, > > > freezeable or not. Otherwise this is just a mess, imho. > > Still, this is imho true. So we'd better do some other changes to be > consistent. Agreed. Greetings, Rafael - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
