On Tue, Aug 03, 2010 at 06:55:20PM +0800, Jan Kara wrote: > On Tue 03-08-10 11:01:25, Wu Fengguang wrote: > > On Tue, Aug 03, 2010 at 04:51:52AM +0800, Jan Kara wrote: > > > On Fri 30-07-10 12:03:06, Wu Fengguang wrote: > > > > On Fri, Jul 30, 2010 at 12:20:27AM +0800, Jan Kara wrote: > > > > > On Thu 29-07-10 19:51:44, Wu Fengguang wrote: > > > > > > The periodic/background writeback can run forever. So when any > > > > > > sync work is enqueued, increase bdi->sync_works to notify the > > > > > > active non-sync works to exit. Non-sync works queued after sync > > > > > > works won't be affected. > > > > > Hmm, wouldn't it be simpler logic to just make for_kupdate and > > > > > for_background work always yield when there's some other work to do (as > > > > > they are livelockable from the definition of the target they have) and > > > > > make sure any other work isn't livelockable? > > > > > > > > Good idea! > > > > > > > > > The only downside is that > > > > > non-livelockable work cannot be "fair" in the sense that we cannot switch > > > > > inodes after writing MAX_WRITEBACK_PAGES. > > > > > > > > Cannot switch indoes _before_ finish with the current > > > > MAX_WRITEBACK_PAGES batch? > > > Well, even after writing all those MAX_WRITEBACK_PAGES. Because what you > > > want to do in a non-livelockable work is: take inode, write it, never look at > > > it again for this work. Because if you later return to the inode, it can > > > have newer dirty pages and thus you cannot really avoid livelock. Of > > > course, this all assumes .nr_to_write isn't set to something small. That > > > avoids the livelock as well. > > > > I do have a poor man's solution that can handle this case. > > https://kerneltrap.org/mailarchive/linux-fsdevel/2009/10/7/6476473/thread > > It may do more extra works, but will stop livelock in theory. > So I don't think sync work on it's own is a problem. There we can just > give up any fairness and just go inode by inode. IMHO it's much simpler that > way. I would like to reserve my opinion here. IMHO small files should better get synced first :) > The remaining types of work we have are "for_reclaim" and then ones > triggered by filesystems to get rid of delayed allocated data. These cases > can easily have well defined and low nr_to_write so they wouldn't be > livelockable either. What do you think? Right. for_reclaim works won't livelock in itself, since it will be bounded by either nr_to_write or some range. They may be delayed for a while by large sync or nr_pages works though. > > A related question is, what if some for_reclaim works get enqueued? > > Shall we postpone the sync work as well? The global sync is not likely > > to hit the dirty pages in a small memcg, or may take long time. It > > seems not a high priority task though. > I see some incentive to do this but the simple thing with for_background > and for_kupdate work is that they are essentially state-less and so they > can be easily (and automatically) restarted. It would be really hard to > implement something like this for sync and still avoid livelocks. So let's ignore the issue for now. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
