On Thu, Jan 19, 2017 at 12:23:36PM +0100, Michal Hocko wrote: > On Thu 19-01-17 10:07:55, Mel Gorman wrote: > [...] > > mm, vmscan: Wait on a waitqueue when too many pages are isolated > > > > When too many pages are isolated, direct reclaim waits on congestion to clear > > for up to a tenth of a second. There is no reason to believe that too many > > pages are isolated due to dirty pages, reclaim efficiency or congestion. > > It may simply be because an extremely large number of processes have entered > > direct reclaim at the same time. However, it is possible for the situation > > to persist forever and never reach OOM. > > > > This patch queues processes a waitqueue when too many pages are isolated. > > When parallel reclaimers finish shrink_page_list, they wake the waiters > > to recheck whether too many pages are isolated. > > > > The wait on the queue has a timeout as not all sites that isolate pages > > will do the wakeup. Depending on every isolation of LRU pages to be perfect > > forever is potentially fragile. The specific wakeups occur for page reclaim > > and compaction. If too many pages are isolated due to memory failure, > > hotplug or directly calling migration from a syscall then the waiting > > processes may wait the full timeout. > > > > Note that the timeout allows the use of waitqueue_active() on the basis > > that a race will cause the full timeout to be reached due to a missed > > wakeup. This is relatively harmless and still a massive improvement over > > unconditionally calling congestion_wait. > > > > Direct reclaimers that cannot isolate pages within the timeout will consider > > return to the caller. This is somewhat clunky as it won't return immediately > > and make go through the other priorities and slab shrinking. Eventually, > > it'll go through a few iterations of should_reclaim_retry and reach the > > MAX_RECLAIM_RETRIES limit and consider going OOM. > > I cannot really say I would like this. It's just much more complex than > necessary. I guess it's a difference in opinion. Miximg per-zone and per-node information for me is complex. I liked the workqueue because it was an example of waiting on a specific event instead of relying completely on time. > I definitely agree that congestion_wait while waiting for > too_many_isolated is a crude hack. This patch doesn't really resolve > my biggest worry, though, that we go OOM with too many pages isolated > as your patch doesn't alter zone_reclaimable_pages to reflect those > numbers. > Indeed, but such cases are also caught by the no_progress_loop logic to avoid a premature OOM. > Anyway, I think both of us are probably overcomplicating things a bit. > Your waitqueue approach is definitely better semantically than the > congestion_wait because we are waiting for a different event than the > API is intended for. On the other hand a mere > schedule_timeout_interruptible might work equally well in the real life. > On the other side I might really over emphasise the role of NR_ISOLATED* > counts. It might really turn out that we can safely ignore them and it > won't be the end of the world. So what do you think about the following > as a starting point. If we ever see oom reports with high number of > NR_ISOLATED* which are part of the oom report then we know we have to do > something about that. Those changes would at least be driven by a real > usecase rather than theoretical scenarios. > > So what do you think about the following? Tetsuo, would you be willing > to run this patch through your torture testing please? I'm fine with treating this as a starting point. Thanks. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>