On Fri 01-12-17 14:56:38, Johannes Weiner wrote: > On Fri, Dec 01, 2017 at 03:46:34PM +0100, Michal Hocko wrote: > > On Fri 01-12-17 14:33:17, Johannes Weiner wrote: > > > On Sat, Nov 25, 2017 at 07:52:47PM +0900, Tetsuo Handa wrote: > > > > @@ -1068,6 +1071,17 @@ bool out_of_memory(struct oom_control *oc) > > > > } > > > > > > > > select_bad_process(oc); > > > > + /* > > > > + * Try really last second allocation attempt after we selected an OOM > > > > + * victim, for somebody might have managed to free memory while we were > > > > + * selecting an OOM victim which can take quite some time. > > > > > > Somebody might free some memory right after this attempt fails. OOM > > > can always be a temporary state that resolves on its own. > > > > > > What keeps us from declaring OOM prematurely is the fact that we > > > already scanned the entire LRU list without success, not last second > > > or last-last second, or REALLY last-last-last-second allocations. > > > > You are right that this is inherently racy. The point here is, however, > > that the race window between the last check and the kill can be _huge_! > > My point is that it's irrelevant. We already sampled the entire LRU > list; compared to that, the delay before the kill is immaterial. Well, I would disagree. I have seen OOM reports with a free memory. Closer debugging shown that an existing process was on the way out and the oom victim selection took way too long and fired after a large process manage. There were different hacks^Wheuristics to cover those cases but they turned out to just cause different corner cases. Moving the existing last moment allocation after a potentially very time consuming action is relatively cheap and safe measure to cover those cases without any negative side effects I can think of. Anyway, if the delay is immaterial than the existing last-retry is even more pointless because it is executed right _after_ we gave up reclaim retries. Compare that to the select_bad_process time window. And really, that can take quite a lot of time. Especially in weird priority inversion situations. > > Another argument is that the allocator itself could have changed its > > allocation capabilities - e.g. become the OOM victim itself since the > > last time it the allocator could have reflected that fact. > > Can you outline how this would happen exactly? http://lkml.kernel.org/r/20171101135855.bqg2kuj6ao2cicqi@xxxxxxxxxxxxxx As I try to explain the workload is really pathological but this (resp. the follow up based on this patch) as a workaround is moderately ugly wrt. it actually can help. > > > Nacked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- Michal Hocko 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>
