On Wed, Sep 27, 2017 at 09:37:44AM +0200, Michal Hocko wrote: > On Tue 26-09-17 14:04:41, David Rientjes wrote: > > On Tue, 26 Sep 2017, Michal Hocko wrote: > > > > > > No, I agree that we shouldn't compare sibling memory cgroups based on > > > > different criteria depending on whether group_oom is set or not. > > > > > > > > I think it would be better to compare siblings based on the same criteria > > > > independent of group_oom if the user has mounted the hierarchy with the > > > > new mode (I think we all agree that the mount option is needed). It's > > > > very easy to describe to the user and the selection is simple to > > > > understand. > > > > > > I disagree. Just take the most simplistic example when cgroups reflect > > > some other higher level organization - e.g. school with teachers, > > > students and admins as the top level cgroups to control the proper cpu > > > share load. Now you want to have a fair OOM selection between different > > > entities. Do you consider selecting students all the time as an expected > > > behavior just because their are the largest group? This just doesn't > > > make any sense to me. > > > > > > > Are you referring to this? > > > > root > > / \ > > students admins > > / \ / \ > > A B C D > > > > If the cumulative usage of all students exceeds the cumulative usage of > > all admins, yes, the choice is to kill from the /students tree. > > Which is wrong IMHO because the number of stutends is likely much more > larger than admins (or teachers) yet it might be the admins one to run > away. This example simply shows how comparing siblinks highly depends > on the way you organize the hierarchy rather than the actual memory > consumer runaways which is the primary goal of the OOM killer to handle. > > > This has been Roman's design from the very beginning. > > I suspect this was the case because deeper hierarchies for > organizational purposes haven't been considered. > > > If the preference is to kill > > the single largest process, which may be attached to either subtree, you > > would not have opted-in to the new heuristic. > > I believe you are making a wrong assumption here. The container cleanup > is sound reason to opt in and deeper hierarchies are simply required in > the cgroup v2 world where you do not have separate hierarchies. > > > > > Then, once a cgroup has been chosen as the victim cgroup, > > > > kill the process with the highest badness, allowing the user to influence > > > > that with /proc/pid/oom_score_adj just as today, if group_oom is disabled; > > > > otherwise, kill all eligible processes if enabled. > > > > > > And now, what should be the semantic of group_oom on an intermediate > > > (non-leaf) memcg? Why should we compare it to other killable entities? > > > Roman was mentioning a setup where a _single_ workload consists of a > > > deeper hierarchy which has to be shut down at once. It absolutely makes > > > sense to consider the cumulative memory of that hierarchy when we are > > > going to kill it all. > > > > > > > If group_oom is enabled on an intermediate memcg, I think the intuitive > > way to handle it would be that all descendants are also implicitly or > > explicitly group_oom. > > This is an interesting point. I would tend to agree here. If somebody > requires all-in clean up up the hierarchy it feels strange that a > subtree would disagree (e.g. during memcg oom on the subtree). I can > hardly see a usecase that would really need a different group_oom policy > depending on where in the hierarchy the oom happened to be honest. > Roman? Yes, I'd say that it's strange to apply settings from outside the OOMing cgroup to the subtree, but actually it's not. The oom_group setting should basically mean that the OOM killer will not kill a random task in the subtree. And it doesn't matter if it was global or memcg-wide OOM. Applied to v9. Thanks! -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
