On Tue 05-09-17 21:23:57, Roman Gushchin wrote: > On Tue, Sep 05, 2017 at 04:57:00PM +0200, Michal Hocko wrote: [...] > > Hmm. The changelog says "By default, it will look for the biggest leaf > > cgroup, and kill the largest task inside." But you are accumulating > > oom_score up the hierarchy and so parents will have higher score than > > the layer of their children and the larger the sub-hierarchy the more > > biased it will become. Say you have > > root > > /\ > > / \ > > A D > > / \ > > B C > > > > B (5), C(15) thus A(20) and D(20). Unless I am missing something we are > > going to go down A path and then chose C even though D is the largest > > leaf group, right? > > You're right, changelog is not accurate, I'll fix it. > The behavior is correct, IMO. Please explain why. This is really a non-intuitive semantic. Why should larger hierarchies be punished more than shallow ones? I would completely agree if the whole hierarchy would be a killable entity (aka A would be kill-all). [...] > > I do not understand why do we have to handle root cgroup specially here. > > select_victim_memcg already iterates all memcgs in the oom hierarchy > > (including root) so if the root memcg is the largest one then we > > should simply consider it no? > > We don't have necessary stats for the root cgroup, so we can't calculate > it's oom_score. We used to charge pages to the root memcg as well so we might resurrect that idea. In any case this is something that could be hidden in memcg_oom_badness rather then special cased somewhere else. > > You are skipping root there because of > > memcg_has_children but I suspect this and the whole accumulate up the > > hierarchy approach just makes the whole thing more complex than necessary. With > > "tasks only in leafs" cgroup policy we should only see any pages on LRUs > > on the global root memcg and leaf cgroups. The same applies to memcg > > stats. So why cannot we simply do the tree walk, calculate > > badness/check the priority and select the largest memcg in one go? > > We have to traverse from top to bottom to make priority-based decision, > but size-based oom_score is calculated as sum of descending leaf cgroup scores. > > For example: > root > /\ > / \ > A D > / \ > B C > A and D have same priorities, B has larger priority than C. > > In this case we need to calculate size-based score for A, which requires > summing oom_score of the sub-tree (B an C), despite we don't need it > for choosing between B and C. > > Maybe I don't see it, but I don't know how to implement it more optimal. I have to think about the priority based oom killing some more to be honest. Do we really want to allow setting a priority to non-leaf memcgs? How are you going to manage the whole tree consistency? Say your above example have prio(A) < prio(D) && prio(C) > prio(D). Your current implementation would kill D, right? Isn't that counter intuitive behavior again. If anything we should prio(A) = max(tree_prio(A)). Again I could understand comparing priorities only on killable entities. -- Michal Hocko SUSE Labs -- 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
