On Wed, Aug 29, 2018 at 2:24 PM Roman Gushchin <guro@xxxxxx> wrote: > > On Tue, Aug 21, 2018 at 03:10:52PM -0700, Shakeel Butt wrote: > > On Tue, Aug 21, 2018 at 2:36 PM Roman Gushchin <guro@xxxxxx> wrote: > > > > > > If CONFIG_VMAP_STACK is set, kernel stacks are allocated > > > using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel > > > stack pages are charged against corresponding memory cgroups > > > on allocation and uncharged on releasing them. > > > > > > The problem is that we do cache kernel stacks in small > > > per-cpu caches and do reuse them for new tasks, which can > > > belong to different memory cgroups. > > > > > > Each stack page still holds a reference to the original cgroup, > > > so the cgroup can't be released until the vmap area is released. > > > > > > To make this happen we need more than two subsequent exits > > > without forks in between on the current cpu, which makes it > > > very unlikely to happen. As a result, I saw a significant number > > > of dying cgroups (in theory, up to 2 * number_of_cpu + > > > number_of_tasks), which can't be released even by significant > > > memory pressure. > > > > > > As a cgroup structure can take a significant amount of memory > > > (first of all, per-cpu data like memcg statistics), it leads > > > to a noticeable waste of memory. > > > > > > Signed-off-by: Roman Gushchin <guro@xxxxxx> > > > > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > > > > BTW this makes a very good use-case for optimizing kmem uncharging > > similar to what you did for skmem uncharging. > > The only thing I'm slightly worried here is that it can make > reclaiming of memory cgroups harder. Probably, it's still ok, > but let me first finish the work I'm doing on optimizing the > whole memcg reclaim process, and then return to this case. > Yes, maybe we can disable that optimization for offlined memcgs. Anyways, we can discuss this later as you have suggested. Shakeel
