Hi Johannes, On Mon, Jul 07, 2014 at 10:25:06AM -0400, Johannes Weiner wrote: > Hi Vladimir, > > On Mon, Jul 07, 2014 at 04:00:05PM +0400, Vladimir Davydov wrote: > > Hi, > > > > This patch set introduces re-parenting of kmem charges on memcg css > > offline. The idea lying behind it is very simple - instead of pointing > > from kmem objects (kmem caches, non-slab kmem pages) directly to the > > memcg which they are charged against, we make them point to a proxy > > object, mem_cgroup_kmem_context, which, in turn, points to the memcg > > which it belongs to. As a result on memcg offline, it's enough to only > > re-parent the memcg's mem_cgroup_kmem_context. > > The motivation for this was to clear out all references to a memcg by > the time it's offlined, so that the unreachable css can be freed soon. > > However, recent cgroup core changes further disconnected the css from > the cgroup object itself, so it's no longer as urgent to free the css. > > In addition, Tejun made offlined css iterable and split css_tryget() > and css_tryget_online(), which would allow memcg to pin the css until > the last charge is gone while continuing to iterate and reclaim it on > hierarchical pressure, even after it was offlined. > > This would obviate the need for reparenting as a whole, not just kmem > pages, but even remaining page cache. Michal already obsoleted the > force_empty knob that reparents as a fallback, and whether the cache > pages are in the parent or in a ghost css after cgroup deletion does > not make a real difference from a user point of view, they still get > reclaimed when the parent experiences pressure. So, that means there's no need in a proxy object between kmem objects and the memcg which they are charged against (mem_cgroup_kmem_context in this patch set), because now it's OK to pin css from kmem allocations. Furthermore there will be no need to reparent per memcg list_lrus when they are introduced. That's nice! > You could then reap dead slab caches as part of the regular per-memcg > slab scanning in reclaim, without having to resort to auxiliary lists, > vmpressure events etc. Do you mean adding a per memcg shrinker that will call kmem_cache_shrink for all memcg caches on memcg/global pressure? Actually I recently made dead caches self-destructive at the cost of slowing down kfrees to dead caches (see https://www.lwn.net/Articles/602330/, it's already in the mmotm tree) so no dead cache reaping is necessary. Do you think if we need it now? > I think it would save us a lot of code and complexity. You want > per-memcg slab scanning *anyway*, all we'd have to change in the > existing code would be to pin the css until the LRUs and kmem caches > are truly empty, and switch mem_cgroup_iter() to css_tryget(). > > Would this make sense to you? Hmm, interesting. Thank you for such a thorough explanation. One question. Do we still need to free mem_cgroup->kmemcg_id on css offline so that it can be reused by new kmem-active cgroups (currently we don't)? If we won't free it the root_cache->memcg_params->memcg_arrays may become really huge due to lots of dead css holding the id. Thanks. -- 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>