Hi, When a memcg is turned offline, some of its kmem caches can still have active objects and therefore cannot be destroyed immediately. Currently, we simply leak such caches along with the owner memcg, which is bad and should be resolved. It would be perfect if we could move all slab pages of such dead caches to the root/parent cache on memcg offline. However, when I tried to implement such re-parenting, I was pointed out by Christoph that the overhead of this would be unacceptable, at least for SLUB (see https://lkml.org/lkml/2014/5/13/446) The problem with re-parenting of individual slabs is that it requires tracking of all slabs allocated to a cache, but SLUB doesn't track full slabs if !debug. Changing this behavior would result in significant performance degradation of regular alloc/free paths, because it would make alloc/free take per node list locks more often. After pondering about this problem for some time, I think we should return to dead caches self-destruction, i.e. scheduling cache destruction work when the last slab page is freed. This is the behavior we had before commit 5bd93da9917f ("memcg, slab: simplify synchronization scheme"). The reason why it was removed was that it simply didn't work, because SL[AU]B are implemented in such a way that they don't discard empty slabs immediately, but prefer keeping them cached for indefinite time to speed up further allocations. However, we can change this w/o noticeable performance impact for both SLAB and SLUB. For SLAB, we can make internal cache reaper shrink dead caches aggressively so that they will die quickly after the last object is freed. For SLUB, we can make free path drop free slabs as soon as they become empty. Since dead caches should never be allocated from, removing empty slabs from them shouldn't degrade performance. So, this patch set reintroduces dead cache self-destruction and adds some tweaks to SL[AU]B to prevent dead caches from hanging around indefinitely. It is organized as follows: - patches 1-3 reintroduce dead memcg cache self-destruction; - patches 4-5 do some cleanup in kmem_cache_shrink; - patch 6 is a minor optimization for SLUB, which makes the following work a bit easier for me; - patches 7 and 8 solves the problem with dead memcg caches for SLUB and SLAB respectively. Even if the whole approach is NAK'ed, patches 4, 5, and 6 are worth applying, IMO, provided Christoph doesn't mind, of course. They don't depend on the rest of the set, BTW. Note, this doesn't resolve the problem of memcgs pinned by dead kmem caches. I'm planning to solve this by re-parenting dead kmem caches to the parent memcg. Thanks, Vladimir Davydov (8): memcg: cleanup memcg_cache_params refcnt usage memcg: destroy kmem caches when last slab is freed memcg: mark caches that belong to offline memcgs as dead slub: never fail kmem_cache_shrink slab: remove kmem_cache_shrink retval slub: do not use cmpxchg for adding cpu partials when irqs disabled slub: make dead caches discard free slabs immediately slab: reap dead memcg caches aggressively include/linux/slab.h | 10 ++- mm/memcontrol.c | 25 +++++- mm/slab.c | 28 +++++-- mm/slab.h | 12 ++- mm/slab_common.c | 8 +- mm/slob.c | 3 +- mm/slub.c | 212 ++++++++++++++++++++++++++++++++++---------------- 7 files changed, 207 insertions(+), 91 deletions(-) -- 1.7.10.4 -- 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>
