|From:||Vladimir Davydov <email@example.com>|
|Subject:||[PATCH -mm v3 0/8] memcg/slab: reintroduce dead cache self-destruction|
|Date:||Fri, 13 Jun 2014 00:38:14 +0400|
|Cc:||<firstname.lastname@example.org>, <email@example.com>, <firstname.lastname@example.org>, <email@example.com>, <firstname.lastname@example.org>, <email@example.com>, <firstname.lastname@example.org>, <email@example.com>|
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 by making them drop free slabs as soon as they become empty. Since dead caches should never be allocated from, removing empty slabs from them shouldn't result in noticeable performance degradation. 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; - patch 4 makes SLUB's version of kmem_cache_shrink always drop empty slabs, even if it fails to allocate a temporary array; - patches 5 and 6 fix possible use-after-free connected with asynchronous cache destruction; - patches 7 and 8 disable caching of empty slabs for dead memcg caches for SLUB and SLAB respectively. 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. v3: - add smp barrier to memcg_cache_dead (Joonsoo); - add comment explaining why kfree has to be non-preemptable (Joonsoo); - do not call flush_all from put_cpu_partial (SLUB), because slab_free is now non-preemptable (Joonsoo); - simplify the patch disabling free slabs/objects caching for dead SLAB caches. v2: https://lkml.org/lkml/2014/6/6/366 - fix use-after-free connected with asynchronous cache destruction; - less intrusive version of SLUB's kmem_cache_shrink fix; - simplify disabling of free slabs caching for SLUB (Joonsoo); - disable free slabs caching instead of using cache_reap for SLAB (Christoph). v1: https://lkml.org/lkml/2014/5/30/264 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: don't fail kmem_cache_shrink if slab placement optimization fails slub: make slab_free non-preemptable memcg: wait for kfree's to finish before destroying cache slub: make dead memcg caches discard free slabs immediately slab: do not keep free objects/slabs on dead memcg caches include/linux/slab.h | 14 +++++++----- mm/memcontrol.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++---- mm/slab.c | 37 ++++++++++++++++++++++++++++++- mm/slab.h | 25 +++++++++++++++++++++ mm/slub.c | 42 ++++++++++++++++++++++++++--------- 5 files changed, 156 insertions(+), 21 deletions(-) -- 184.108.40.206 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to firstname.lastname@example.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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