[PATCH 1/2] mm/slab: skip memcg reclaim only if in atomic context
From: Vladimir Davydov <hidden>
Date: 2015-08-30 19:02:46
Also in:
lkml
Subsystem:
memory management, the rest · Maintainers:
Andrew Morton, Linus Torvalds
SLAB's implementation of kmem_cache_alloc() works as follows:
1. First, it tries to allocate from the preferred NUMA node without
issuing reclaim.
2. If step 1 fails, it tries all nodes in the order of preference,
again without invoking reclaimer
3. Only if steps 1 and 2 fails, it falls back on allocation from any
allowed node with reclaim enabled.
Before commit 4167e9b2cf10f ("mm: remove GFP_THISNODE"), GFP_THISNODE
combination, which equaled __GFP_THISNODE|__GFP_NOWARN|__GFP_NORETRY on
NUMA enabled builds, was used in order to avoid reclaim during steps 1
and 2. If __alloc_pages_slowpath() saw this combination in gfp flags, it
aborted immediately even if __GFP_WAIT flag was set. So there was no
need in clearing __GFP_WAIT flag while performing steps 1 and 2 and
hence we could invoke memcg reclaim when allocating a slab page if the
context allowed.
Commit 4167e9b2cf10f zapped GFP_THISNODE combination. Instead of OR-ing
the gfp mask with GFP_THISNODE, gfp_exact_node() helper should now be
used. The latter sets __GFP_THISNODE and __GFP_NOWARN flags and clears
__GFP_WAIT on the current gfp mask. As a result, it effectively
prohibits invoking memcg reclaim on steps 1 and 2. This breaks
memcg/memory.high logic when kmem accounting is enabled. The memory.high
threshold is supposed to work as a soft limit, i.e. it does not fail an
allocation on breaching it, but it still forces the caller to invoke
direct reclaim to compensate for the excess. Without __GFP_WAIT flag
direct reclaim is impossible so the caller will go on without being
pushed back to the threshold.
To fix this issue, we get rid of gfp_exact_node() helper and move gfp
flags filtering to kmem_getpages() after memcg_charge_slab() is called.
To understand the patch, note that:
- In fallback_alloc() the only effect of using gfp_exact_node() is
preventing recursion fallback_alloc() -> ____cache_alloc_node() ->
fallback_alloc().
- Aside from fallback_alloc(), gfp_exact_node() is only used along with
cache_grow(). Moreover, the only place where cache_grow() is used
without it is fallback_alloc(), which, in contrast to other
cache_grow() users, preallocates a page and passes it to cache_grow()
so that the latter does not need to invoke kmem_getpages() by itself.
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Vladimir Davydov <redacted>
---
mm/slab.c | 32 +++++++++++---------------------
1 file changed, 11 insertions(+), 21 deletions(-)
diff --git a/mm/slab.c b/mm/slab.c
index d890750ec31e..9ee809d2ed8b 100644
--- a/mm/slab.c
+++ b/mm/slab.c@@ -857,11 +857,6 @@ static inline void *____cache_alloc_node(struct kmem_cache *cachep, return NULL; } -static inline gfp_t gfp_exact_node(gfp_t flags) -{ - return flags; -} - #else /* CONFIG_NUMA */ static void *____cache_alloc_node(struct kmem_cache *, gfp_t, int);
@@ -1028,15 +1023,6 @@ static inline int cache_free_alien(struct kmem_cache *cachep, void *objp) return __cache_free_alien(cachep, objp, node, page_node); } - -/* - * Construct gfp mask to allocate from a specific node but do not invoke reclaim - * or warn about failures. - */ -static inline gfp_t gfp_exact_node(gfp_t flags) -{ - return (flags | __GFP_THISNODE | __GFP_NOWARN) & ~__GFP_WAIT; -} #endif /*
@@ -1583,7 +1569,7 @@ slab_out_of_memory(struct kmem_cache *cachep, gfp_t gfpflags, int nodeid) * would be relatively rare and ignorable. */ static struct page *kmem_getpages(struct kmem_cache *cachep, gfp_t flags, - int nodeid) + int nodeid, bool fallback) { struct page *page; int nr_pages;
@@ -1595,6 +1581,9 @@ static struct page *kmem_getpages(struct kmem_cache *cachep, gfp_t flags, if (memcg_charge_slab(cachep, flags, cachep->gfporder)) return NULL; + if (!fallback) + flags = (flags | __GFP_THISNODE | __GFP_NOWARN) & ~__GFP_WAIT; + page = __alloc_pages_node(nodeid, flags | __GFP_NOTRACK, cachep->gfporder); if (!page) { memcg_uncharge_slab(cachep, cachep->gfporder);
@@ -2641,7 +2630,8 @@ static int cache_grow(struct kmem_cache *cachep, * 'nodeid'. */ if (!page) - page = kmem_getpages(cachep, local_flags, nodeid); + page = kmem_getpages(cachep, local_flags, nodeid, + !IS_ENABLED(CONFIG_NUMA)); if (!page) goto failed;
@@ -2840,7 +2830,7 @@ alloc_done: if (unlikely(!ac->avail)) { int x; force_grow: - x = cache_grow(cachep, gfp_exact_node(flags), node, NULL); + x = cache_grow(cachep, flags, node, NULL); /* cache_grow can reenable interrupts, then ac could change. */ ac = cpu_cache_get(cachep);
@@ -3034,7 +3024,7 @@ retry: get_node(cache, nid) && get_node(cache, nid)->free_objects) { obj = ____cache_alloc_node(cache, - gfp_exact_node(flags), nid); + flags | __GFP_THISNODE, nid); if (obj) break; }
@@ -3052,7 +3042,7 @@ retry: if (local_flags & __GFP_WAIT) local_irq_enable(); kmem_flagcheck(cache, flags); - page = kmem_getpages(cache, local_flags, numa_mem_id()); + page = kmem_getpages(cache, local_flags, numa_mem_id(), true); if (local_flags & __GFP_WAIT) local_irq_disable(); if (page) {
@@ -3062,7 +3052,7 @@ retry: nid = page_to_nid(page); if (cache_grow(cache, flags, nid, page)) { obj = ____cache_alloc_node(cache, - gfp_exact_node(flags), nid); + flags | __GFP_THISNODE, nid); if (!obj) /* * Another processor may allocate the
@@ -3133,7 +3123,7 @@ retry: must_grow: spin_unlock(&n->list_lock); - x = cache_grow(cachep, gfp_exact_node(flags), nodeid, NULL); + x = cache_grow(cachep, flags, nodeid, NULL); if (x) goto retry;
--
2.1.4
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