mm: slub: optimise the SLUB fast path to avoid pfmemalloc checks
This patch removes the check for pfmemalloc from the alloc hotpath and
puts the logic after the election of a new per cpu slab. For a pfmemalloc
page we do not use the fast path but force the use of the slow path which
is also used for the debug case.
This has the side-effect of weakening pfmemalloc processing in the
following way;
1. A process that is allocating for network swap calls __slab_alloc.
pfmemalloc_match is true so the freelist is loaded and c->freelist is
now pointing to a pfmemalloc page.
2. A process that is attempting normal allocations calls slab_alloc,
finds the pfmemalloc page on the freelist and uses it because it did
not check pfmemalloc_match()
The patch allows non-pfmemalloc allocations to use pfmemalloc pages with
the kmalloc slabs being the most vunerable caches on the grounds they
are most likely to have a mix of pfmemalloc and !pfmemalloc requests. A
later patch will still protect the system as processes will get throttled
if the pfmemalloc reserves get depleted but performance will not degrade
as smoothly.
[mgorman@suse.de: Expanded changelog]
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: David Miller <davem@davemloft.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/slub.c b/mm/slub.c
index c3f05e1..8f78e25 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2281,11 +2281,11 @@
}
page = c->page;
- if (likely(!kmem_cache_debug(s)))
+ if (likely(!kmem_cache_debug(s) && pfmemalloc_match(page, gfpflags)))
goto load_freelist;
/* Only entered in the debug case */
- if (!alloc_debug_processing(s, page, freelist, addr))
+ if (kmem_cache_debug(s) && !alloc_debug_processing(s, page, freelist, addr))
goto new_slab; /* Slab failed checks. Next slab needed */
deactivate_slab(s, page, get_freepointer(s, freelist));
@@ -2337,8 +2337,7 @@
object = c->freelist;
page = c->page;
- if (unlikely(!object || !node_match(page, node) ||
- !pfmemalloc_match(page, gfpflags)))
+ if (unlikely(!object || !node_match(page, node)))
object = __slab_alloc(s, gfpflags, node, addr, c);
else {
