This patch introduce a new API call kfree_bulk() for bulk freeing
memory objects not bound to a single kmem_cache.
Christoph pointed out that it is possible to implement freeing of
objects, without knowing the kmem_cache pointer as that information is
available from the object's page->slab_cache. Proposing to remove the
kmem_cache argument from the bulk free API.
Jesper demonstrated that these extra steps per object comes at a
performance cost. It is only in the case CONFIG_MEMCG_KMEM is
compiled in and activated runtime that these steps are done anyhow.
The extra cost is most visible for SLAB allocator, because the SLUB
allocator does the page lookup (virt_to_head_page()) anyhow.
Thus, the conclusion was to keep the kmem_cache free bulk API with a
kmem_cache pointer, but we can still implement a kfree_bulk() API
fairly easily. Simply by handling if kmem_cache_free_bulk() gets
called with a kmem_cache NULL pointer.
This does increase the code size a bit, but implementing a separate
kfree_bulk() call would likely increase code size even more.
Below benchmarks cost of alloc+free (obj size 256 bytes) on
CPU i7-4790K @ 4.00GHz, no PREEMPT and CONFIG_MEMCG_KMEM=y.
Code size increase for SLAB:
add/remove: 0/0 grow/shrink: 1/0 up/down: 74/0 (74)
function old new delta
kmem_cache_free_bulk 660 734 +74
SLAB fastpath: 85 cycles(tsc) 21.468 ns (step:0)
sz - fallback - kmem_cache_free_bulk - kfree_bulk
1 - 101 cycles 25.291 ns - 41 cycles 10.499 ns - 130 cycles 32.522 ns
2 - 95 cycles 23.964 ns - 26 cycles 6.558 ns - 56 cycles 14.134 ns
3 - 93 cycles 23.281 ns - 20 cycles 5.244 ns - 41 cycles 10.393 ns
4 - 92 cycles 23.123 ns - 18 cycles 4.589 ns - 26 cycles 6.612 ns
8 - 90 cycles 22.696 ns - 24 cycles 6.211 ns - 32 cycles 8.175 ns
16 - 108 cycles 27.175 ns - 13 cycles 3.418 ns - 21 cycles 5.480 ns
30 - 90 cycles 22.708 ns - 14 cycles 3.667 ns - 20 cycles 5.222 ns
32 - 90 cycles 22.687 ns - 13 cycles 3.337 ns - 20 cycles 5.170 ns
34 - 90 cycles 22.699 ns - 14 cycles 3.622 ns - 21 cycles 5.269 ns
48 - 90 cycles 22.585 ns - 14 cycles 3.525 ns - 21 cycles 5.261 ns
64 - 90 cycles 22.523 ns - 13 cycles 3.440 ns - 20 cycles 5.190 ns
128 - 91 cycles 22.962 ns - 15 cycles 3.883 ns - 22 cycles 5.622 ns
158 - 91 cycles 22.877 ns - 15 cycles 3.770 ns - 22 cycles 5.582 ns
250 - 93 cycles 23.282 ns - 16 cycles 4.133 ns - 24 cycles 6.047 ns
SLAB when enabling MEMCG_KMEM runtime:
- kmemcg fastpath: 134 cycles(tsc) 33.514 ns (step:0)
1 - 146 cycles 36.634 ns - 66 cycles 16.705 ns - 67 cycles 16.793 ns
2 - 137 cycles 34.452 ns - 41 cycles 10.326 ns - 42 cycles 10.736 ns
3 - 135 cycles 33.856 ns - 34 cycles 8.545 ns - 36 cycles 9.049 ns
4 - 132 cycles 33.232 ns - 33 cycles 8.306 ns - 29 cycles 7.368 ns
8 - 134 cycles 33.612 ns - 24 cycles 6.250 ns - 24 cycles 6.130 ns
16 - 131 cycles 33.003 ns - 23 cycles 5.981 ns - 22 cycles 5.552 ns
30 - 131 cycles 32.920 ns - 21 cycles 5.499 ns - 21 cycles 5.397 ns
32 - 131 cycles 32.861 ns - 21 cycles 5.482 ns - 21 cycles 5.301 ns
34 - 131 cycles 32.837 ns - 21 cycles 5.461 ns - 20 cycles 5.236 ns
48 - 130 cycles 32.725 ns - 23 cycles 5.878 ns - 21 cycles 5.367 ns
64 - 130 cycles 32.625 ns - 21 cycles 5.374 ns - 21 cycles 5.251 ns
128 - 132 cycles 33.048 ns - 22 cycles 5.725 ns - 22 cycles 5.662 ns
158 - 132 cycles 33.235 ns - 22 cycles 5.641 ns - 22 cycles 5.579 ns
250 - 134 cycles 33.557 ns - 24 cycles 6.035 ns - 23 cycles 5.934 ns
Code size increase for SLUB:
function old new delta
kmem_cache_free_bulk 717 799 +82
SLUB benchmark:
SLUB fastpath: 46 cycles(tsc) 11.691 ns (step:0)
sz - fallback - kmem_cache_free_bulk - kfree_bulk
1 - 61 cycles 15.486 ns - 53 cycles 13.364 ns - 57 cycles 14.464 ns
2 - 54 cycles 13.703 ns - 32 cycles 8.110 ns - 33 cycles 8.482 ns
3 - 53 cycles 13.272 ns - 25 cycles 6.362 ns - 27 cycles 6.947 ns
4 - 51 cycles 12.994 ns - 24 cycles 6.087 ns - 24 cycles 6.078 ns
8 - 50 cycles 12.576 ns - 21 cycles 5.354 ns - 22 cycles 5.513 ns
16 - 49 cycles 12.368 ns - 20 cycles 5.054 ns - 20 cycles 5.042 ns
30 - 49 cycles 12.273 ns - 18 cycles 4.748 ns - 19 cycles 4.758 ns
32 - 49 cycles 12.401 ns - 19 cycles 4.821 ns - 19 cycles 4.810 ns
34 - 98 cycles 24.519 ns - 24 cycles 6.154 ns - 24 cycles 6.157 ns
48 - 83 cycles 20.833 ns - 21 cycles 5.446 ns - 21 cycles 5.429 ns
64 - 75 cycles 18.891 ns - 20 cycles 5.247 ns - 20 cycles 5.238 ns
128 - 93 cycles 23.271 ns - 27 cycles 6.856 ns - 27 cycles 6.823 ns
158 - 102 cycles 25.581 ns - 30 cycles 7.714 ns - 30 cycles 7.695 ns
250 - 107 cycles 26.917 ns - 38 cycles 9.514 ns - 38 cycles 9.506 ns
SLUB when enabling MEMCG_KMEM runtime:
- kmemcg fastpath: 71 cycles(tsc) 17.897 ns (step:0)
1 - 85 cycles 21.484 ns - 78 cycles 19.569 ns - 75 cycles 18.938 ns
2 - 81 cycles 20.363 ns - 45 cycles 11.258 ns - 44 cycles 11.076 ns
3 - 78 cycles 19.709 ns - 33 cycles 8.354 ns - 32 cycles 8.044 ns
4 - 77 cycles 19.430 ns - 28 cycles 7.216 ns - 28 cycles 7.003 ns
8 - 101 cycles 25.288 ns - 23 cycles 5.849 ns - 23 cycles 5.787 ns
16 - 76 cycles 19.148 ns - 20 cycles 5.162 ns - 20 cycles 5.081 ns
30 - 76 cycles 19.067 ns - 19 cycles 4.868 ns - 19 cycles 4.821 ns
32 - 76 cycles 19.052 ns - 19 cycles 4.857 ns - 19 cycles 4.815 ns
34 - 121 cycles 30.291 ns - 25 cycles 6.333 ns - 25 cycles 6.268 ns
48 - 108 cycles 27.111 ns - 21 cycles 5.498 ns - 21 cycles 5.458 ns
64 - 100 cycles 25.164 ns - 20 cycles 5.242 ns - 20 cycles 5.229 ns
128 - 155 cycles 38.976 ns - 27 cycles 6.886 ns - 27 cycles 6.892 ns
158 - 132 cycles 33.034 ns - 30 cycles 7.711 ns - 30 cycles 7.728 ns
250 - 130 cycles 32.612 ns - 38 cycles 9.560 ns - 38 cycles 9.549 ns
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
include/linux/slab.h | 8 ++++++++
mm/slab.c | 5 ++++-
mm/slab_common.c | 8 ++++++--
mm/slub.c | 22 +++++++++++++++++++---
4 files changed, 37 insertions(+), 6 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 2037a861e367..599b47f02b27 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -318,6 +318,14 @@ void kmem_cache_free(struct kmem_cache *, void *);
void kmem_cache_free_bulk(struct kmem_cache *, size_t, void **);
int kmem_cache_alloc_bulk(struct kmem_cache *, gfp_t, size_t, void **);
+static __always_inline void kfree_bulk(size_t size, void **p)
+{
+ /* Reusing call to kmem_cache_free_bulk() allow kfree_bulk to
+ * use same code icache
+ */
+ kmem_cache_free_bulk(NULL, size, p);
+}
+
#ifdef CONFIG_NUMA
void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_kmalloc_alignment;
void *kmem_cache_alloc_node(struct kmem_cache *, gfp_t flags, int node) __assume_slab_alignment;
diff --git a/mm/slab.c b/mm/slab.c
index 1358f86c0684..d4dc4836918f 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3587,7 +3587,10 @@ void kmem_cache_free_bulk(struct kmem_cache *orig_s, size_t size, void **p)
for (i = 0; i < size; i++) {
void *objp = p[i];
- s = cache_from_obj(orig_s, objp);
+ if (!orig_s) /* called via kfree_bulk */
+ s = virt_to_cache(objp);
+ else
+ s = cache_from_obj(orig_s, objp);
debug_check_no_locks_freed(objp, s->object_size);
if (!(s->flags & SLAB_DEBUG_OBJECTS))
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 3c6a86b4ec25..963c25589949 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -108,8 +108,12 @@ void __kmem_cache_free_bulk(struct kmem_cache *s, size_t nr, void **p)
{
size_t i;
- for (i = 0; i < nr; i++)
- kmem_cache_free(s, p[i]);
+ for (i = 0; i < nr; i++) {
+ if (s)
+ kmem_cache_free(s, p[i]);
+ else
+ kfree(p[i]);
+ }
}
int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
diff --git a/mm/slub.c b/mm/slub.c
index 3697f216d7c7..c33d2e1f011e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2786,23 +2786,39 @@ int build_detached_freelist(struct kmem_cache **s, size_t size,
size_t first_skipped_index = 0;
int lookahead = 3;
void *object;
+ struct page *page;
/* Always re-init detached_freelist */
df->page = NULL;
do {
object = p[--size];
+ /* Do we need !ZERO_OR_NULL_PTR(object) here? (for kfree) */
} while (!object && size);
if (!object)
return 0;
- /* Support for memcg, compiler can optimize this out */
- *s = cache_from_obj(*s, object);
+ page = virt_to_head_page(object);
+ if (!*s) {
+ /* Handle kalloc'ed objects */
+ if (unlikely(!PageSlab(page))) {
+ BUG_ON(!PageCompound(page));
+ kfree_hook(object);
+ __free_kmem_pages(page, compound_order(page));
+ p[size] = NULL; /* mark object processed */
+ return size;
+ }
+ /* Derive kmem_cache from object */
+ *s = page->slab_cache;
+ } else {
+ /* Support for memcg, compiler can optimize this out */
+ *s = cache_from_obj(*s, object);
+ }
/* Start new detached freelist */
+ df->page = page;
set_freepointer(*s, object, NULL);
- df->page = virt_to_head_page(object);
df->tail = object;
df->freelist = object;
p[size] = NULL; /* mark object processed */
--
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