[PATCH v2 1/2] mm, slub: change percpu partial accounting from objects to pages

From: Vlastimil Babka
Date: Tue Oct 12 2021 - 09:47:07 EST

With CONFIG_SLUB_CPU_PARTIAL enabled, SLUB keeps a percpu list of partial
slabs that can be promoted to cpu slab when the previous one is depleted,
without accessing the shared partial list. A slab can be added to this list
by 1) refill of an empty list from get_partial_node() - once we really have to
access the shared partial list, we acquire multiple slabs to amortize the cost
of locking, and 2) first free to a previously full slab - instead of putting
the slab on a shared partial list, we can more cheaply freeze it and put it on
the per-cpu list.

To control how large a percpu partial list can grow for a kmem cache,
set_cpu_partial() calculates a target number of free objects on each cpu's
percpu partial list, and this can be also set by the sysfs file cpu_partial.

However, the tracking of actual number of objects is imprecise, in order to
limit overhead from cpu X freeing an objects to a slab on percpu partial
list of cpu Y. Basically, the percpu partial slabs form a single linked list,
and when we add a new slab to the list with current head "oldpage", we set in
the struct page of the slab we're adding:

page->pages = oldpage->pages + 1; // this is precise
page->pobjects = oldpage->pobjects + (page->objects - page->inuse);
page->next = oldpage;

Thus the real number of free objects in the slab (objects - inuse) is only
determined at the moment of adding the slab to the percpu partial list, and
further freeing doesn't update the pobjects counter nor propagate it to the
current list head. As Jann reports [1], this can easily lead to large
inaccuracies, where the target number of objects (up to 30 by default) can
translate to the same number of (empty) slab pages on the list. In case 2)
above, we put a slab with 1 free object on the list, thus only increase
page->pobjects by 1, even if there are subsequent frees on the same slab. Jann
has noticed this in practice and so did we [2] when investigating significant
increase of kmemcg usage after switching from SLAB to SLUB.

While this is no longer a problem in kmemcg context thanks to the accounting
rewrite in 5.9, the memory waste is still not ideal and it's questionable
whether it makes sense to perform free object count based control when object
counts can easily become so much inaccurate. So this patch converts the
accounting to be based on number of pages only (which is precise) and removes
the page->pobjects field completely. This is also ultimately simpler.

To retain the existing set_cpu_partial() heuristic, first calculate the target
number of objects as previously, but then convert it to target number of pages
by assuming the pages will be half-filled on average. This assumption might
obviously also be inaccurate in practice, but cannot degrade to actual number of
pages being equal to the target number of objects.

We could also skip the intermediate step with target number of objects and
rewrite the heuristic in terms of pages. However we still have the sysfs file
cpu_partial which uses number of objects and could break existing users if it
suddenly becomes number of pages, so this patch doesn't do that.

In practice, after this patch the heuristics limit the size of percpu partial
list up to 2 pages. In case of a reported regression (which would mean some
workload has benefited from the previous imprecise object based counting), we
can tune the heuristics to get a better compromise within the new scheme, while
still avoid the unexpectedly long percpu partial lists.

[1] https://lore.kernel.org/linux-mm/CAG48ez2Qx5K1Cab-m8BdSibp6wLTip6ro4=-umR7BLsEgjEYzA@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/2f0f46e8-2535-410a-1859-e9cfa4e57c18@xxxxxxx/

==========
Evaluation
==========

Mel was kind enough to run v1 through mmtests machinery for netperf (localhost)
and hackbench and, for most significant results see below. So there are some
apparent regressions, especially with hackbench, which I think ultimately boils
down to having shorter percpu partial lists on average and some benchmarks
benefiting from longer ones. Monitoring slab usage also indicated less memory
usage by slab. Based on that, the following patch will bump the defaults to
allow longer percpu partial lists than after this patch.

However the goal is certainly not such that we would limit the percpu partial
lists to 30 pages just because previously a specific alloc/free pattern could
lead to the limit of 30 objects translate to a limit to 30 pages - that would
make little sense. This is a correctness patch, and if a workload benefits from
larger lists, the sysfs tuning knobs are still there to allow that.

Netperf

2-socket Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz (20 cores, 40 threads
per socket), 384GB RAM
TCP-RR:
hmean before 127045.79 after 121092.94 (-4.69%, worse)
stddev before 2634.37 after 1254.08
UDP-RR:
hmean before 166985.45 after 160668.94 ( -3.78%, worse)
stddev before 4059.69 after 1943.63

2-socket Intel(R) Xeon(R) CPU E5-2698 v4 @ 2.20GHz (20 cores, 40 threads
per socket), 512GB RAM
TCP-RR:
hmean before 84173.25 after 76914.72 ( -8.62%, worse)
UDP-RR:
hmean before 93571.12 after 96428.69 ( 3.05%, better)
stddev before 23118.54 after 16828.14

2-socket Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz (12 cores, 24 threads
per socket), 64GB RAM
TCP-RR:
hmean before 49984.92 after 48922.27 ( -2.13%, worse)
stddev before 6248.15 after 4740.51
UDP-RR:
hmean before 61854.31 after 68761.81 ( 11.17%, better)
stddev before 4093.54 after 5898.91

other machines - within 2%

Hackbench

(results before and after the patch, negative % means worse)

2-socket AMD EPYC 7713 (64 cores, 128 threads per core), 256GB RAM
hackbench-process-sockets
Amean 1 0.5380 0.5583 ( -3.78%)
Amean 4 0.7510 0.8150 ( -8.52%)
Amean 7 0.7930 0.9533 ( -20.22%)
Amean 12 0.7853 1.1313 ( -44.06%)
Amean 21 1.1520 1.4993 ( -30.15%)
Amean 30 1.6223 1.9237 ( -18.57%)
Amean 48 2.6767 2.9903 ( -11.72%)
Amean 79 4.0257 5.1150 ( -27.06%)
Amean 110 5.5193 7.4720 ( -35.38%)
Amean 141 7.2207 9.9840 ( -38.27%)
Amean 172 8.4770 12.1963 ( -43.88%)
Amean 203 9.6473 14.3137 ( -48.37%)
Amean 234 11.3960 18.7917 ( -64.90%)
Amean 265 13.9627 22.4607 ( -60.86%)
Amean 296 14.9163 26.0483 ( -74.63%)

hackbench-thread-sockets
Amean 1 0.5597 0.5877 ( -5.00%)
Amean 4 0.7913 0.8960 ( -13.23%)
Amean 7 0.8190 1.0017 ( -22.30%)
Amean 12 0.9560 1.1727 ( -22.66%)
Amean 21 1.7587 1.5660 ( 10.96%)
Amean 30 2.4477 1.9807 ( 19.08%)
Amean 48 3.4573 3.0630 ( 11.41%)
Amean 79 4.7903 5.1733 ( -8.00%)
Amean 110 6.1370 7.4220 ( -20.94%)
Amean 141 7.5777 9.2617 ( -22.22%)
Amean 172 9.2280 11.0907 ( -20.18%)
Amean 203 10.2793 13.3470 ( -29.84%)
Amean 234 11.2410 17.1070 ( -52.18%)
Amean 265 12.5970 23.3323 ( -85.22%)
Amean 296 17.1540 24.2857 ( -41.57%)

2-socket Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz (20 cores, 40 threads
per socket), 384GB RAM
hackbench-process-sockets
Amean 1 0.5760 0.4793 ( 16.78%)
Amean 4 0.9430 0.9707 ( -2.93%)
Amean 7 1.5517 1.8843 ( -21.44%)
Amean 12 2.4903 2.7267 ( -9.49%)
Amean 21 3.9560 4.2877 ( -8.38%)
Amean 30 5.4613 5.8343 ( -6.83%)
Amean 48 8.5337 9.2937 ( -8.91%)
Amean 79 14.0670 15.2630 ( -8.50%)
Amean 110 19.2253 21.2467 ( -10.51%)
Amean 141 23.7557 25.8550 ( -8.84%)
Amean 172 28.4407 29.7603 ( -4.64%)
Amean 203 33.3407 33.9927 ( -1.96%)
Amean 234 38.3633 39.1150 ( -1.96%)
Amean 265 43.4420 43.8470 ( -0.93%)
Amean 296 48.3680 48.9300 ( -1.16%)

hackbench-thread-sockets
Amean 1 0.6080 0.6493 ( -6.80%)
Amean 4 1.0000 1.0513 ( -5.13%)
Amean 7 1.6607 2.0260 ( -22.00%)
Amean 12 2.7637 2.9273 ( -5.92%)
Amean 21 5.0613 4.5153 ( 10.79%)
Amean 30 6.3340 6.1140 ( 3.47%)
Amean 48 9.0567 9.5577 ( -5.53%)
Amean 79 14.5657 15.7983 ( -8.46%)
Amean 110 19.6213 21.6333 ( -10.25%)
Amean 141 24.1563 26.2697 ( -8.75%)
Amean 172 28.9687 30.2187 ( -4.32%)
Amean 203 33.9763 34.6970 ( -2.12%)
Amean 234 38.8647 39.3207 ( -1.17%)
Amean 265 44.0813 44.1507 ( -0.16%)
Amean 296 49.2040 49.4330 ( -0.47%)

2-socket Intel(R) Xeon(R) CPU E5-2698 v4 @ 2.20GHz (20 cores, 40 threads
per socket), 512GB RAM
hackbench-process-sockets
Amean 1 0.5027 0.5017 ( 0.20%)
Amean 4 1.1053 1.2033 ( -8.87%)
Amean 7 1.8760 2.1820 ( -16.31%)
Amean 12 2.9053 3.1810 ( -9.49%)
Amean 21 4.6777 4.9920 ( -6.72%)
Amean 30 6.5180 6.7827 ( -4.06%)
Amean 48 10.0710 10.5227 ( -4.48%)
Amean 79 16.4250 17.5053 ( -6.58%)
Amean 110 22.6203 24.4617 ( -8.14%)
Amean 141 28.0967 31.0363 ( -10.46%)
Amean 172 34.4030 36.9233 ( -7.33%)
Amean 203 40.5933 43.0850 ( -6.14%)
Amean 234 46.6477 48.7220 ( -4.45%)
Amean 265 53.0530 53.9597 ( -1.71%)
Amean 296 59.2760 59.9213 ( -1.09%)

hackbench-thread-sockets
Amean 1 0.5363 0.5330 ( 0.62%)
Amean 4 1.1647 1.2157 ( -4.38%)
Amean 7 1.9237 2.2833 ( -18.70%)
Amean 12 2.9943 3.3110 ( -10.58%)
Amean 21 4.9987 5.1880 ( -3.79%)
Amean 30 6.7583 7.0043 ( -3.64%)
Amean 48 10.4547 10.8353 ( -3.64%)
Amean 79 16.6707 17.6790 ( -6.05%)
Amean 110 22.8207 24.4403 ( -7.10%)
Amean 141 28.7090 31.0533 ( -8.17%)
Amean 172 34.9387 36.8260 ( -5.40%)
Amean 203 41.1567 43.0450 ( -4.59%)
Amean 234 47.3790 48.5307 ( -2.43%)
Amean 265 53.9543 54.6987 ( -1.38%)
Amean 296 60.0820 60.2163 ( -0.22%)

1-socket Intel(R) Xeon(R) CPU E3-1240 v5 @ 3.50GHz (4 cores, 8 threads),
32 GB RAM
hackbench-process-sockets
Amean 1 1.4760 1.5773 ( -6.87%)
Amean 3 3.9370 4.0910 ( -3.91%)
Amean 5 6.6797 6.9357 ( -3.83%)
Amean 7 9.3367 9.7150 ( -4.05%)
Amean 12 15.7627 16.1400 ( -2.39%)
Amean 18 23.5360 23.6890 ( -0.65%)
Amean 24 31.0663 31.3137 ( -0.80%)
Amean 30 38.7283 39.0037 ( -0.71%)
Amean 32 41.3417 41.6097 ( -0.65%)

hackbench-thread-sockets
Amean 1 1.5250 1.6043 ( -5.20%)
Amean 3 4.0897 4.2603 ( -4.17%)
Amean 5 6.7760 7.0933 ( -4.68%)
Amean 7 9.4817 9.9157 ( -4.58%)
Amean 12 15.9610 16.3937 ( -2.71%)
Amean 18 23.9543 24.3417 ( -1.62%)
Amean 24 31.4400 31.7217 ( -0.90%)
Amean 30 39.2457 39.5467 ( -0.77%)
Amean 32 41.8267 42.1230 ( -0.71%)

2-socket Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz (12 cores, 24 threads
per socket), 64GB RAM
hackbench-process-sockets
Amean 1 1.0347 1.0880 ( -5.15%)
Amean 4 1.7267 1.8527 ( -7.30%)
Amean 7 2.6707 2.8110 ( -5.25%)
Amean 12 4.1617 4.3383 ( -4.25%)
Amean 21 7.0070 7.2600 ( -3.61%)
Amean 30 9.9187 10.2397 ( -3.24%)
Amean 48 15.6710 16.3923 ( -4.60%)
Amean 79 24.7743 26.1247 ( -5.45%)
Amean 110 34.3000 35.9307 ( -4.75%)
Amean 141 44.2043 44.8010 ( -1.35%)
Amean 172 54.2430 54.7260 ( -0.89%)
Amean 192 60.6557 60.9777 ( -0.53%)

hackbench-thread-sockets
Amean 1 1.0610 1.1353 ( -7.01%)
Amean 4 1.7543 1.9140 ( -9.10%)
Amean 7 2.7840 2.9573 ( -6.23%)
Amean 12 4.3813 4.4937 ( -2.56%)
Amean 21 7.3460 7.5350 ( -2.57%)
Amean 30 10.2313 10.5190 ( -2.81%)
Amean 48 15.9700 16.5940 ( -3.91%)
Amean 79 25.3973 26.6637 ( -4.99%)
Amean 110 35.1087 36.4797 ( -3.91%)
Amean 141 45.8220 46.3053 ( -1.05%)
Amean 172 55.4917 55.7320 ( -0.43%)
Amean 192 62.7490 62.5410 ( 0.33%)

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
Changes in v2:
- added evaluation results to changelog
- added patch 2 bumping the defaults
include/linux/mm_types.h | 2 -
include/linux/slub_def.h | 13 +-----
mm/slub.c | 89 ++++++++++++++++++++++++++--------------
3 files changed, 61 insertions(+), 43 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7f8ee09c711f..68ffa064b7a8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -124,10 +124,8 @@ struct page {
struct page *next;
#ifdef CONFIG_64BIT
int pages; /* Nr of pages left */
- int pobjects; /* Approximate count */
#else
short int pages;
- short int pobjects;
#endif
};
};
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 85499f0586b0..0fa751b946fa 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -99,6 +99,8 @@ struct kmem_cache {
#ifdef CONFIG_SLUB_CPU_PARTIAL
/* Number of per cpu partial objects to keep around */
unsigned int cpu_partial;
+ /* Number of per cpu partial pages to keep around */
+ unsigned int cpu_partial_pages;
#endif
struct kmem_cache_order_objects oo;

@@ -141,17 +143,6 @@ struct kmem_cache {
struct kmem_cache_node *node[MAX_NUMNODES];
};

-#ifdef CONFIG_SLUB_CPU_PARTIAL
-#define slub_cpu_partial(s) ((s)->cpu_partial)
-#define slub_set_cpu_partial(s, n) \
-({ \
- slub_cpu_partial(s) = (n); \
-})
-#else
-#define slub_cpu_partial(s) (0)
-#define slub_set_cpu_partial(s, n)
-#endif /* CONFIG_SLUB_CPU_PARTIAL */
-
#ifdef CONFIG_SYSFS
#define SLAB_SUPPORTS_SYSFS
void sysfs_slab_unlink(struct kmem_cache *);
diff --git a/mm/slub.c b/mm/slub.c
index 3d2025f7163b..3757f31c5d97 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -414,6 +414,29 @@ static inline unsigned int oo_objects(struct kmem_cache_order_objects x)
return x.x & OO_MASK;
}

+#ifdef CONFIG_SLUB_CPU_PARTIAL
+static void slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
+{
+ unsigned int nr_pages;
+
+ s->cpu_partial = nr_objects;
+
+ /*
+ * We take the number of objects but actually limit the number of
+ * pages on the per cpu partial list, in order to limit excessive
+ * growth of the list. For simplicity we assume that the pages will
+ * be half-full.
+ */
+ nr_pages = DIV_ROUND_UP(nr_objects * 2, oo_objects(s->oo));
+ s->cpu_partial_pages = nr_pages;
+}
+#else
+static inline void
+slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
+{
+}
+#endif /* CONFIG_SLUB_CPU_PARTIAL */
+
/*
* Per slab locking using the pagelock
*/
@@ -2045,7 +2068,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
*/
static inline void *acquire_slab(struct kmem_cache *s,
struct kmem_cache_node *n, struct page *page,
- int mode, int *objects)
+ int mode)
{
void *freelist;
unsigned long counters;
@@ -2061,7 +2084,6 @@ static inline void *acquire_slab(struct kmem_cache *s,
freelist = page->freelist;
counters = page->counters;
new.counters = counters;
- *objects = new.objects - new.inuse;
if (mode) {
new.inuse = page->objects;
new.freelist = NULL;
@@ -2099,9 +2121,8 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
{
struct page *page, *page2;
void *object = NULL;
- unsigned int available = 0;
unsigned long flags;
- int objects;
+ unsigned int partial_pages = 0;

/*
* Racy check. If we mistakenly see no partial slabs then we
@@ -2119,11 +2140,10 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
if (!pfmemalloc_match(page, gfpflags))
continue;

- t = acquire_slab(s, n, page, object == NULL, &objects);
+ t = acquire_slab(s, n, page, object == NULL);
if (!t)
break;

- available += objects;
if (!object) {
*ret_page = page;
stat(s, ALLOC_FROM_PARTIAL);
@@ -2131,10 +2151,15 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
} else {
put_cpu_partial(s, page, 0);
stat(s, CPU_PARTIAL_NODE);
+ partial_pages++;
}
+#ifdef CONFIG_SLUB_CPU_PARTIAL
if (!kmem_cache_has_cpu_partial(s)
- || available > slub_cpu_partial(s) / 2)
+ || partial_pages > s->cpu_partial_pages / 2)
break;
+#else
+ break;
+#endif

}
spin_unlock_irqrestore(&n->list_lock, flags);
@@ -2539,14 +2564,13 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
struct page *page_to_unfreeze = NULL;
unsigned long flags;
int pages = 0;
- int pobjects = 0;

local_lock_irqsave(&s->cpu_slab->lock, flags);

oldpage = this_cpu_read(s->cpu_slab->partial);

if (oldpage) {
- if (drain && oldpage->pobjects > slub_cpu_partial(s)) {
+ if (drain && oldpage->pages >= s->cpu_partial_pages) {
/*
* Partial array is full. Move the existing set to the
* per node partial list. Postpone the actual unfreezing
@@ -2555,16 +2579,13 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
page_to_unfreeze = oldpage;
oldpage = NULL;
} else {
- pobjects = oldpage->pobjects;
pages = oldpage->pages;
}
}

pages++;
- pobjects += page->objects - page->inuse;

page->pages = pages;
- page->pobjects = pobjects;
page->next = oldpage;

this_cpu_write(s->cpu_slab->partial, page);
@@ -3980,6 +4001,8 @@ static void set_min_partial(struct kmem_cache *s, unsigned long min)
static void set_cpu_partial(struct kmem_cache *s)
{
#ifdef CONFIG_SLUB_CPU_PARTIAL
+ unsigned int nr_objects;
+
/*
* cpu_partial determined the maximum number of objects kept in the
* per cpu partial lists of a processor.
@@ -3989,24 +4012,22 @@ static void set_cpu_partial(struct kmem_cache *s)
* filled up again with minimal effort. The slab will never hit the
* per node partial lists and therefore no locking will be required.
*
- * This setting also determines
- *
- * A) The number of objects from per cpu partial slabs dumped to the
- * per node list when we reach the limit.
- * B) The number of objects in cpu partial slabs to extract from the
- * per node list when we run out of per cpu objects. We only fetch
- * 50% to keep some capacity around for frees.
+ * For backwards compatibility reasons, this is determined as number
+ * of objects, even though we now limit maximum number of pages, see
+ * slub_set_cpu_partial()
*/
if (!kmem_cache_has_cpu_partial(s))
- slub_set_cpu_partial(s, 0);
+ nr_objects = 0;
else if (s->size >= PAGE_SIZE)
- slub_set_cpu_partial(s, 2);
+ nr_objects = 2;
else if (s->size >= 1024)
- slub_set_cpu_partial(s, 6);
+ nr_objects = 6;
else if (s->size >= 256)
- slub_set_cpu_partial(s, 13);
+ nr_objects = 13;
else
- slub_set_cpu_partial(s, 30);
+ nr_objects = 30;
+
+ slub_set_cpu_partial(s, nr_objects);
#endif
}

@@ -5379,7 +5400,12 @@ SLAB_ATTR(min_partial);

static ssize_t cpu_partial_show(struct kmem_cache *s, char *buf)
{
- return sysfs_emit(buf, "%u\n", slub_cpu_partial(s));
+ unsigned int nr_partial = 0;
+#ifdef CONFIG_SLUB_CPU_PARTIAL
+ nr_partial = s->cpu_partial;
+#endif
+
+ return sysfs_emit(buf, "%u\n", nr_partial);
}

static ssize_t cpu_partial_store(struct kmem_cache *s, const char *buf,
@@ -5450,12 +5476,12 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)

page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));

- if (page) {
+ if (page)
pages += page->pages;
- objects += page->pobjects;
- }
}

+ /* Approximate half-full pages , see slub_set_cpu_partial() */
+ objects = (pages * oo_objects(s->oo)) / 2;
len += sysfs_emit_at(buf, len, "%d(%d)", objects, pages);

#ifdef CONFIG_SMP
@@ -5463,9 +5489,12 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
struct page *page;

page = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
- if (page)
+ if (page) {
+ pages = READ_ONCE(page->pages);
+ objects = (pages * oo_objects(s->oo)) / 2;
len += sysfs_emit_at(buf, len, " C%d=%d(%d)",
- cpu, page->pobjects, page->pages);
+ cpu, objects, pages);
+ }
}
#endif
len += sysfs_emit_at(buf, len, "\n");
--
2.33.0