[RFC v2 0/7] SLUB percpu array caches and maple tree nodes

[RFC v2 0/7] SLUB percpu array caches and maple tree nodes

[Vlastimil Babka]

Also in git [1]. Changes since v1 [2]:

- fix a few bugs
- SLAB marked as BROKEN so bots dont complain about missing functions
- incorporate Liam's patches, which allows getting rid of preallocations
  in mas_prealloc() completely. This has reduced the allocation stats
  further, with the whole series.

More notes wrt v1 RFC feedback:

- locking is still done as in v1, as it allows remote draining, which
  should be added before this is suitable for merging
- there's currently no bulk freeing/refill of the percpu array, which
  will eventually be added, but I expect most perf gain for the maple
  tree use case to come from the avoided preallocations anyway

----

At LSF/MM I've mentioned that I see several use cases for introducing
opt-in percpu arrays for caching alloc/free objects in SLUB. This is my
first exploration of this idea, speficially for the use case of maple
tree nodes. We have brainstormed this use case on IRC last week with
Liam and Matthew and this how I understood the requirements:

- percpu arrays will be faster thank bulk alloc/free which needs
  relatively long freelists to work well. Especially in the freeing case
  we need the nodes to come from the same slab (or small set of those)

- preallocation for the worst case of needed nodes for a tree operation
  that can't reclaim due to locks is wasteful. We could instead expect
  that most of the time percpu arrays would satisfy the constained
  allocations, and in the rare cases it does not we can dip into
  GFP_ATOMIC reserves temporarily. Instead of preallocation just prefill
  the arrays.

- NUMA locality is not a concern as the nodes of a process's VMA tree
  end up all over the place anyway.

So this RFC patchset adds such percpu array in Patch 2. Locking is
stolen from Mel's recent page allocator's pcplists implementation so it
can avoid disabling IRQs and just disable preemption, but the trylocks
can fail in rare situations.

Then maple tree is modified in patches 3-6 to benefit from this. This is
done in a very crude way as I'm not so familiar with the code.

I've briefly tested this with virtme VM boot and checking the stats from
CONFIG_SLUB_STATS in sysfs.

Patch 2:

slub changes implemented including new counters alloc_cpu_cache
and free_cpu_cache but maple tree doesn't use them yet

(none):/sys/kernel/slab/maple_node # grep . alloc_cpu_cache alloc_*path free_cpu_cache free_*path | cut -d' ' -f1
alloc_cpu_cache:0
alloc_fastpath:54842
alloc_slowpath:8142
free_cpu_cache:0
free_fastpath:32336
free_slowpath:23484

Patch 3:

maple node cache creates percpu array with 32 entries,
not changed anything else

-> some allocs/free satisfied by the array

alloc_cpu_cache:11956
alloc_fastpath:40675
alloc_slowpath:7308
free_cpu_cache:12082
free_fastpath:23617
free_slowpath:17956

Patch 4:

maple tree nodes bulk alloc/free converted to loop of normal alloc to use
percpu array more, because bulk alloc bypasses it

-> majority alloc/free now satisfied by percpu array

alloc_cpu_cache:54673
alloc_fastpath:4491
alloc_slowpath:737
free_cpu_cache:54759
free_fastpath:332
free_slowpath:4723

Patch 5+6:

mas_preallocate() just prefills the percpu array, doesn't preallocate anything
mas_store_prealloc() gains a retry loop with mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)

-> major drop of alloc/free
(the prefills are included in the accounting)

alloc_cpu_cache:15036
alloc_fastpath:4651
alloc_slowpath:656
free_cpu_cache:15102
free_fastpath:299
free_slowpath:4835

It would be interesting to see how it affects the workloads that saw
regressions from the maple tree introduction, as the slab operations
were suspected to be a major factor and now they should be both reduced
and made cheaper.

Liam R. Howlett (2):
  maple_tree: Remove MA_STATE_PREALLOC
  tools: Add SLUB percpu array functions for testing

Vlastimil Babka (5):
  mm, slub: fix bulk alloc and free stats
  mm, slub: add opt-in slub_percpu_array
  maple_tree: use slub percpu array
  maple_tree: avoid bulk alloc/free to use percpu array more
  maple_tree: replace preallocation with slub percpu array prefill

 include/linux/slab.h                    |   4 +
 include/linux/slub_def.h                |  10 ++
 lib/maple_tree.c                        |  60 ++++---
 mm/Kconfig                              |   1 +
 mm/slub.c                               | 221 +++++++++++++++++++++++-
 tools/include/linux/slab.h              |   4 +
 tools/testing/radix-tree/linux.c        |  14 ++
 tools/testing/radix-tree/linux/kernel.h |   1 +
 8 files changed, 286 insertions(+), 29 deletions(-)

-- 
2.41.0

[RFC v2 1/7] mm, slub: fix bulk alloc and free stats

[Vlastimil Babka]

The SLUB sysfs stats enabled CONFIG_SLUB_STATS have two deficiencies
identified wrt bulk alloc/free operations:

- Bulk allocations from cpu freelist are not counted. Add the
  ALLOC_FASTPATH counter there.

- Bulk fastpath freeing will count a list of multiple objects with a
  single FREE_FASTPATH inc. Add a stat_add() variant to count them all.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index e3b5d5c0eb3a..a9437d48840c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -341,6 +341,14 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
 #endif
 }
 
+static inline void stat_add(const struct kmem_cache *s, enum stat_item si, int v)
+{
+#ifdef CONFIG_SLUB_STATS
+	raw_cpu_add(s->cpu_slab->stat[si], v);
+#endif
+}
+
+
 /*
  * Tracks for which NUMA nodes we have kmem_cache_nodes allocated.
  * Corresponds to node_state[N_NORMAL_MEMORY], but can temporarily
@@ -3776,7 +3784,7 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 
 		local_unlock(&s->cpu_slab->lock);
 	}
-	stat(s, FREE_FASTPATH);
+	stat_add(s, FREE_FASTPATH, cnt);
 }
 #else /* CONFIG_SLUB_TINY */
 static void do_slab_free(struct kmem_cache *s,
@@ -3978,6 +3986,7 @@ static inline int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
 		c->freelist = get_freepointer(s, object);
 		p[i] = object;
 		maybe_wipe_obj_freeptr(s, p[i]);
+		stat(s, ALLOC_FASTPATH);
 	}
 	c->tid = next_tid(c->tid);
 	local_unlock_irqrestore(&s->cpu_slab->lock, irqflags);
-- 
2.41.0

[RFC v2 2/7] mm, slub: add opt-in slub_percpu_array

[Vlastimil Babka]

kmem_cache_setup_percpu_array() will allocate a per-cpu array for
caching alloc/free objects of given size for the cache. The cache
has to be created with SLAB_NO_MERGE flag.

The array is filled by freeing. When empty for alloc or full for
freeing, it's simply bypassed by the operation, there's currently no
batch freeing/allocations.

The locking is copied from the page allocator's pcplists, based on
embedded spin locks. Interrupts are not disabled, only preemption (cpu
migration on RT). Trylock is attempted to avoid deadlock due to
an intnerrupt, trylock failure means the array is bypassed.

Sysfs stat counters alloc_cpu_cache and free_cpu_cache count operations
that used the percpu array.

Bulk allocation bypasses the array, bulk freeing does not.

kmem_cache_prefill_percpu_array() can be called to ensure the array on
the current cpu to at least the given number of objects. However this is
only opportunistic as there's no cpu pinning and the trylocks may always
fail. Therefore allocations cannot rely on the array for success even
after the prefill. But misses should be rare enough that e.g. GFP_ATOMIC
allocations should be acceptable after the refill.
The operation is currently not optimized.

Mark SLAB_DEPRECATED as BROKEN so the new APIs don't need to be
reimplemented there and the bots don't complain. SLAB has percpu arrays
by design but their sizes are determined internally.

More TODO/FIXMEs:

- NUMA awareness - preferred node currently ignored, __GFP_THISNODE not
  honored
- slub_debug - will not work for allocations from the array. Normally in
  SLUB implementation the slub_debug kills all fast paths, but that
  could lead to depleting the reserves if we ignore the prefill and use
  GFP_ATOMIC. Needs more thought.
---
 include/linux/slab.h     |   4 +
 include/linux/slub_def.h |  10 ++
 mm/Kconfig               |   1 +
 mm/slub.c                | 210 ++++++++++++++++++++++++++++++++++++++-
 4 files changed, 224 insertions(+), 1 deletion(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 848c7c82ad5a..f6c91cbc1544 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -196,6 +196,8 @@ struct kmem_cache *kmem_cache_create_usercopy(const char *name,
 void kmem_cache_destroy(struct kmem_cache *s);
 int kmem_cache_shrink(struct kmem_cache *s);
 
+int kmem_cache_setup_percpu_array(struct kmem_cache *s, unsigned int count);
+
 /*
  * Please use this macro to create slab caches. Simply specify the
  * name of the structure and maybe some flags that are listed above.
@@ -494,6 +496,8 @@ void kmem_cache_free(struct kmem_cache *s, void *objp);
 void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);
 int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size, void **p);
 
+int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count, gfp_t gfp);
+
 static __always_inline void kfree_bulk(size_t size, void **p)
 {
 	kmem_cache_free_bulk(NULL, size, p);
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index deb90cf4bffb..c85434668419 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -13,8 +13,10 @@
 #include <linux/local_lock.h>
 
 enum stat_item {
+	ALLOC_PERCPU_CACHE,	/* Allocation from percpu array cache */
 	ALLOC_FASTPATH,		/* Allocation from cpu slab */
 	ALLOC_SLOWPATH,		/* Allocation by getting a new cpu slab */
+	FREE_PERCPU_CACHE,	/* Free to percpu array cache */
 	FREE_FASTPATH,		/* Free to cpu slab */
 	FREE_SLOWPATH,		/* Freeing not to cpu slab */
 	FREE_FROZEN,		/* Freeing to frozen slab */
@@ -66,6 +68,13 @@ struct kmem_cache_cpu {
 };
 #endif /* CONFIG_SLUB_TINY */
 
+struct slub_percpu_array {
+	spinlock_t lock;
+	unsigned int count;
+	unsigned int used;
+	void * objects[];
+};
+
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 #define slub_percpu_partial(c)		((c)->partial)
 
@@ -99,6 +108,7 @@ struct kmem_cache {
 #ifndef CONFIG_SLUB_TINY
 	struct kmem_cache_cpu __percpu *cpu_slab;
 #endif
+	struct slub_percpu_array __percpu *cpu_array;
 	/* Used for retrieving partial slabs, etc. */
 	slab_flags_t flags;
 	unsigned long min_partial;
diff --git a/mm/Kconfig b/mm/Kconfig
index 09130434e30d..84f4dff70d39 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -237,6 +237,7 @@ choice
 config SLAB_DEPRECATED
 	bool "SLAB (DEPRECATED)"
 	depends on !PREEMPT_RT
+	depends on BROKEN
 	help
 	  Deprecated and scheduled for removal in a few cycles. Replaced by
 	  SLUB.
diff --git a/mm/slub.c b/mm/slub.c
index a9437d48840c..f41c69bac07d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -188,6 +188,79 @@ do {					\
 #define USE_LOCKLESS_FAST_PATH()	(false)
 #endif
 
+/* copy/pasted  from mm/page_alloc.c */
+
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
+/*
+ * On SMP, spin_trylock is sufficient protection.
+ * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
+ */
+#define pcp_trylock_prepare(flags)	do { } while (0)
+#define pcp_trylock_finish(flag)	do { } while (0)
+#else
+
+/* UP spin_trylock always succeeds so disable IRQs to prevent re-entrancy. */
+#define pcp_trylock_prepare(flags)	local_irq_save(flags)
+#define pcp_trylock_finish(flags)	local_irq_restore(flags)
+#endif
+
+/*
+ * Locking a pcp requires a PCP lookup followed by a spinlock. To avoid
+ * a migration causing the wrong PCP to be locked and remote memory being
+ * potentially allocated, pin the task to the CPU for the lookup+lock.
+ * preempt_disable is used on !RT because it is faster than migrate_disable.
+ * migrate_disable is used on RT because otherwise RT spinlock usage is
+ * interfered with and a high priority task cannot preempt the allocator.
+ */
+#ifndef CONFIG_PREEMPT_RT
+#define pcpu_task_pin()		preempt_disable()
+#define pcpu_task_unpin()	preempt_enable()
+#else
+#define pcpu_task_pin()		migrate_disable()
+#define pcpu_task_unpin()	migrate_enable()
+#endif
+
+/*
+ * Generic helper to lookup and a per-cpu variable with an embedded spinlock.
+ * Return value should be used with equivalent unlock helper.
+ */
+#define pcpu_spin_lock(type, member, ptr)				\
+({									\
+	type *_ret;							\
+	pcpu_task_pin();						\
+	_ret = this_cpu_ptr(ptr);					\
+	spin_lock(&_ret->member);					\
+	_ret;								\
+})
+
+#define pcpu_spin_trylock(type, member, ptr)				\
+({									\
+	type *_ret;							\
+	pcpu_task_pin();						\
+	_ret = this_cpu_ptr(ptr);					\
+	if (!spin_trylock(&_ret->member)) {				\
+		pcpu_task_unpin();					\
+		_ret = NULL;						\
+	}								\
+	_ret;								\
+})
+
+#define pcpu_spin_unlock(member, ptr)					\
+({									\
+	spin_unlock(&ptr->member);					\
+	pcpu_task_unpin();						\
+})
+
+/* struct slub_percpu_array specific helpers. */
+#define pca_spin_lock(ptr)						\
+	pcpu_spin_lock(struct slub_percpu_array, lock, ptr)
+
+#define pca_spin_trylock(ptr)						\
+	pcpu_spin_trylock(struct slub_percpu_array, lock, ptr)
+
+#define pca_spin_unlock(ptr)						\
+	pcpu_spin_unlock(lock, ptr)
+
 #ifndef CONFIG_SLUB_TINY
 #define __fastpath_inline __always_inline
 #else
@@ -3440,6 +3513,32 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
 			0, sizeof(void *));
 }
 
+static inline void *alloc_from_pca(struct kmem_cache *s)
+{
+	unsigned long __maybe_unused UP_flags;
+	struct slub_percpu_array *pca;
+	void *object = NULL;
+
+	pcp_trylock_prepare(UP_flags);
+	pca = pca_spin_trylock(s->cpu_array);
+
+	if (unlikely(!pca))
+		goto failed;
+
+	if (likely(pca->used > 0)) {
+		object = pca->objects[--pca->used];
+		pca_spin_unlock(pca);
+		pcp_trylock_finish(UP_flags);
+		stat(s, ALLOC_PERCPU_CACHE);
+		return object;
+	}
+	pca_spin_unlock(pca);
+
+failed:
+	pcp_trylock_finish(UP_flags);
+	return NULL;
+}
+
 /*
  * Inlined fastpath so that allocation functions (kmalloc, kmem_cache_alloc)
  * have the fastpath folded into their functions. So no function call
@@ -3465,7 +3564,11 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
 	if (unlikely(object))
 		goto out;
 
-	object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
+	if (s->cpu_array)
+		object = alloc_from_pca(s);
+
+	if (!object)
+		object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
 
 	maybe_wipe_obj_freeptr(s, object);
 	init = slab_want_init_on_alloc(gfpflags, s);
@@ -3715,6 +3818,34 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
 	discard_slab(s, slab);
 }
 
+static inline bool free_to_pca(struct kmem_cache *s, void *object)
+{
+	unsigned long __maybe_unused UP_flags;
+	struct slub_percpu_array *pca;
+	bool ret = false;
+
+	pcp_trylock_prepare(UP_flags);
+	pca = pca_spin_trylock(s->cpu_array);
+
+	if (!pca) {
+		pcp_trylock_finish(UP_flags);
+		return false;
+	}
+
+	if (pca->used < pca->count) {
+		pca->objects[pca->used++] = object;
+		ret = true;
+	}
+
+	pca_spin_unlock(pca);
+	pcp_trylock_finish(UP_flags);
+
+	if (ret)
+		stat(s, FREE_PERCPU_CACHE);
+
+	return ret;
+}
+
 #ifndef CONFIG_SLUB_TINY
 /*
  * Fastpath with forced inlining to produce a kfree and kmem_cache_free that
@@ -3740,6 +3871,11 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
 	unsigned long tid;
 	void **freelist;
 
+	if (s->cpu_array && cnt == 1) {
+		if (free_to_pca(s, head))
+			return;
+	}
+
 redo:
 	/*
 	 * Determine the currently cpus per cpu slab.
@@ -3793,6 +3929,11 @@ static void do_slab_free(struct kmem_cache *s,
 {
 	void *tail_obj = tail ? : head;
 
+	if (s->cpu_array && cnt == 1) {
+		if (free_to_pca(s, head))
+			return;
+	}
+
 	__slab_free(s, slab, head, tail_obj, cnt, addr);
 }
 #endif /* CONFIG_SLUB_TINY */
@@ -4060,6 +4201,45 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 }
 EXPORT_SYMBOL(kmem_cache_alloc_bulk);
 
+int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
+		gfp_t gfp)
+{
+	struct slub_percpu_array *pca;
+	void *objects[32];
+	unsigned int used;
+	unsigned int allocated;
+
+	if (!s->cpu_array)
+		return -EINVAL;
+
+	/* racy but we don't care */
+	pca = raw_cpu_ptr(s->cpu_array);
+
+	used = READ_ONCE(pca->used);
+
+	if (used >= count)
+		return 0;
+
+	if (pca->count < count)
+		return -EINVAL;
+
+	count -= used;
+
+	/* TODO fix later */
+	if (count > 32)
+		count = 32;
+
+	for (int i = 0; i < count; i++)
+		objects[i] = NULL;
+	allocated = kmem_cache_alloc_bulk(s, gfp, count, &objects[0]);
+
+	for (int i = 0; i < count; i++) {
+		if (objects[i]) {
+			kmem_cache_free(s, objects[i]);
+		}
+	}
+	return allocated;
+}
 
 /*
  * Object placement in a slab is made very easy because we always start at
@@ -5131,6 +5311,30 @@ int __kmem_cache_create(struct kmem_cache *s, slab_flags_t flags)
 	return 0;
 }
 
+int kmem_cache_setup_percpu_array(struct kmem_cache *s, unsigned int count)
+{
+	int cpu;
+
+	if (WARN_ON_ONCE(!(s->flags & SLAB_NO_MERGE)))
+		return -EINVAL;
+
+	s->cpu_array = __alloc_percpu(struct_size(s->cpu_array, objects, count),
+					sizeof(void *));
+
+	if (!s->cpu_array)
+		return -ENOMEM;
+
+	for_each_possible_cpu(cpu) {
+		struct slub_percpu_array *pca = per_cpu_ptr(s->cpu_array, cpu);
+
+		spin_lock_init(&pca->lock);
+		pca->count = count;
+		pca->used = 0;
+	}
+
+	return 0;
+}
+
 #ifdef SLAB_SUPPORTS_SYSFS
 static int count_inuse(struct slab *slab)
 {
@@ -5908,8 +6112,10 @@ static ssize_t text##_store(struct kmem_cache *s,		\
 }								\
 SLAB_ATTR(text);						\
 
+STAT_ATTR(ALLOC_PERCPU_CACHE, alloc_cpu_cache);
 STAT_ATTR(ALLOC_FASTPATH, alloc_fastpath);
 STAT_ATTR(ALLOC_SLOWPATH, alloc_slowpath);
+STAT_ATTR(FREE_PERCPU_CACHE, free_cpu_cache);
 STAT_ATTR(FREE_FASTPATH, free_fastpath);
 STAT_ATTR(FREE_SLOWPATH, free_slowpath);
 STAT_ATTR(FREE_FROZEN, free_frozen);
@@ -5995,8 +6201,10 @@ static struct attribute *slab_attrs[] = {
 	&remote_node_defrag_ratio_attr.attr,
 #endif
 #ifdef CONFIG_SLUB_STATS
+	&alloc_cpu_cache_attr.attr,
 	&alloc_fastpath_attr.attr,
 	&alloc_slowpath_attr.attr,
+	&free_cpu_cache_attr.attr,
 	&free_fastpath_attr.attr,
 	&free_slowpath_attr.attr,
 	&free_frozen_attr.attr,
-- 
2.41.0

[RFC v2 3/7] maple_tree: use slub percpu array

[Vlastimil Babka]

Just make sure the maple_node_cache has a percpu array of size 32.

Will break with CONFIG_SLAB.
---
 lib/maple_tree.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 4dd73cf936a6..1196d0a17f03 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -6180,9 +6180,16 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
 
 void __init maple_tree_init(void)
 {
+	int ret;
+
 	maple_node_cache = kmem_cache_create("maple_node",
 			sizeof(struct maple_node), sizeof(struct maple_node),
-			SLAB_PANIC, NULL);
+			SLAB_PANIC | SLAB_NO_MERGE, NULL);
+
+	ret = kmem_cache_setup_percpu_array(maple_node_cache, 32);
+
+	if (ret)
+		pr_warn("error %d creating percpu_array for maple_node_cache\n", ret);
 }
 
 /**
-- 
2.41.0

[RFC v2 4/7] maple_tree: avoid bulk alloc/free to use percpu array more

[Vlastimil Babka]

Using bulk alloc/free on a cache with percpu array should not be
necessary and the bulk alloc actually bypasses the array (the prefill
functionality currently relies on this).

The simplest change for now is just to convert the respective maple tree
wrappers to do a loop of normal alloc/free. We can optimize later
versions if needed.
---
 lib/maple_tree.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 1196d0a17f03..926bee7a274a 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -161,12 +161,20 @@ static inline struct maple_node *mt_alloc_one(gfp_t gfp)
 
 static inline int mt_alloc_bulk(gfp_t gfp, size_t size, void **nodes)
 {
-	return kmem_cache_alloc_bulk(maple_node_cache, gfp, size, nodes);
+	int i;
+
+	for (i = 0; i < size; i++) {
+		nodes[i] = kmem_cache_alloc(maple_node_cache, gfp);
+		if (!nodes[i])
+			break;
+	}
+	return i;
 }
 
 static inline void mt_free_bulk(size_t size, void __rcu **nodes)
 {
-	kmem_cache_free_bulk(maple_node_cache, size, (void **)nodes);
+	for (size_t i = 0; i < size; i++)
+		kmem_cache_free(maple_node_cache, nodes[i]);
 }
 
 static void mt_free_rcu(struct rcu_head *head)
-- 
2.41.0

[RFC v2 5/7] maple_tree: Remove MA_STATE_PREALLOC

[Vlastimil Babka]

From: "Liam R. Howlett" <Liam.Howlett@oracle.com>

MA_SATE_PREALLOC was added to catch any writes that try to allocate when
the maple state is being used in preallocation mode.  This can safely be
removed in favour of the percpu array of nodes.

Note that mas_expected_entries() still expects no allocations during
operation and so MA_STATE_BULK can be used in place of preallocations
for this case, which is primarily used for forking.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 lib/maple_tree.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 926bee7a274a..8bd4a79537d8 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -68,11 +68,9 @@
  * Maple state flags
  * * MA_STATE_BULK		- Bulk insert mode
  * * MA_STATE_REBALANCE		- Indicate a rebalance during bulk insert
- * * MA_STATE_PREALLOC		- Preallocated nodes, WARN_ON allocation
  */
 #define MA_STATE_BULK		1
 #define MA_STATE_REBALANCE	2
-#define MA_STATE_PREALLOC	4
 
 #define ma_parent_ptr(x) ((struct maple_pnode *)(x))
 #define ma_mnode_ptr(x) ((struct maple_node *)(x))
@@ -1280,11 +1278,8 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
 		return;
 
 	mas_set_alloc_req(mas, 0);
-	if (mas->mas_flags & MA_STATE_PREALLOC) {
-		if (allocated)
-			return;
-		WARN_ON(!allocated);
-	}
+	if (mas->mas_flags & MA_STATE_BULK)
+		return;
 
 	if (!allocated || mas->alloc->node_count == MAPLE_ALLOC_SLOTS) {
 		node = (struct maple_alloc *)mt_alloc_one(gfp);
@@ -5596,7 +5591,7 @@ void mas_destroy(struct ma_state *mas)
 
 		mas->mas_flags &= ~MA_STATE_REBALANCE;
 	}
-	mas->mas_flags &= ~(MA_STATE_BULK|MA_STATE_PREALLOC);
+	mas->mas_flags &= ~MA_STATE_BULK;
 
 	total = mas_allocated(mas);
 	while (total) {
@@ -5645,9 +5640,6 @@ int mas_expected_entries(struct ma_state *mas, unsigned long nr_entries)
 	 * of nodes during the operation.
 	 */
 
-	/* Optimize splitting for bulk insert in-order */
-	mas->mas_flags |= MA_STATE_BULK;
-
 	/*
 	 * Avoid overflow, assume a gap between each entry and a trailing null.
 	 * If this is wrong, it just means allocation can happen during
@@ -5664,8 +5656,9 @@ int mas_expected_entries(struct ma_state *mas, unsigned long nr_entries)
 	/* Add working room for split (2 nodes) + new parents */
 	mas_node_count(mas, nr_nodes + 3);
 
-	/* Detect if allocations run out */
-	mas->mas_flags |= MA_STATE_PREALLOC;
+	/* Optimize splitting for bulk insert in-order */
+	mas->mas_flags |= MA_STATE_BULK;
+
 
 	if (!mas_is_err(mas))
 		return 0;
-- 
2.41.0

[RFC v2 6/7] maple_tree: replace preallocation with slub percpu array prefill

[Vlastimil Babka]

With the percpu array we can try not doing the preallocations in maple
tree, and instead make sure the percpu array is prefilled, and using
GFP_ATOMIC in places that relied on the preallocation (in case we miss
or fail trylock on the array), i.e. mas_store_prealloc(). For now simply
add __GFP_NOFAIL there as well.
---
 lib/maple_tree.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 8bd4a79537d8..304f2453fac9 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5530,7 +5530,12 @@ void mas_store_prealloc(struct ma_state *mas, void *entry)
 
 	mas_wr_store_setup(&wr_mas);
 	trace_ma_write(__func__, mas, 0, entry);
+
+retry:
 	mas_wr_store_entry(&wr_mas);
+	if (unlikely(mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)))
+		goto retry;
+
 	MAS_WR_BUG_ON(&wr_mas, mas_is_err(mas));
 	mas_destroy(mas);
 }
@@ -5545,19 +5550,12 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc);
  */
 int mas_preallocate(struct ma_state *mas, gfp_t gfp)
 {
-	int ret;
+	int count = 1 + mas_mt_height(mas) * 3;
 
-	mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp);
-	mas->mas_flags |= MA_STATE_PREALLOC;
-	if (likely(!mas_is_err(mas)))
-		return 0;
+	// TODO: should probably indicate if it failed the prefill?
+	kmem_cache_prefill_percpu_array(maple_node_cache, count, gfp);
 
-	mas_set_alloc_req(mas, 0);
-	ret = xa_err(mas->node);
-	mas_reset(mas);
-	mas_destroy(mas);
-	mas_reset(mas);
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(mas_preallocate);
 
-- 
2.41.0

[RFC v2 7/7] tools: Add SLUB percpu array functions for testing

[Vlastimil Babka]

From: "Liam R. Howlett" <Liam.Howlett@oracle.com>

Support new percpu array functions to the test code so they can be used
in the maple tree testing.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 tools/include/linux/slab.h              |  4 ++++
 tools/testing/radix-tree/linux.c        | 14 ++++++++++++++
 tools/testing/radix-tree/linux/kernel.h |  1 +
 3 files changed, 19 insertions(+)

diff --git a/tools/include/linux/slab.h b/tools/include/linux/slab.h
index 311759ea25e9..1043f9c5ef4e 100644
--- a/tools/include/linux/slab.h
+++ b/tools/include/linux/slab.h
@@ -7,6 +7,7 @@
 
 #define SLAB_PANIC 2
 #define SLAB_RECLAIM_ACCOUNT    0x00020000UL            /* Objects are reclaimable */
+#define SLAB_NO_MERGE		0x01000000UL		/* Prevent merging with compatible kmem caches */
 
 #define kzalloc_node(size, flags, node) kmalloc(size, flags)
 
@@ -45,4 +46,7 @@ void kmem_cache_free_bulk(struct kmem_cache *cachep, size_t size, void **list);
 int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 			  void **list);
 
+int kmem_cache_setup_percpu_array(struct kmem_cache *s, unsigned int count);
+int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
+		gfp_t gfp);
 #endif		/* _TOOLS_SLAB_H */
diff --git a/tools/testing/radix-tree/linux.c b/tools/testing/radix-tree/linux.c
index d587a558997f..cbe7937fdd5e 100644
--- a/tools/testing/radix-tree/linux.c
+++ b/tools/testing/radix-tree/linux.c
@@ -187,6 +187,20 @@ int kmem_cache_alloc_bulk(struct kmem_cache *cachep, gfp_t gfp, size_t size,
 	return size;
 }
 
+int kmem_cache_setup_percpu_array(struct kmem_cache *s, unsigned int count)
+{
+	return 0;
+}
+
+int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
+		gfp_t gfp)
+{
+	if (count > s->non_kernel)
+		return s->non_kernel;
+
+	return count;
+}
+
 struct kmem_cache *
 kmem_cache_create(const char *name, unsigned int size, unsigned int align,
 		unsigned int flags, void (*ctor)(void *))
diff --git a/tools/testing/radix-tree/linux/kernel.h b/tools/testing/radix-tree/linux/kernel.h
index c5c9d05f29da..fc75018974de 100644
--- a/tools/testing/radix-tree/linux/kernel.h
+++ b/tools/testing/radix-tree/linux/kernel.h
@@ -15,6 +15,7 @@
 
 #define printk printf
 #define pr_err printk
+#define pr_warn printk
 #define pr_info printk
 #define pr_debug printk
 #define pr_cont printk
-- 
2.41.0

Re: [RFC v2 0/7] SLUB percpu array caches and maple tree nodes

[Suren Baghdasaryan]

On Thu, Aug 10, 2023 at 9:36 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> Also in git [1]. Changes since v1 [2]:
>
> - fix a few bugs
> - SLAB marked as BROKEN so bots dont complain about missing functions
> - incorporate Liam's patches, which allows getting rid of preallocations
>   in mas_prealloc() completely. This has reduced the allocation stats
>   further, with the whole series.
>
> More notes wrt v1 RFC feedback:
>
> - locking is still done as in v1, as it allows remote draining, which
>   should be added before this is suitable for merging
> - there's currently no bulk freeing/refill of the percpu array, which
>   will eventually be added, but I expect most perf gain for the maple
>   tree use case to come from the avoided preallocations anyway
>
> ----
>
> At LSF/MM I've mentioned that I see several use cases for introducing
> opt-in percpu arrays for caching alloc/free objects in SLUB. This is my
> first exploration of this idea, speficially for the use case of maple
> tree nodes. We have brainstormed this use case on IRC last week with
> Liam and Matthew and this how I understood the requirements:
>
> - percpu arrays will be faster thank bulk alloc/free which needs
>   relatively long freelists to work well. Especially in the freeing case
>   we need the nodes to come from the same slab (or small set of those)
>
> - preallocation for the worst case of needed nodes for a tree operation
>   that can't reclaim due to locks is wasteful. We could instead expect
>   that most of the time percpu arrays would satisfy the constained
>   allocations, and in the rare cases it does not we can dip into
>   GFP_ATOMIC reserves temporarily. Instead of preallocation just prefill
>   the arrays.
>
> - NUMA locality is not a concern as the nodes of a process's VMA tree
>   end up all over the place anyway.
>
> So this RFC patchset adds such percpu array in Patch 2. Locking is
> stolen from Mel's recent page allocator's pcplists implementation so it
> can avoid disabling IRQs and just disable preemption, but the trylocks
> can fail in rare situations.
>
> Then maple tree is modified in patches 3-6 to benefit from this. This is
> done in a very crude way as I'm not so familiar with the code.
>
> I've briefly tested this with virtme VM boot and checking the stats from
> CONFIG_SLUB_STATS in sysfs.
>
> Patch 2:
>
> slub changes implemented including new counters alloc_cpu_cache
> and free_cpu_cache but maple tree doesn't use them yet
>
> (none):/sys/kernel/slab/maple_node # grep . alloc_cpu_cache alloc_*path free_cpu_cache free_*path | cut -d' ' -f1
> alloc_cpu_cache:0
> alloc_fastpath:54842
> alloc_slowpath:8142
> free_cpu_cache:0
> free_fastpath:32336
> free_slowpath:23484
>
> Patch 3:
>
> maple node cache creates percpu array with 32 entries,
> not changed anything else
>
> -> some allocs/free satisfied by the array
>
> alloc_cpu_cache:11956
> alloc_fastpath:40675
> alloc_slowpath:7308
> free_cpu_cache:12082
> free_fastpath:23617
> free_slowpath:17956
>
> Patch 4:
>
> maple tree nodes bulk alloc/free converted to loop of normal alloc to use
> percpu array more, because bulk alloc bypasses it
>
> -> majority alloc/free now satisfied by percpu array
>
> alloc_cpu_cache:54673
> alloc_fastpath:4491
> alloc_slowpath:737
> free_cpu_cache:54759
> free_fastpath:332
> free_slowpath:4723
>
> Patch 5+6:
>
> mas_preallocate() just prefills the percpu array, doesn't preallocate anything
> mas_store_prealloc() gains a retry loop with mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)
>
> -> major drop of alloc/free
> (the prefills are included in the accounting)
>
> alloc_cpu_cache:15036
> alloc_fastpath:4651
> alloc_slowpath:656
> free_cpu_cache:15102
> free_fastpath:299
> free_slowpath:4835
>
> It would be interesting to see how it affects the workloads that saw
> regressions from the maple tree introduction, as the slab operations
> were suspected to be a major factor and now they should be both reduced
> and made cheaper.

Hi Vlastimil,
I backported your patchset to 6.1 and tested it on Android with my
mmap stress test (mmap a file-backed page, read-fault, unmap all in a
tight loop). The performance of such tests is important for Android
because that's what is being done during application launch and app
launch time is an important metric for us. I recorded 1.8% performance
improvement with this test.
Thanks,
Suren.

>
> Liam R. Howlett (2):
>   maple_tree: Remove MA_STATE_PREALLOC
>   tools: Add SLUB percpu array functions for testing
>
> Vlastimil Babka (5):
>   mm, slub: fix bulk alloc and free stats
>   mm, slub: add opt-in slub_percpu_array
>   maple_tree: use slub percpu array
>   maple_tree: avoid bulk alloc/free to use percpu array more
>   maple_tree: replace preallocation with slub percpu array prefill
>
>  include/linux/slab.h                    |   4 +
>  include/linux/slub_def.h                |  10 ++
>  lib/maple_tree.c                        |  60 ++++---
>  mm/Kconfig                              |   1 +
>  mm/slub.c                               | 221 +++++++++++++++++++++++-
>  tools/include/linux/slab.h              |   4 +
>  tools/testing/radix-tree/linux.c        |  14 ++
>  tools/testing/radix-tree/linux/kernel.h |   1 +
>  8 files changed, 286 insertions(+), 29 deletions(-)
>
> --
> 2.41.0
>

Re: [RFC v2 2/7] mm, slub: add opt-in slub_percpu_array

[Hyeonggon Yoo]

Hi,

On Fri, Aug 11, 2023 at 1:36 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> kmem_cache_setup_percpu_array() will allocate a per-cpu array for
> caching alloc/free objects of given size for the cache. The cache
> has to be created with SLAB_NO_MERGE flag.
>
> The array is filled by freeing. When empty for alloc or full for
> freeing, it's simply bypassed by the operation, there's currently no
> batch freeing/allocations.
>
> The locking is copied from the page allocator's pcplists, based on
> embedded spin locks. Interrupts are not disabled, only preemption (cpu
> migration on RT). Trylock is attempted to avoid deadlock due to
> an intnerrupt, trylock failure means the array is bypassed.

nit: s/intnerrupt/interrupt/

>  /*
>   * Inlined fastpath so that allocation functions (kmalloc, kmem_cache_alloc)
>   * have the fastpath folded into their functions. So no function call
> @@ -3465,7 +3564,11 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
>         if (unlikely(object))
>                 goto out;
>
> -       object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
> +       if (s->cpu_array)
> +               object = alloc_from_pca(s);
> +
> +       if (!object)
> +               object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
>
>         maybe_wipe_obj_freeptr(s, object);
>         init = slab_want_init_on_alloc(gfpflags, s);
> @@ -3715,6 +3818,34 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>         discard_slab(s, slab);
>  }

>  #ifndef CONFIG_SLUB_TINY
>  /*
>   * Fastpath with forced inlining to produce a kfree and kmem_cache_free that
> @@ -3740,6 +3871,11 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
>         unsigned long tid;
>         void **freelist;
>
> +       if (s->cpu_array && cnt == 1) {
> +               if (free_to_pca(s, head))
> +                       return;
> +       }
> +
>  redo:
>         /*
>          * Determine the currently cpus per cpu slab.
> @@ -3793,6 +3929,11 @@ static void do_slab_free(struct kmem_cache *s,
>  {
>         void *tail_obj = tail ? : head;
>
> +       if (s->cpu_array && cnt == 1) {
> +               if (free_to_pca(s, head))
> +                       return;
> +       }
> +
>         __slab_free(s, slab, head, tail_obj, cnt, addr);
>  }
>  #endif /* CONFIG_SLUB_TINY */

Is this functionality needed for SLUB_TINY?

> @@ -4060,6 +4201,45 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  }
>  EXPORT_SYMBOL(kmem_cache_alloc_bulk);
>
> +int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
> +               gfp_t gfp)
> +{
> +       struct slub_percpu_array *pca;
> +       void *objects[32];
> +       unsigned int used;
> +       unsigned int allocated;
> +
> +       if (!s->cpu_array)
> +               return -EINVAL;
> +
> +       /* racy but we don't care */
> +       pca = raw_cpu_ptr(s->cpu_array);
> +
> +       used = READ_ONCE(pca->used);

Hmm for the prefill to be meaningful,
remote allocation should be possible, right?

Otherwise it only prefills for the CPU that requested it.

> +       if (used >= count)
> +               return 0;
> +
> +       if (pca->count < count)
> +               return -EINVAL;
> +
> +       count -= used;
> +
> +       /* TODO fix later */
> +       if (count > 32)
> +               count = 32;
> +
> +       for (int i = 0; i < count; i++)
> +               objects[i] = NULL;
> +       allocated = kmem_cache_alloc_bulk(s, gfp, count, &objects[0]);
> +
> +       for (int i = 0; i < count; i++) {
> +               if (objects[i]) {
> +                       kmem_cache_free(s, objects[i]);
> +               }
> +       }

nit: why not

for (int i = 0; i < allocated; i++) {
    kmem_cache_free(s, objects[i]);
}

and skip objects[i] = NULL

> +       return allocated;
> +}

And a question:
Does SLUB still need to maintain per-cpu partial slab lists even when
an opt-in percpu array is used?

Re: [RFC v2 2/7] mm, slub: add opt-in slub_percpu_array

[Vlastimil Babka]

On 8/21/23 16:57, Hyeonggon Yoo wrote:
> Hi,
> 
> On Fri, Aug 11, 2023 at 1:36 AM Vlastimil Babka <vbabka@suse.cz> wrote:

Oops, looks like I forgot reply, sorry (preparing v3 now).

>>
>> kmem_cache_setup_percpu_array() will allocate a per-cpu array for
>> caching alloc/free objects of given size for the cache. The cache
>> has to be created with SLAB_NO_MERGE flag.
>>
>> The array is filled by freeing. When empty for alloc or full for
>> freeing, it's simply bypassed by the operation, there's currently no
>> batch freeing/allocations.
>>
>> The locking is copied from the page allocator's pcplists, based on
>> embedded spin locks. Interrupts are not disabled, only preemption (cpu
>> migration on RT). Trylock is attempted to avoid deadlock due to
>> an intnerrupt, trylock failure means the array is bypassed.
> 
> nit: s/intnerrupt/interrupt/

Thanks.

> 
>>  /*
>>   * Inlined fastpath so that allocation functions (kmalloc, kmem_cache_alloc)
>>   * have the fastpath folded into their functions. So no function call
>> @@ -3465,7 +3564,11 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
>>         if (unlikely(object))
>>                 goto out;
>>
>> -       object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
>> +       if (s->cpu_array)
>> +               object = alloc_from_pca(s);
>> +
>> +       if (!object)
>> +               object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
>>
>>         maybe_wipe_obj_freeptr(s, object);
>>         init = slab_want_init_on_alloc(gfpflags, s);
>> @@ -3715,6 +3818,34 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>>         discard_slab(s, slab);
>>  }
> 
>>  #ifndef CONFIG_SLUB_TINY
>>  /*
>>   * Fastpath with forced inlining to produce a kfree and kmem_cache_free that
>> @@ -3740,6 +3871,11 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
>>         unsigned long tid;
>>         void **freelist;
>>
>> +       if (s->cpu_array && cnt == 1) {
>> +               if (free_to_pca(s, head))
>> +                       return;
>> +       }
>> +
>>  redo:
>>         /*
>>          * Determine the currently cpus per cpu slab.
>> @@ -3793,6 +3929,11 @@ static void do_slab_free(struct kmem_cache *s,
>>  {
>>         void *tail_obj = tail ? : head;
>>
>> +       if (s->cpu_array && cnt == 1) {
>> +               if (free_to_pca(s, head))
>> +                       return;
>> +       }
>> +
>>         __slab_free(s, slab, head, tail_obj, cnt, addr);
>>  }
>>  #endif /* CONFIG_SLUB_TINY */
> 
> Is this functionality needed for SLUB_TINY?

Due to the prefill semantics, I think it has to be be even in TINY, or we
risk running out of memory reserves. Also later I want to investigate
extending this approach for supporting allocations in very constrained
contexts (NMI) so e.g. bpf doesn't have to reimplement the slab allocator,
and that would also not be good to limit to !SLUB_TINY.

>> @@ -4060,6 +4201,45 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>>  }
>>  EXPORT_SYMBOL(kmem_cache_alloc_bulk);
>>
>> +int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
>> +               gfp_t gfp)
>> +{
>> +       struct slub_percpu_array *pca;
>> +       void *objects[32];
>> +       unsigned int used;
>> +       unsigned int allocated;
>> +
>> +       if (!s->cpu_array)
>> +               return -EINVAL;
>> +
>> +       /* racy but we don't care */
>> +       pca = raw_cpu_ptr(s->cpu_array);
>> +
>> +       used = READ_ONCE(pca->used);
> 
> Hmm for the prefill to be meaningful,
> remote allocation should be possible, right?

Remote in what sense?

> Otherwise it only prefills for the CPU that requested it.

If there's a cpu migration between the prefill and usage, it might run out
of the cached array, but assumption is to be rare enough to become an issue.

>> +       if (used >= count)
>> +               return 0;
>> +
>> +       if (pca->count < count)
>> +               return -EINVAL;
>> +
>> +       count -= used;
>> +
>> +       /* TODO fix later */
>> +       if (count > 32)
>> +               count = 32;
>> +
>> +       for (int i = 0; i < count; i++)
>> +               objects[i] = NULL;
>> +       allocated = kmem_cache_alloc_bulk(s, gfp, count, &objects[0]);
>> +
>> +       for (int i = 0; i < count; i++) {
>> +               if (objects[i]) {
>> +                       kmem_cache_free(s, objects[i]);
>> +               }
>> +       }
> 
> nit: why not
> 
> for (int i = 0; i < allocated; i++) {
>     kmem_cache_free(s, objects[i]);
> }
> 
> and skip objects[i] = NULL
> 

This is rewritten significantly in v3 so I think it doesn't apply anymore.

>> +       return allocated;
>> +}
> 
> And a question:
> Does SLUB still need to maintain per-cpu partial slab lists even when
> an opt-in percpu array is used?

Good question :) didn't investigate it yet. We can, once this settles.

Thanks.

Re: [RFC v2 2/7] mm, slub: add opt-in slub_percpu_array

[Hyeonggon Yoo]

On Wed, Nov 29, 2023 at 2:37 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 8/21/23 16:57, Hyeonggon Yoo wrote:
> > Hi,
> >
> > On Fri, Aug 11, 2023 at 1:36 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> Oops, looks like I forgot reply, sorry (preparing v3 now).

It's fine, you were busy removing SLAB :)
thanks for replying.

> >
> >>  /*
> >>   * Inlined fastpath so that allocation functions (kmalloc, kmem_cache_alloc)
> >>   * have the fastpath folded into their functions. So no function call
> >> @@ -3465,7 +3564,11 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
> >>         if (unlikely(object))
> >>                 goto out;
> >>
> >> -       object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
> >> +       if (s->cpu_array)
> >> +               object = alloc_from_pca(s);
> >> +
> >> +       if (!object)
> >> +               object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
> >>
> >>         maybe_wipe_obj_freeptr(s, object);
> >>         init = slab_want_init_on_alloc(gfpflags, s);
> >> @@ -3715,6 +3818,34 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
> >>         discard_slab(s, slab);
> >>  }
> >
> >>  #ifndef CONFIG_SLUB_TINY
> >>  /*
> >>   * Fastpath with forced inlining to produce a kfree and kmem_cache_free that
> >> @@ -3740,6 +3871,11 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
> >>         unsigned long tid;
> >>         void **freelist;
> >>
> >> +       if (s->cpu_array && cnt == 1) {
> >> +               if (free_to_pca(s, head))
> >> +                       return;
> >> +       }
> >> +
> >>  redo:
> >>         /*
> >>          * Determine the currently cpus per cpu slab.
> >> @@ -3793,6 +3929,11 @@ static void do_slab_free(struct kmem_cache *s,
> >>  {
> >>         void *tail_obj = tail ? : head;
> >>
> >> +       if (s->cpu_array && cnt == 1) {
> >> +               if (free_to_pca(s, head))
> >> +                       return;
> >> +       }
> >> +
> >>         __slab_free(s, slab, head, tail_obj, cnt, addr);
> >>  }
> >>  #endif /* CONFIG_SLUB_TINY */
> >
> > Is this functionality needed for SLUB_TINY?
>
> Due to the prefill semantics, I think it has to be be even in TINY, or we
> risk running out of memory reserves. Also later I want to investigate
> extending this approach for supporting allocations in very constrained
> contexts (NMI) so e.g. bpf doesn't have to reimplement the slab allocator,
> and that would also not be good to limit to !SLUB_TINY.

I've got the point, thanks for the explanation!

> >> @@ -4060,6 +4201,45 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> >>  }
> >>  EXPORT_SYMBOL(kmem_cache_alloc_bulk);
> >>
> >> +int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
> >> +               gfp_t gfp)
> >> +{
> >> +       struct slub_percpu_array *pca;
> >> +       void *objects[32];
> >> +       unsigned int used;
> >> +       unsigned int allocated;
> >> +
> >> +       if (!s->cpu_array)
> >> +               return -EINVAL;
> >> +
> >> +       /* racy but we don't care */
> >> +       pca = raw_cpu_ptr(s->cpu_array);
> >> +
> >> +       used = READ_ONCE(pca->used);
> >
> > Hmm for the prefill to be meaningful,
> > remote allocation should be possible, right?
>
> Remote in what sense?

TL;DR) What I wanted to ask was:
"How pre-filling a number of objects works when the pre-filled objects
are not shared between CPUs"

IIUC the prefill is opportunistically filling the array so (hopefully)
expecting there are
some objects filled in it.

Let's say CPU X calls kmem_cache_prefill_percpu_array(32) and all 32
objects are filled into CPU X's array.
But if CPU Y can't allocate from CPU X's array (which I referred to as
"remote allocation"), the semantics differ from
the maple tree's perspective because preallocated objects were shared
between CPUs before, but now it's not?

Thanks!

--
Hyeonggon

Re: [RFC v2 2/7] mm, slub: add opt-in slub_percpu_array

[Vlastimil Babka]

On 11/29/23 01:46, Hyeonggon Yoo wrote:
> On Wed, Nov 29, 2023 at 2:37 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> 
>> >> @@ -4060,6 +4201,45 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>> >>  }
>> >>  EXPORT_SYMBOL(kmem_cache_alloc_bulk);
>> >>
>> >> +int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count,
>> >> +               gfp_t gfp)
>> >> +{
>> >> +       struct slub_percpu_array *pca;
>> >> +       void *objects[32];
>> >> +       unsigned int used;
>> >> +       unsigned int allocated;
>> >> +
>> >> +       if (!s->cpu_array)
>> >> +               return -EINVAL;
>> >> +
>> >> +       /* racy but we don't care */
>> >> +       pca = raw_cpu_ptr(s->cpu_array);
>> >> +
>> >> +       used = READ_ONCE(pca->used);
>> >
>> > Hmm for the prefill to be meaningful,
>> > remote allocation should be possible, right?
>>
>> Remote in what sense?
> 
> TL;DR) What I wanted to ask was:
> "How pre-filling a number of objects works when the pre-filled objects
> are not shared between CPUs"
> 
> IIUC the prefill is opportunistically filling the array so (hopefully)
> expecting there are
> some objects filled in it.

Yes.

> Let's say CPU X calls kmem_cache_prefill_percpu_array(32) and all 32
> objects are filled into CPU X's array.
> But if CPU Y can't allocate from CPU X's array (which I referred to as
> "remote allocation"), the semantics differ from
> the maple tree's perspective because preallocated objects were shared
> between CPUs before, but now it's not?

The assumption is that the operation will prefill on CPU X and then consume
it also on X, because shortly after prefill it will enter some restricted
context (i.e. spin_lock_irqsave or whatnot) that prevents it from migrating.
That's not guaranteed of course, but migration in a bad moment and
subsequent depleted array should be rare enough that we'll just handle it in
the slow paths, and if it results in dipping into reserves, it won't be too
disruptive.

> Thanks!
> 
> --
> Hyeonggon