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Subject[PATCH] kretprobe scalability improvement
Date
From: wuqiang <wuqiang.matt@bytedance.com>

The original freelist is a LIFO queue based on singly linked list, which lacks
of scalability, and thus becomes bottleneck under high workloads. freelist was
introduced by Masami Hiramatsu's work of removing kretprobe hash lock:
url: https://lkml.org/lkml/2020/8/29/209.

Here an array-based MPMC lockless queue is proposed. The solution of bounded
array can nicely avoid ABA issue, while freelist or circular queue etc. have
to perform 2 CAS loops. The other advantage is that order and fairness can be
ignored, the only concern is to retrieve kretprobe instance records as fast
as possible, i.e. performance and correctness. Tests of kretprobe on 96-CORE
ARM64 show the biggest gain as 466.7x of the original freelist throughput.
The raw queue throughput can be 1,975 times of freelist. Here are the results:

Ubuntu 20.04, 5.13.0-rc6 (XEON E5-2660V3 2.4G, DDR4 2133MT/s, 10 CORES/20 THREADS):
1x 2x 4x 8x 10x 16x 20x 32x 40x
freelist: 13086080 22493637 32773854 20129772 18455899 18435561 18980332 18988603 18991334
array : 13144036 26059941 47449954 94517172 115856027 116414714 125692971 125553061 125685981

Ubuntu 21.04 - 5.12.10 QEMU 96 CORES (HUAWEI TaiShan 2280V2 KP920 96 CORES 2.6G, DDR4 2944 MT/s):
1x 2x 4x 8x 16x 24x 48x 96x 192x
freelist: 17,233,640 10,296,664 8,095,309 6,993,545 5,050,817 4,295,283 3,382,013 2,738,050 2,743,345
array: 19,360,905 37,395,225 56,417,463 10,020,136 209,876,209 328,940,014 632,754,916 1,277,862,473 1,169,076,739

So linear scalability is still not available, limited by the following two
considerations:

1. keep it simple: best solution could be an implementation of per-cpu queues,
but it's not applicable for this case due to complexity. After all for
most cases the number of pre-allocated kretprobe instances (maxactive) is
only a small value. If not specified by user during registering, maxactive
is set as CPU cores or 2x when preemption is enabled
2. keep it compact: cache-line-alignment can solve false-sharing and minimize
cache thrashing, but it introduces memory wasting, considering the small
body of structure kretprobe_instance. Secondly the performance improvement
of cache-line-aligned is not significant as expected

With a pre-built kernel, further performance tuning can be done by increasing
maxactive when registering kretprobe. Tests show 4x cores number is a fair
choice for both performance and memory efficiency.

More info is available at: https://github.com/mattwuq/kretprobe

Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
---
include/linux/freelist.h | 187 +++++++++++++++++++--------------------
kernel/kprobes.c | 29 +++---
2 files changed, 107 insertions(+), 109 deletions(-)

diff --git a/include/linux/freelist.h b/include/linux/freelist.h
index fc1842b96469..3d4a0bc425b2 100644
--- a/include/linux/freelist.h
+++ b/include/linux/freelist.h
@@ -1,129 +1,122 @@
-/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
+/* SPDX-License-Identifier: GPL-2.0-or-later */
#ifndef FREELIST_H
#define FREELIST_H

+#include <linux/slab.h>
#include <linux/atomic.h>

/*
- * Copyright: cameron@moodycamel.com
+ * lockless queue for kretprobe instances
*
- * A simple CAS-based lock-free free list. Not the fastest thing in the world
- * under heavy contention, but simple and correct (assuming nodes are never
- * freed until after the free list is destroyed), and fairly speedy under low
- * contention.
- *
- * Adapted from: https://moodycamel.com/blog/2014/solving-the-aba-problem-for-lock-free-free-lists
+ * It's an array-based MPMC lockless queue, solely for better scalability
+ * and contention mitigation. It's simple in implementation and compact in
+ * memory efficiency. The only concern is to retrieve kretprobe instance
+ * records as fast as possible, with both order and fairness ignored.
*/

struct freelist_node {
- atomic_t refs;
- struct freelist_node *next;
+ struct freelist_node *next;
};
-
struct freelist_head {
- struct freelist_node *head;
+ uint32_t fh_size; /* rounded to power of 2 */
+ uint32_t fh_mask; /* (fh_size - 1) */
+ uint16_t fh_bits; /* log2(fh_size) */
+ uint16_t fh_step; /* per-core shift stride */
+ uint32_t fh_used; /* num of elements in list */
+ struct freelist_node **fh_ents; /* array for krp instances */
};

-#define REFS_ON_FREELIST 0x80000000
-#define REFS_MASK 0x7FFFFFFF
+static inline int freelist_init(struct freelist_head *list, int max)
+{
+ uint32_t size, cores = num_possible_cpus();
+
+ list->fh_used = 0;
+ list->fh_step = ilog2(L1_CACHE_BYTES / sizeof(void *));
+ if (max < (cores << list->fh_step))
+ list->fh_size = roundup_pow_of_two(cores) << list->fh_step;
+ else
+ list->fh_size = roundup_pow_of_two(max);
+ list->fh_mask = list->fh_size - 1;
+ list->fh_bits = (uint16_t)ilog2(list->fh_size);
+ size = list->fh_size * sizeof(struct freelist_node *);
+ list->fh_ents = kzalloc(size, GFP_KERNEL);
+ if (!list->fh_ents)
+ return -ENOMEM;
+
+ return 0;
+}

-static inline void __freelist_add(struct freelist_node *node, struct freelist_head *list)
+static inline int freelist_try_add(struct freelist_node *node, struct freelist_head *list)
{
- /*
- * Since the refcount is zero, and nobody can increase it once it's
- * zero (except us, and we run only one copy of this method per node at
- * a time, i.e. the single thread case), then we know we can safely
- * change the next pointer of the node; however, once the refcount is
- * back above zero, then other threads could increase it (happens under
- * heavy contention, when the refcount goes to zero in between a load
- * and a refcount increment of a node in try_get, then back up to
- * something non-zero, then the refcount increment is done by the other
- * thread) -- so if the CAS to add the node to the actual list fails,
- * decrese the refcount and leave the add operation to the next thread
- * who puts the refcount back to zero (which could be us, hence the
- * loop).
- */
- struct freelist_node *head = READ_ONCE(list->head);
-
- for (;;) {
- WRITE_ONCE(node->next, head);
- atomic_set_release(&node->refs, 1);
-
- if (!try_cmpxchg_release(&list->head, &head, node)) {
- /*
- * Hmm, the add failed, but we can only try again when
- * the refcount goes back to zero.
- */
- if (atomic_fetch_add_release(REFS_ON_FREELIST - 1, &node->refs) == 1)
- continue;
+ uint32_t i, hint = list->fh_used << list->fh_step;
+
+ for (i = 0; i < list->fh_size; i++) {
+ struct freelist_node *item = NULL;
+ uint32_t slot = (i + hint) & list->fh_mask;
+ if (try_cmpxchg_release(&list->fh_ents[slot], &item, node)) {
+ list->fh_used++;
+ break;
}
- return;
}
+
+ return (i >= list->fh_size);
}

-static inline void freelist_add(struct freelist_node *node, struct freelist_head *list)
+static inline int freelist_add(struct freelist_node *node, struct freelist_head *list)
{
- /*
- * We know that the should-be-on-freelist bit is 0 at this point, so
- * it's safe to set it using a fetch_add.
- */
- if (!atomic_fetch_add_release(REFS_ON_FREELIST, &node->refs)) {
- /*
- * Oh look! We were the last ones referencing this node, and we
- * know we want to add it to the free list, so let's do it!
- */
- __freelist_add(node, list);
- }
+ uint32_t hint = raw_smp_processor_id() << list->fh_step;
+ uint32_t slot = ((uint32_t) hint) & list->fh_mask;
+
+ do {
+ struct freelist_node *item = NULL;
+ if (try_cmpxchg_release(&list->fh_ents[slot], &item, node))
+ return 0;
+ slot = (slot + 1) & list->fh_mask;
+ } while (1);
+
+ return -1;
}

static inline struct freelist_node *freelist_try_get(struct freelist_head *list)
{
- struct freelist_node *prev, *next, *head = smp_load_acquire(&list->head);
- unsigned int refs;
-
- while (head) {
- prev = head;
- refs = atomic_read(&head->refs);
- if ((refs & REFS_MASK) == 0 ||
- !atomic_try_cmpxchg_acquire(&head->refs, &refs, refs+1)) {
- head = smp_load_acquire(&list->head);
- continue;
+ struct freelist_node *node = NULL;
+ uint32_t i, hint = raw_smp_processor_id() << list->fh_step;
+
+ for (i = 0; i < list->fh_size; i++) {
+ uint32_t slot = (hint + i) & list->fh_mask;
+ struct freelist_node *item = smp_load_acquire(&list->fh_ents[slot]);
+ if (item && try_cmpxchg_release(&list->fh_ents[slot], &item, NULL)) {
+ node = item;
+ break;
}
+ }

- /*
- * Good, reference count has been incremented (it wasn't at
- * zero), which means we can read the next and not worry about
- * it changing between now and the time we do the CAS.
- */
- next = READ_ONCE(head->next);
- if (try_cmpxchg_acquire(&list->head, &head, next)) {
- /*
- * Yay, got the node. This means it was on the list,
- * which means should-be-on-freelist must be false no
- * matter the refcount (because nobody else knows it's
- * been taken off yet, it can't have been put back on).
- */
- WARN_ON_ONCE(atomic_read(&head->refs) & REFS_ON_FREELIST);
-
- /*
- * Decrease refcount twice, once for our ref, and once
- * for the list's ref.
- */
- atomic_fetch_add(-2, &head->refs);
-
- return head;
- }
+ return node;
+}

- /*
- * OK, the head must have changed on us, but we still need to decrement
- * the refcount we increased.
- */
- refs = atomic_fetch_add(-1, &prev->refs);
- if (refs == REFS_ON_FREELIST + 1)
- __freelist_add(prev, list);
+static inline void freelist_destroy(struct freelist_head *list, void *context,
+ int (*release)(void *, void *))
+{
+ uint32_t i;
+
+ if (!list->fh_ents)
+ return;
+
+ for (i = 0; i < list->fh_size; i++) {
+ uint32_t slot = i & list->fh_mask;
+ struct freelist_node *item = smp_load_acquire(&list->fh_ents[slot]);
+ while (item) {
+ if (try_cmpxchg_release(&list->fh_ents[slot], &item, NULL)) {
+ if (release)
+ release(context, item);
+ break;
+ }
+ }
}

- return NULL;
+ if (list->fh_ents) {
+ kfree(list->fh_ents);
+ list->fh_ents = NULL;
+ }
}
-
#endif /* FREELIST_H */
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 471b1d18a92f..5c41bee25983 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1277,20 +1277,21 @@ void kprobe_flush_task(struct task_struct *tk)
}
NOKPROBE_SYMBOL(kprobe_flush_task);

-static inline void free_rp_inst(struct kretprobe *rp)
+static int release_ri(void *context, void *node)
{
struct kretprobe_instance *ri;
- struct freelist_node *node;
- int count = 0;
+ ri = container_of(node, struct kretprobe_instance, freelist);
+ kfree(ri);
+ if (context)
+ (*((int *)context))++;
+ return 0;
+}

- node = rp->freelist.head;
- while (node) {
- ri = container_of(node, struct kretprobe_instance, freelist);
- node = node->next;
+static inline void free_rp_inst(struct kretprobe *rp)
+{
+ int count = 0;

- kfree(ri);
- count++;
- }
+ freelist_destroy(&rp->freelist, &count, release_ri);

if (refcount_sub_and_test(count, &rp->rph->ref)) {
kfree(rp->rph);
@@ -2015,10 +2016,14 @@ int register_kretprobe(struct kretprobe *rp)
rp->maxactive = num_possible_cpus();
#endif
}
- rp->freelist.head = NULL;
+ if (freelist_init(&rp->freelist, rp->maxactive))
+ return -ENOMEM;
+
rp->rph = kzalloc(sizeof(struct kretprobe_holder), GFP_KERNEL);
- if (!rp->rph)
+ if (!rp->rph) {
+ freelist_destroy(&rp->freelist, NULL, NULL);
return -ENOMEM;
+ }

rp->rph->rp = rp;
for (i = 0; i < rp->maxactive; i++) {
--
2.25.1
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