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Date: Wed, 18 May 2022 14:27:15 +0800
From: Feng zhou <zhoufeng.zf@...edance.com>
To: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
kafai@...com, songliubraving@...com, yhs@...com,
john.fastabend@...il.com, kpsingh@...nel.org
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
songmuchun@...edance.com, wangdongdong.6@...edance.com,
cong.wang@...edance.com, zhouchengming@...edance.com,
zhoufeng.zf@...edance.com
Subject: [PATCH] bpf: avoid grabbing spin_locks of all cpus when no free elems
From: Feng Zhou <zhoufeng.zf@...edance.com>
We encountered bad case on big system with 96 CPUs that
alloc_htab_elem() would last for 1ms. The reason is that after the
prealloc hashtab has no free elems, when trying to update, it will still
grab spin_locks of all cpus. If there are multiple update users, the
competition is very serious.
So this patch add is_empty in pcpu_freelist_head to check freelist
having free or not. If having, grab spin_lock, or check next cpu's
freelist.
Before patch: hash_map performance
./map_perf_test 1
0:hash_map_perf pre-alloc 975345 events per sec
4:hash_map_perf pre-alloc 855367 events per sec
12:hash_map_perf pre-alloc 860862 events per sec
8:hash_map_perf pre-alloc 849561 events per sec
3:hash_map_perf pre-alloc 849074 events per sec
6:hash_map_perf pre-alloc 847120 events per sec
10:hash_map_perf pre-alloc 845047 events per sec
5:hash_map_perf pre-alloc 841266 events per sec
14:hash_map_perf pre-alloc 849740 events per sec
2:hash_map_perf pre-alloc 839598 events per sec
9:hash_map_perf pre-alloc 838695 events per sec
11:hash_map_perf pre-alloc 845390 events per sec
7:hash_map_perf pre-alloc 834865 events per sec
13:hash_map_perf pre-alloc 842619 events per sec
1:hash_map_perf pre-alloc 804231 events per sec
15:hash_map_perf pre-alloc 795314 events per sec
hash_map the worst: no free
./map_perf_test 2048
6:worse hash_map_perf pre-alloc 28628 events per sec
5:worse hash_map_perf pre-alloc 28553 events per sec
11:worse hash_map_perf pre-alloc 28543 events per sec
3:worse hash_map_perf pre-alloc 28444 events per sec
1:worse hash_map_perf pre-alloc 28418 events per sec
7:worse hash_map_perf pre-alloc 28427 events per sec
13:worse hash_map_perf pre-alloc 28330 events per sec
14:worse hash_map_perf pre-alloc 28263 events per sec
9:worse hash_map_perf pre-alloc 28211 events per sec
15:worse hash_map_perf pre-alloc 28193 events per sec
12:worse hash_map_perf pre-alloc 28190 events per sec
10:worse hash_map_perf pre-alloc 28129 events per sec
8:worse hash_map_perf pre-alloc 28116 events per sec
4:worse hash_map_perf pre-alloc 27906 events per sec
2:worse hash_map_perf pre-alloc 27801 events per sec
0:worse hash_map_perf pre-alloc 27416 events per sec
3:worse hash_map_perf pre-alloc 28188 events per sec
ftrace trace
0) | htab_map_update_elem() {
0) 0.198 us | migrate_disable();
0) | _raw_spin_lock_irqsave() {
0) 0.157 us | preempt_count_add();
0) 0.538 us | }
0) 0.260 us | lookup_elem_raw();
0) | alloc_htab_elem() {
0) | __pcpu_freelist_pop() {
0) | _raw_spin_lock() {
0) 0.152 us | preempt_count_add();
0) 0.352 us | native_queued_spin_lock_slowpath();
0) 1.065 us | }
| ...
0) | _raw_spin_unlock() {
0) 0.254 us | preempt_count_sub();
0) 0.555 us | }
0) + 25.188 us | }
0) + 25.486 us | }
0) | _raw_spin_unlock_irqrestore() {
0) 0.155 us | preempt_count_sub();
0) 0.454 us | }
0) 0.148 us | migrate_enable();
0) + 28.439 us | }
The test machine is 16C, trying to get spin_lock 17 times, in addition
to 16c, there is an extralist.
after patch: hash_map performance
./map_perf_test 1
0:hash_map_perf pre-alloc 969348 events per sec
10:hash_map_perf pre-alloc 906526 events per sec
11:hash_map_perf pre-alloc 904557 events per sec
9:hash_map_perf pre-alloc 902384 events per sec
15:hash_map_perf pre-alloc 912287 events per sec
14:hash_map_perf pre-alloc 905689 events per sec
12:hash_map_perf pre-alloc 903680 events per sec
13:hash_map_perf pre-alloc 902631 events per sec
8:hash_map_perf pre-alloc 875369 events per sec
4:hash_map_perf pre-alloc 862808 events per sec
1:hash_map_perf pre-alloc 857218 events per sec
2:hash_map_perf pre-alloc 852875 events per sec
5:hash_map_perf pre-alloc 846497 events per sec
6:hash_map_perf pre-alloc 828467 events per sec
3:hash_map_perf pre-alloc 812542 events per sec
7:hash_map_perf pre-alloc 805336 events per sec
hash_map worst: no free
./map_perf_test 2048
7:worse hash_map_perf pre-alloc 391104 events per sec
4:worse hash_map_perf pre-alloc 388073 events per sec
5:worse hash_map_perf pre-alloc 387038 events per sec
1:worse hash_map_perf pre-alloc 386546 events per sec
0:worse hash_map_perf pre-alloc 384590 events per sec
11:worse hash_map_perf pre-alloc 379378 events per sec
10:worse hash_map_perf pre-alloc 375480 events per sec
12:worse hash_map_perf pre-alloc 372394 events per sec
6:worse hash_map_perf pre-alloc 367692 events per sec
3:worse hash_map_perf pre-alloc 363970 events per sec
9:worse hash_map_perf pre-alloc 364008 events per sec
8:worse hash_map_perf pre-alloc 363759 events per sec
2:worse hash_map_perf pre-alloc 360743 events per sec
14:worse hash_map_perf pre-alloc 361195 events per sec
13:worse hash_map_perf pre-alloc 360276 events per sec
15:worse hash_map_perf pre-alloc 360057 events per sec
0:worse hash_map_perf pre-alloc 378177 events per sec
ftrace trace
0) | htab_map_update_elem() {
0) 0.317 us | migrate_disable();
0) | _raw_spin_lock_irqsave() {
0) 0.260 us | preempt_count_add();
0) 1.803 us | }
0) 0.276 us | lookup_elem_raw();
0) | alloc_htab_elem() {
0) 0.586 us | __pcpu_freelist_pop();
0) 0.945 us | }
0) | _raw_spin_unlock_irqrestore() {
0) 0.160 us | preempt_count_sub();
0) 0.972 us | }
0) 0.657 us | migrate_enable();
0) 8.669 us | }
It can be seen that after adding this patch, the map performance is
almost not degraded, and when free=0, first check is_empty instead of
directly acquiring spin_lock.
As for why to add is_empty instead of directly judging head->first, my
understanding is this, head->first is frequently modified during updating
map, which will lead to invalid other cpus's cache, and is_empty is after
freelist having no free elems will be changed, the performance will be
better.
Co-developed-by: Chengming Zhou <zhouchengming@...edance.com>
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
Signed-off-by: Feng Zhou <zhoufeng.zf@...edance.com>
---
kernel/bpf/percpu_freelist.c | 68 ++++++++++++++++++++++++------------
kernel/bpf/percpu_freelist.h | 1 +
2 files changed, 46 insertions(+), 23 deletions(-)
diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
index 3d897de89061..abc148c8cae1 100644
--- a/kernel/bpf/percpu_freelist.c
+++ b/kernel/bpf/percpu_freelist.c
@@ -16,9 +16,11 @@ int pcpu_freelist_init(struct pcpu_freelist *s)
raw_spin_lock_init(&head->lock);
head->first = NULL;
+ head->is_empty = true;
}
raw_spin_lock_init(&s->extralist.lock);
s->extralist.first = NULL;
+ s->extralist.is_empty = true;
return 0;
}
@@ -32,6 +34,8 @@ static inline void pcpu_freelist_push_node(struct pcpu_freelist_head *head,
{
node->next = head->first;
head->first = node;
+ if (head->is_empty)
+ head->is_empty = false;
}
static inline void ___pcpu_freelist_push(struct pcpu_freelist_head *head,
@@ -130,14 +134,18 @@ static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
orig_cpu = cpu = raw_smp_processor_id();
while (1) {
head = per_cpu_ptr(s->freelist, cpu);
- raw_spin_lock(&head->lock);
- node = head->first;
- if (node) {
- head->first = node->next;
+ if (!head->is_empty) {
+ raw_spin_lock(&head->lock);
+ node = head->first;
+ if (node) {
+ head->first = node->next;
+ if (!head->first)
+ head->is_empty = true;
+ raw_spin_unlock(&head->lock);
+ return node;
+ }
raw_spin_unlock(&head->lock);
- return node;
}
- raw_spin_unlock(&head->lock);
cpu = cpumask_next(cpu, cpu_possible_mask);
if (cpu >= nr_cpu_ids)
cpu = 0;
@@ -146,11 +154,16 @@ static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
}
/* per cpu lists are all empty, try extralist */
- raw_spin_lock(&s->extralist.lock);
- node = s->extralist.first;
- if (node)
- s->extralist.first = node->next;
- raw_spin_unlock(&s->extralist.lock);
+ if (!s->extralist.is_empty) {
+ raw_spin_lock(&s->extralist.lock);
+ node = s->extralist.first;
+ if (node) {
+ s->extralist.first = node->next;
+ if (!s->extralist.first)
+ s->extralist.is_empty = true;
+ }
+ raw_spin_unlock(&s->extralist.lock);
+ }
return node;
}
@@ -164,14 +177,18 @@ ___pcpu_freelist_pop_nmi(struct pcpu_freelist *s)
orig_cpu = cpu = raw_smp_processor_id();
while (1) {
head = per_cpu_ptr(s->freelist, cpu);
- if (raw_spin_trylock(&head->lock)) {
- node = head->first;
- if (node) {
- head->first = node->next;
+ if (!head->is_empty) {
+ if (raw_spin_trylock(&head->lock)) {
+ node = head->first;
+ if (node) {
+ head->first = node->next;
+ if (!head->first)
+ head->is_empty = true;
+ raw_spin_unlock(&head->lock);
+ return node;
+ }
raw_spin_unlock(&head->lock);
- return node;
}
- raw_spin_unlock(&head->lock);
}
cpu = cpumask_next(cpu, cpu_possible_mask);
if (cpu >= nr_cpu_ids)
@@ -181,12 +198,17 @@ ___pcpu_freelist_pop_nmi(struct pcpu_freelist *s)
}
/* cannot pop from per cpu lists, try extralist */
- if (!raw_spin_trylock(&s->extralist.lock))
- return NULL;
- node = s->extralist.first;
- if (node)
- s->extralist.first = node->next;
- raw_spin_unlock(&s->extralist.lock);
+ if (!s->extralist.is_empty) {
+ if (!raw_spin_trylock(&s->extralist.lock))
+ return NULL;
+ node = s->extralist.first;
+ if (node) {
+ s->extralist.first = node->next;
+ if (!s->extralist.first)
+ s->extralist.is_empty = true;
+ }
+ raw_spin_unlock(&s->extralist.lock);
+ }
return node;
}
diff --git a/kernel/bpf/percpu_freelist.h b/kernel/bpf/percpu_freelist.h
index 3c76553cfe57..9e4545631ed5 100644
--- a/kernel/bpf/percpu_freelist.h
+++ b/kernel/bpf/percpu_freelist.h
@@ -9,6 +9,7 @@
struct pcpu_freelist_head {
struct pcpu_freelist_node *first;
raw_spinlock_t lock;
+ bool is_empty;
};
struct pcpu_freelist {
--
2.20.1
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