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Subject[PATCH v5 1/2] bpf: avoid grabbing spin_locks of all cpus when no free elems
Date
From: Feng Zhou <zhoufeng.zf@bytedance.com>

This patch use head->first 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 head->first instead of
directly acquiring spin_lock.

Co-developed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
---
kernel/bpf/percpu_freelist.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
index 3d897de89061..00b874c8e889 100644
--- a/kernel/bpf/percpu_freelist.c
+++ b/kernel/bpf/percpu_freelist.c
@@ -31,7 +31,7 @@ static inline void pcpu_freelist_push_node(struct pcpu_freelist_head *head,
struct pcpu_freelist_node *node)
{
node->next = head->first;
- head->first = node;
+ WRITE_ONCE(head->first, node);
}

static inline void ___pcpu_freelist_push(struct pcpu_freelist_head *head,
@@ -130,14 +130,17 @@ 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);
+ if (!READ_ONCE(head->first))
+ goto next_cpu;
raw_spin_lock(&head->lock);
node = head->first;
if (node) {
- head->first = node->next;
+ WRITE_ONCE(head->first, node->next);
raw_spin_unlock(&head->lock);
return node;
}
raw_spin_unlock(&head->lock);
+next_cpu:
cpu = cpumask_next(cpu, cpu_possible_mask);
if (cpu >= nr_cpu_ids)
cpu = 0;
@@ -146,10 +149,12 @@ static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
}

/* per cpu lists are all empty, try extralist */
+ if (!READ_ONCE(s->extralist.first))
+ return NULL;
raw_spin_lock(&s->extralist.lock);
node = s->extralist.first;
if (node)
- s->extralist.first = node->next;
+ WRITE_ONCE(s->extralist.first, node->next);
raw_spin_unlock(&s->extralist.lock);
return node;
}
@@ -164,15 +169,18 @@ ___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 (!READ_ONCE(head->first))
+ goto next_cpu;
if (raw_spin_trylock(&head->lock)) {
node = head->first;
if (node) {
- head->first = node->next;
+ WRITE_ONCE(head->first, node->next);
raw_spin_unlock(&head->lock);
return node;
}
raw_spin_unlock(&head->lock);
}
+next_cpu:
cpu = cpumask_next(cpu, cpu_possible_mask);
if (cpu >= nr_cpu_ids)
cpu = 0;
@@ -181,11 +189,11 @@ ___pcpu_freelist_pop_nmi(struct pcpu_freelist *s)
}

/* cannot pop from per cpu lists, try extralist */
- if (!raw_spin_trylock(&s->extralist.lock))
+ if (!READ_ONCE(s->extralist.first) || !raw_spin_trylock(&s->extralist.lock))
return NULL;
node = s->extralist.first;
if (node)
- s->extralist.first = node->next;
+ WRITE_ONCE(s->extralist.first, node->next);
raw_spin_unlock(&s->extralist.lock);
return node;
}
--
2.20.1
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