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Date: Tue, 31 May 2022 11:24:30 +0800
From: Feng Zhou <zhoufeng.zf@...edance.com>
To: Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
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
Subject: Re: Re: [PATCH v3 1/2] bpf: avoid grabbing spin_locks of all cpus
when no free elems
在 2022/5/31 上午5:20, Daniel Borkmann 写道:
> On 5/30/22 11:13 AM, Feng zhou wrote:
>> From: Feng Zhou <zhoufeng.zf@...edance.com>
>>
>> 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 | 28 +++++++++++++++++++++++++---
>> kernel/bpf/percpu_freelist.h | 1 +
>> 2 files changed, 26 insertions(+), 3 deletions(-)
> [...]
>> /* per cpu lists are all empty, try extralist */
>> + if (s->extralist.is_empty)
>> + return NULL;
>> raw_spin_lock(&s->extralist.lock);
>> node = s->extralist.first;
>> - if (node)
>> + if (node) {
>> s->extralist.first = node->next;
>> + if (!s->extralist.first)
>> + s->extralist.is_empty = true;
>> + }
>> raw_spin_unlock(&s->extralist.lock);
>> return node;
>> }
>> @@ -164,15 +178,20 @@ ___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 (head->is_empty)
>
> This should use READ_ONCE/WRITE_ONCE pair for head->is_empty.
Yes, will do. Thanks.
>
>> + goto next_cpu;
>> 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);
>> }
>> +next_cpu:
>> cpu = cpumask_next(cpu, cpu_possible_mask);
>> if (cpu >= nr_cpu_ids)
>> cpu = 0;
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