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Message-ID: <380fa11e-f15d-da1a-51f7-70e14ed58ffc@bytedance.com>
Date: Thu, 19 May 2022 11:12:48 +0800
From: Feng Zhou <zhoufeng.zf@...edance.com>
To: Yonghong Song <yhs@...com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Network Development <netdev@...r.kernel.org>,
bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
Xiongchun Duan <duanxiongchun@...edance.com>,
Muchun Song <songmuchun@...edance.com>,
Dongdong Wang <wangdongdong.6@...edance.com>,
Cong Wang <cong.wang@...edance.com>,
Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [External] Re: [PATCH] bpf: avoid grabbing spin_locks of all cpus
when no free elems
在 2022/5/19 上午4:39, Yonghong Song 写道:
>
>
> On 5/17/22 11:57 PM, Feng Zhou wrote:
>> 在 2022/5/18 下午2:32, Alexei Starovoitov 写道:
>>> On Tue, May 17, 2022 at 11:27 PM Feng zhou
>>> <zhoufeng.zf@...edance.com> wrote:
>>>> 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
>
> could you explain what parameter '1' means here?
This code is here:
samples/bpf/map_perf_test_user.c
samples/bpf/map_perf_test_kern.c
parameter '1' means testcase flag, test hash_map's performance
parameter '2048' means test hash_map's performance when free=0.
testcase flag '2048' is added by myself to reproduce the problem phenomenon.
>
>>>> 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.
>>> Is this with small max_entries and a large number of cpus?
>>>
>>> If so, probably better to fix would be to artificially
>>> bump max_entries to be 4x of num_cpus.
>>> Racy is_empty check still wastes the loop.
>>
>> This hash_map worst testcase with 16 CPUs, map's max_entries is 1000.
>>
>> This is the test case I constructed, it is to fill the map on
>> purpose, and then
>>
>> continue to update, just to reproduce the problem phenomenon.
>>
>> The bad case we encountered with 96 CPUs, map's max_entries is 10240.
>
> For such cases, most likely the map is *almost* full. What is the
> performance if we increase map size, e.g., from 10240 to 16K(16192)?
Yes, increasing max_entries can temporarily solve this problem, but when
16k is used up,
it will still encounter this problem. This patch is to try to fix this
corner case.
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