linux-kernel - Re: [External] Re: [PATCH] bpf: avoid grabbing spin

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Date:   Fri, 20 May 2022 11:02:24 +0800
From:   Feng Zhou <zhoufeng.zf@...edance.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Yonghong Song <yhs@...com>, 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 2022/5/20 上午12:12, Alexei Starovoitov 写道:
> On Wed, May 18, 2022 at 8:12 PM Feng Zhou <zhoufeng.zf@...edance.com> wrote:
>> 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.
> Please convert it to selftests/bpf/bench,
> so that everyone can reproduce the issue you're seeing
> and can assess whether it's a real issue or a corner case.
>
> Also please avoid adding indent in the patch.
> Instead of
>   if (!s->extralist.is_empty) {
>    .. churn
>
> do
>
>   if (s->extralist.is_empty)

Ok, will do. Thanks.