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Message-ID: <6aedba89-51f4-c889-d3cc-5f513defb920@bytedance.com>
Date:   Thu, 2 Jun 2022 11:27:49 +0800
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Subject: Re: Re: [PATCH v4 1/2] bpf: avoid grabbing spin_locks of all cpus
 when no free elems
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Xiongchun Duan <duanxiongchun@bytedance.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Dongdong Wang <wangdongdong.6@bytedance.com>,
        Cong Wang <cong.wang@bytedance.com>,
        Chengming Zhou <zhouchengming@bytedance.com>
References: <20220601084149.13097-1-zhoufeng.zf@bytedance.com>
 <20220601084149.13097-2-zhoufeng.zf@bytedance.com>
 <CAADnVQJcbDXtQsYNn=j0NzKx3SFSPE1YTwbmtkxkpzmFt-zh9Q@mail.gmail.com>
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 <CAADnVQKdU-3uBE9tKifChUunmr=c=32M4GwP8qG1-S=Atf7fvw@mail.gmail.com>
From:   Feng Zhou <zhoufeng.zf@bytedance.com>
In-Reply-To: <CAADnVQKdU-3uBE9tKifChUunmr=c=32M4GwP8qG1-S=Atf7fvw@mail.gmail.com>
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在 2022/6/1 下午7:35, Alexei Starovoitov 写道:
> On Wed, Jun 1, 2022 at 1:11 PM Feng Zhou <zhoufeng.zf@bytedance.com> wrote:
>> 在 2022/6/1 下午5:50, Alexei Starovoitov 写道:
>>> On Wed, Jun 1, 2022 at 10:42 AM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>>>    static inline void ___pcpu_freelist_push(struct pcpu_freelist_head *head,
>>>> @@ -130,14 +134,19 @@ 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->is_empty))
>>>> +                       goto next_cpu;
>>>>                   raw_spin_lock(&head->lock);
>>>>                   node = head->first;
>>>>                   if (node) {
>>> extra bool is unnecessary.
>>> just READ_ONCE(head->first)
>> 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.
> maybe. pls benchmark it.
> imo wasting a bool for the corner case is not a good trade off.

Yes, I will do and post the results as soon as possible, Thanks.