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Message-ID: <21ec90e3-2e89-09c1-fd22-de76e6794d68@bytedance.com>
Date:   Wed, 1 Jun 2022 19:10:54 +0800
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Subject: Re: [External] 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>
From:   Feng Zhou <zhoufeng.zf@bytedance.com>
In-Reply-To: <CAADnVQJcbDXtQsYNn=j0NzKx3SFSPE1YTwbmtkxkpzmFt-zh9Q@mail.gmail.com>
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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.
If I'm thinking wrong, please tell me why.