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Message-ID: <CAADnVQKdU-3uBE9tKifChUunmr=c=32M4GwP8qG1-S=Atf7fvw@mail.gmail.com>
Date: Wed, 1 Jun 2022 13:35:19 +0200
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Feng Zhou <zhoufeng.zf@...edance.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>, Yonghong Song <yhs@...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 v4 1/2] bpf: avoid grabbing spin_locks of
all cpus when no free elems
On Wed, Jun 1, 2022 at 1:11 PM Feng Zhou <zhoufeng.zf@...edance.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@...edance.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.
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