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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 9 Nov 2022 15:56:26 -0800
Message-ID: <CAEf4BzYjkP_xjaEErXwe5mG9pV+HQHKwY3hTamKH6zQTZrobLw@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2] bpf: Initialize same number of free nodes for
 each pcpu_freelist
To:     Xu Kuohai <xukuohai@huaweicloud.com>
Cc:     bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>
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On Tue, Nov 8, 2022 at 6:05 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> From: Xu Kuohai <xukuohai@huawei.com>
>
> pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
> free nodes for some CPUs, and then possibly one CPU with fewer nodes,
> followed by remaining cpus with 0 nodes. For example, when nr_elems == 256
> and num_possible_cpus() == 32, if CPU 0 is the current cpu, CPU 0~27
> each gets 9 free nodes, CPU 28 gets 4 free nodes, CPU 29~31 get 0 free
> nodes, while in fact each CPU should get 8 nodes equally.
>
> This patch initializes nr_elems / num_possible_cpus() free nodes for each
> CPU firstly, then allocates the remaining free nodes by one for each CPU
> until no free nodes left.
>
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> Acked-by: Yonghong Song <yhs@fb.com>
> ---
> v2: Update commit message and add Yonghong's ack
> ---
>  kernel/bpf/percpu_freelist.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> index b6e7f5c5b9ab..89e84f7381cc 100644
> --- a/kernel/bpf/percpu_freelist.c
> +++ b/kernel/bpf/percpu_freelist.c
> @@ -100,12 +100,15 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
>                             u32 nr_elems)
>  {
>         struct pcpu_freelist_head *head;
> -       int i, cpu, pcpu_entries;
> +       int i, cpu, pcpu_entries, remain_entries;
> +
> +       pcpu_entries = nr_elems / num_possible_cpus();
> +       remain_entries = nr_elems % num_possible_cpus();
>
> -       pcpu_entries = nr_elems / num_possible_cpus() + 1;
>         i = 0;
>
>         for_each_possible_cpu(cpu) {
> +               int j = i + pcpu_entries + (remain_entries-- > 0 ? 1 : 0);
>  again:
>                 head = per_cpu_ptr(s->freelist, cpu);
>                 /* No locking required as this is not visible yet. */
> @@ -114,7 +117,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
>                 buf += elem_size;
>                 if (i == nr_elems)
>                         break;
> -               if (i % pcpu_entries)
> +               if (i < j)
>                         goto again;
>         }

this loop's logic is quite hard to follow, if we are fixing it, can we
simplify it maybe? something like:

int cpu, cpu_idx, i, j, n, m;

n = nr_elems / num_possible_cpus();
m = nr_elems % num_possible_cpus();

for_each_possible_cpu(cpu) {
    i = n + (cpu_idx < m ? 1 : 0);
    for (j = 0; j < i; j++) {
        head = per_cpu_ptr(s->freelist, cpu);
        pcpu_freelist_push_node(head, buf);
        buf += elem_size;
    }
    cpu_idx++;
}


no gotos, no extra ifs: for each cpu we determine correct number of
elements to allocate, then just allocate them in a straightforward
loop

>  }
> --
> 2.30.2
>