linux-kernel - Re: [PATCH bpf-next v4] bpf: Initialize same number of free nodes for each pcpu

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Date:   Fri, 11 Nov 2022 12:12:49 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Xu Kuohai <xukuohai@...wei.com>
Cc:     wuqiang <wuqiang.matt@...edance.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH bpf-next v4] bpf: Initialize same number of free nodes for
 each pcpu_freelist

On Thu, Nov 10, 2022 at 11:00 PM Xu Kuohai <xukuohai@...wei.com> wrote:
>
> On 11/11/2022 11:53 AM, wuqiang wrote:
> > On 2022/11/10 20:21, Xu Kuohai wrote:
> >> 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, 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@...wei.com>
> >> Acked-by: Yonghong Song <yhs@...com>
> >> ---
> >> v4: Remove unneeded min()
> >> v3: Simplify code as suggested by Andrii
> >> v2: Update commit message and add Yonghong's ack
> >> ---
> >>   kernel/bpf/percpu_freelist.c | 26 +++++++++++++-------------
> >>   1 file changed, 13 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> >> index b6e7f5c5b9ab..27f2c4aff623 100644
> >> --- a/kernel/bpf/percpu_freelist.c
> >> +++ b/kernel/bpf/percpu_freelist.c
> >> @@ -100,22 +100,22 @@ 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;
> >> +    unsigned int cpu, cpu_idx, i, j, n, m;
> >> -    pcpu_entries = nr_elems / num_possible_cpus() + 1;
> >> -    i = 0;
> >> +    n = nr_elems / num_possible_cpus();
> >> +    m = nr_elems % num_possible_cpus();
> >> +
> >> +    cpu_idx = 0;
> >>       for_each_possible_cpu(cpu) {
> >> -again:
> >> -        head = per_cpu_ptr(s->freelist, cpu);
> >> -        /* No locking required as this is not visible yet. */
> >> -        pcpu_freelist_push_node(head, buf);
> >> -        i++;
> >> -        buf += elem_size;
> >> -        if (i == nr_elems)
> >> -            break;
> >> -        if (i % pcpu_entries)
> >> -            goto again;
> >> +        j = n + (cpu_idx < m ? 1 : 0);
> >> +        for (i = 0; i < j; i++) {
> >> +            head = per_cpu_ptr(s->freelist, cpu);
> >
> > Better move it out of "i-loop",
>
> OK, will do
>

I did that while applying. Also added

Fixes: e19494edab82 ("bpf: introduce percpu_freelist")

Please don't forget to add Fixes tag for future patches.

Applied to bpf tree.

> > and rename "j" to a meaningful name to avoid
> > possible misuse.
> >
> The loop is short enough to be readable and "j" is not used elsewhere, so I
> think it's good to keep the name simple.
>
> >> +            /* No locking required as this is not visible yet. */
> >> +            pcpu_freelist_push_node(head, buf);
> >> +            buf += elem_size;
> >> +        }
> >> +        cpu_idx++;
> >>       }
> >>   }
> >
> > .
>