linux-kernel - Re: [PATCH bpf-next v3] bpf: Initialize same number of free nodes for each pcpu

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Message-ID: <80252c82-9c34-aefe-8422-fa247092dcec@huawei.com>
Date:   Thu, 10 Nov 2022 14:23:46 +0800
From:   Xu Kuohai <xukuohai@...wei.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     <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 v3] bpf: Initialize same number of free nodes for
 each pcpu_freelist

On 11/10/2022 12:05 PM, Andrii Nakryiko wrote:
> On Wed, Nov 9, 2022 at 7:33 PM Xu Kuohai <xukuohai@...wei.com> 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>
>> ---
>> v3: Simplify code as suggested by Andrii
>> v2: Update commit message and add Yonghong's ack
>> ---
>>   kernel/bpf/percpu_freelist.c | 27 ++++++++++++++-------------
>>   1 file changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
>> index b6e7f5c5b9ab..bd60070c079f 100644
>> --- a/kernel/bpf/percpu_freelist.c
>> +++ b/kernel/bpf/percpu_freelist.c
>> @@ -100,22 +100,23 @@ 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 = min(n + (cpu_idx < m ? 1 : 0), nr_elems);
> 
> why the min() here?
> 

to avoid out-of-bounds in case nr_elems is less than the total number of CPUs,
seems not very necessary, but the original code avoids this as well, I just kept
the logic

>> +               for (i = 0; i < j; i++) {
>> +                       head = per_cpu_ptr(s->freelist, cpu);
>> +                       /* No locking required as this is not visible yet. */
>> +                       pcpu_freelist_push_node(head, buf);
>> +                       buf += elem_size;
>> +               }
>> +               nr_elems -= j;
>> +               cpu_idx++;
>>          }
>>   }
>>
>> --
>> 2.30.2
>>
> .