linux-kernel - Re: [PATCH bpf-next v2] bpf: Initialize same number of free nodes for each pcpu

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Date:   Thu, 10 Nov 2022 10:39:20 +0800
From:   Xu Kuohai <xukuohai@...weicloud.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 v2] bpf: Initialize same number of free nodes for
 each pcpu_freelist

On 11/10/2022 7:56 AM, Andrii Nakryiko wrote:
> On Tue, Nov 8, 2022 at 6:05 AM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>
>> From: Xu Kuohai <xukuohai@...wei.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@...wei.com>
>> Acked-by: Yonghong Song <yhs@...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
> 

that's great, will update to:

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

>>   }
>> --
>> 2.30.2
>>