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

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Date:   Tue, 8 Nov 2022 21:38:02 +0800
From:   Xu Kuohai <xukuohai@...weicloud.com>
To:     Yonghong Song <yhs@...a.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     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] bpf: Initialize same number of free nodes for
 each pcpu_freelist

On 11/8/2022 12:40 AM, Yonghong Song wrote:
> 
> 
> On 11/7/22 12:50 AM, Xu Kuohai wrote:
>> From: Xu Kuohai <xukuohai@...wei.com>
>>
>> pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
>> free nodes for each CPU except the last initialized CPU, always making
>> the last CPU get fewer free nodes. For example, when nr_elems == 256
> 
> ... free nodes for some cpus, and then possibly one cpu with fewer nodes, followed by remaining cpus with 0 nodes.
> 

Will update the commit message to describe it more accurately, thanks.

>> 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, and then allocates the remaining free nodes by one for each
>> CPU until no free nodes left.
>>
>> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
> 
> LGTM. Did you observe any performance issues?
>

No. I ran map_perf_test and did not observe any performance issues. I think
it's because the test cases are repeated in loops, so the pcpu_freelists become
stable and balanced after the first few loops.

> Acked-by: Yonghong Song <yhs@...com>
> 
>> ---
>>   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;
>>       }
>>   }