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

Xu Kuohai
2022-11-11T07:00:51Z

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@huawei.com>
>> Acked-by: Yonghong Song <yhs@fb.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

> 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++;
>>       }
>>   }
>
> .