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SubjectRe: [PATCH bpf-next v3] bpf: Initialize same number of free nodes for each pcpu_freelist
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On 11/10/2022 12:05 PM, Andrii Nakryiko wrote:
> On Wed, Nov 9, 2022 at 7:33 PM Xu Kuohai <xukuohai@huawei.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@huawei.com>
>> Acked-by: Yonghong Song <yhs@fb.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
>>
> .

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