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SubjectRe: [PATCH bpf-next] bpf: Initialize same number of free nodes for each pcpu_freelist
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On 11/7/22 12:50 AM, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@huawei.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.

> 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@huawei.com>

LGTM. Did you observe any performance issues?

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

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