On Tue, Nov 8, 2022 at 6:05 AM Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> wrote:
>
> From: Xu Kuohai <xukuohai@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> ---
> 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
> }
> --
> 2.30.2
>
