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

[wuqiang <wuqiang.matt@bytedance.com>]

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", and rename "j" to a meaningful name to avoid
possible misuse.

> +			/* No locking required as this is not visible yet. */
> +			pcpu_freelist_push_node(head, buf);
> +			buf += elem_size;
> +		}
> +		cpu_idx++;
>   	}
>   }
>