linux-kernel - Re: [PATCH bpf-next] bpf: Initialize same number of free nodes for each pcpu

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Date:   Mon, 7 Nov 2022 08:40:14 -0800
From:   Yonghong Song <yhs@...a.com>
To:     Xu Kuohai <xukuohai@...weicloud.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH bpf-next] bpf: Initialize same number of free nodes for
 each pcpu_freelist



On 11/7/22 12:50 AM, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@...wei.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@...wei.com>

LGTM. Did you observe any performance issues?

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