netdev - Re: [PATCH 15/18] bpf: Sort bpf kallsyms symbols

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Date:   Tue, 18 Feb 2020 15:18:17 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...com>, Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 15/18] bpf: Sort bpf kallsyms symbols

On Sun, Feb 16, 2020 at 08:30:02PM +0100, Jiri Olsa wrote:
> Currently we don't sort bpf_kallsyms and display symbols
> in proc/kallsyms as they come in via __bpf_ksym_add.
> 
> Using the latch tree to get the next bpf_ksym object
> and insert the new symbol ahead of it.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/core.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 2f857bbfe05c..fa814179730c 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -651,9 +651,28 @@ static struct latch_tree_root bpf_progs_tree __cacheline_aligned;
>  
>  static void __bpf_ksym_add(struct bpf_ksym *ksym)
>  {
> +	struct list_head *head = &bpf_kallsyms;
> +	struct rb_node *next;
> +
>  	WARN_ON_ONCE(!list_empty(&ksym->lnode));
> -	list_add_tail_rcu(&ksym->lnode, &bpf_kallsyms);
>  	latch_tree_insert(&ksym->tnode, &bpf_ksym_tree, &bpf_ksym_tree_ops);
> +
> +	/*
> +	 * Add ksym into bpf_kallsyms in ordered position,
> +	 * which is prepared for us by latch tree addition.
> +	 *
> +	 * Find out the next symbol and insert ksym right
> +	 * ahead of it. If ksym is the last one, just tail
> +	 * add to the bpf_kallsyms.
> +	 */
> +	next = rb_next(&ksym->tnode.node[0]);
> +	if (next) {
> +		struct bpf_ksym *ptr;
> +
> +		ptr = container_of(next, struct bpf_ksym, tnode.node[0]);
> +		head = &ptr->lnode;
> +	}
> +	list_add_tail_rcu(&ksym->lnode, head);

what is the motivation for sorting? do you want perf and other user space
to depend on it? Or purely aesthetics?