netdev - Re: [PATCH 06/15] bpf: Add bpf_ksym

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Date:   Tue, 3 Mar 2020 21:12:26 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...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>,
        Song Liu <song@...nel.org>
Subject: Re: [PATCH 06/15] bpf: Add bpf_ksym_tree tree

On Tue, Mar 03, 2020 at 10:03:19AM -0800, Alexei Starovoitov wrote:
> On Mon, Mar 02, 2020 at 03:31:45PM +0100, Jiri Olsa wrote:
> > The bpf_tree is used both for kallsyms iterations and searching
> > for exception tables of bpf programs, which is needed only for
> > bpf programs.
> > 
> > Adding bpf_ksym_tree that will hold symbols for all bpf_prog
> > bpf_trampoline and bpf_dispatcher objects and keeping bpf_tree
> > only for bpf_prog objects to keep it fast.
> 
> ...
> 
> >  static void bpf_prog_ksym_node_add(struct bpf_prog_aux *aux)
> > @@ -616,6 +650,7 @@ static void bpf_prog_ksym_node_add(struct bpf_prog_aux *aux)
> >  	WARN_ON_ONCE(!list_empty(&aux->ksym.lnode));
> >  	list_add_tail_rcu(&aux->ksym.lnode, &bpf_kallsyms);
> >  	latch_tree_insert(&aux->ksym_tnode, &bpf_tree, &bpf_tree_ops);
> > +	latch_tree_insert(&aux->ksym.tnode, &bpf_ksym_tree, &bpf_ksym_tree_ops);
> >  }
> >  
> >  static void bpf_prog_ksym_node_del(struct bpf_prog_aux *aux)
> > @@ -624,6 +659,7 @@ static void bpf_prog_ksym_node_del(struct bpf_prog_aux *aux)
> >  		return;
> >  
> >  	latch_tree_erase(&aux->ksym_tnode, &bpf_tree, &bpf_tree_ops);
> > +	latch_tree_erase(&aux->ksym.tnode, &bpf_ksym_tree, &bpf_ksym_tree_ops);
> 
> I have to agree with Daniel here.
> Having bpf prog in two latch trees is unnecessary.
> Especially looking at the patch 7 that moves update to the other tree.
> The whole thing becomes assymetrical and harder to follow.
> Consider that walking extable is slow anyway. It's a page fault.
> Having trampoline and dispatch in the same tree will not be measurable
> on the speed of search_bpf_extables->bpf_prog_kallsyms_find.
> So please consolidate.

ok

> 
> Also I don't see a hunk that deletes tnode from 'struct bpf_image'.
> These patches suppose to generalize it too, no?

__bpf_ksym_del function added in patch:

    bpf: Separate kallsyms add/del functions

> And at the end kernel_text_address() suppose to call
> is_bpf_text_address() only, right?
> Instead of is_bpf_text_address() || is_bpf_image_address() ?
> That _will_ actually speed up backtrace collection.

right, this one could have already used just the ksym tree

will send new version.. meanwhile I was checking struct_ops,
so will include kallsyms support them as well

thanks,
jirka