Re: [PATCH 06/15] bpf: Add bpf_ksym_tree tree

Alexei Starovoitov
2020-03-03T18:03:26Z

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.

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