netdev - Re: [PATCH bpf-next v2 4/4] libbpf: Add option to auto-pin maps when opening BPF object

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Date:   Sun, 27 Oct 2019 13:04:35 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 4/4] libbpf: Add option to auto-pin maps when opening BPF object

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> On Thu, Oct 24, 2019 at 6:11 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> From: Toke Høiland-Jørgensen <toke@...hat.com>
>>
>> With the functions added in previous commits that can automatically pin
>> maps based on their 'pinning' setting, we can support auto-pinning of maps
>> by the simple setting of an option to bpf_object__open.
>>
>> Since auto-pinning only does something if any maps actually have a
>> 'pinning' BTF attribute set, we default the new option to enabled, on the
>> assumption that seamless pinning is what most callers want.
>>
>> When a map has a pin_path set at load time, libbpf will compare the map
>> pinned at that location (if any), and if the attributes match, will re-use
>> that map instead of creating a new one. If no existing map is found, the
>> newly created map will instead be pinned at the location.
>>
>> Programs wanting to customise the pinning can override the pinning paths
>> using bpf_map__set_pin_path() before calling bpf_object__load().
>>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>
> How have you tested this? From reading the code, all the maps will be
> pinned irregardless of their .pinning setting?

No, build_pin_path() checks map->pinning :)

> Please add proper tests to test_progs, testing various modes and
> overrides.

Can do.

> You keep trying to add more and more knobs :) Please stop doing that,
> even if we have a good mechanism for extensibility, it doesn't mean we
> need to increase a proliferation of options.

But I like options! ;)

> Each option has to be tested. In current version of your patches, you
> have something like 4 or 5 different knobs, do you really want to
> write tests testing each of them? ;)

Heh, I guess I can cut down the number of options to the number of tests :P

> Another high-level feedback. I think having separate passes over all
> maps (build_map_pin_paths, reuse, then we already have create_maps) is
> actually making everything more verbose and harder to extend. I'm
> thinking about all these as sub-steps of map creation. Can you please
> try refactoring so all these steps are happening per each map in one
> place: if map needs to be pinned, check if it can be reused, if not -
> create it. This actually will allow to handle races better, because
> you will be able to retry easily, while if it's all spread in
> independent passes, it becomes much harder. Please consider that.

We'll need at least two passes: set pin_path on open, and check reuse /
create / pin on load. Don't have any objections to consolidating the
other passes into create_maps; will fix, along with your comments below.

-Toke