linux-kernel - Re: [PATCH] bpf: Separate bpf_local_storage

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Date: Mon, 5 Feb 2024 16:00:15 +0100
From: Marco Elver <elver@...gle.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, 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>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] bpf: Separate bpf_local_storage_lookup() fast and slow paths

On Wed, 31 Jan 2024 at 20:52, Martin KaFai Lau <martin.lau@...ux.dev> wrote:
[...]
> > | num_maps: 1000
> > |  local_storage cache sequential  get:
> > |                              <before>                | <after>
> > |   hits throughput:           0.357 ± 0.005 M ops/s   | 0.325 ± 0.005 M ops/s        (-9.0%)
> > |   hits latency:              2803.738 ns/op          | 3076.923 ns/op               (+9.7%)
>
> Is it understood why the slow down here? The same goes for the "num_maps: 32"
> case above but not as bad as here.

It turned out that there's a real slowdown due to the outlined
slowpath. If I inline everything except for inserting the entry into
the cache (cacheit_lockit codepath is still outlined), the results
look much better even for the case where it always misses the cache.

[...]
> > diff --git a/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c b/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
> > index a043d8fefdac..9895087a9235 100644
> > --- a/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
> > +++ b/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
> > @@ -21,7 +21,7 @@ struct {
> >       __type(value, long);
> >   } map_b SEC(".maps");
> >
> > -SEC("fentry/bpf_local_storage_lookup")
> > +SEC("fentry/bpf_local_storage_lookup_slowpath")
>
> The selftest is trying to catch recursion. The change here cannot test the same
> thing because the slowpath will never be hit in the test_progs.  I don't have a
> better idea for now also.

Trying to prepare a v2, and for the test, the only option I see is to
introduce a tracepoint ("bpf_local_storage_lookup"). If unused, should
be a no-op due to static branch.

Or can you suggest different functions to hook to for the recursion test?

Preferences?