Re: [PATCH bpf-next 5/5] selftests/bpf: add benchmark for uprobe vs. user_prog

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: open list <linux-kernel@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <Kernel-team@fb.com>,
	john fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Daniel Xu <dlxu@fb.com>
Subject: Re: [PATCH bpf-next 5/5] selftests/bpf: add benchmark for uprobe vs. user_prog
Date: Sun, 2 Aug 2020 22:10:31 -0700	[thread overview]
Message-ID: <CAEf4BzY5RYMM6w8wn3qEB3AsuKWv-TMaD5NVFj=YqbCW4DLjqA@mail.gmail.com> (raw)
In-Reply-To: <DDCD362E-21D3-46BF-90A6-8F3221CBB54E@fb.com>

On Sun, Aug 2, 2020 at 9:47 PM Song Liu <songliubraving@fb.com> wrote:
>
>
> > On Aug 2, 2020, at 6:51 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > On Sat, Aug 1, 2020 at 1:50 AM Song Liu <songliubraving@fb.com> wrote:
> >>
> >> Add a benchmark to compare performance of
> >>  1) uprobe;
> >>  2) user program w/o args;
> >>  3) user program w/ args;
> >>  4) user program w/ args on random cpu.
> >>
> >
> > Can you please add it to the existing benchmark runner instead, e.g.,
> > along the other bench_trigger benchmarks? No need to re-implement
> > benchmark setup. And also that would also allow to compare existing
> > ways of cheaply triggering a program vs this new _USER program?
>
> Will try.
>
> >
> > If the performance is not significantly better than other ways, do you
> > think it still makes sense to add a new BPF program type? I think
> > triggering KPROBE/TRACEPOINT from bpf_prog_test_run() would be very
> > nice, maybe it's possible to add that instead of a new program type?
> > Either way, let's see comparison with other program triggering
> > mechanisms first.
>
> Triggering KPROBE and TRACEPOINT from bpf_prog_test_run() will be useful.
> But I don't think they can be used instead of user program, for a couple
> reasons. First, KPROBE/TRACEPOINT may be triggered by other programs
> running in the system, so user will have to filter those noise out in
> each program. Second, it is not easy to specify CPU for KPROBE/TRACEPOINT,
> while this feature could be useful in many cases, e.g. get stack trace
> on a given CPU.
>

Right, it's not as convenient with KPROBE/TRACEPOINT as with the USER
program you've added specifically with that feature in mind. But if
you pin user-space thread on the needed CPU and trigger kprobe/tp,
then you'll get what you want. As for the "noise", see how
bench_trigger() deals with that: it records thread ID and filters
everything not matching. You can do the same with CPU ID. It's not as
automatic as with a special BPF program type, but still pretty simple,
which is why I'm still deciding (for myself) whether USER program type
is necessary :)

> Thanks,
> Song