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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Song Liu <songliubraving@fb.com>,
	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: Wed, 5 Aug 2020 10:27:28 -0700	[thread overview]
Message-ID: <CAEf4BzYFfAubxo1QY6Axth=gwS9DfzwRkvnYLspfk9tLia0LPg@mail.gmail.com> (raw)
In-Reply-To: <20200805171639.tsqjmifd7eb3htou@ast-mbp.dhcp.thefacebook.com>

On Wed, Aug 5, 2020 at 10:16 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Aug 05, 2020 at 04:47:30AM +0000, Song Liu wrote:
> >
> > Being able to trigger BPF program on a different CPU could enable many
> > use cases and optimizations. The use case I am looking at is to access
> > perf_event and percpu maps on the target CPU. For example:
> >       0. trigger the program
> >       1. read perf_event on cpu x;
> >       2. (optional) check which process is running on cpu x;
> >       3. add perf_event value to percpu map(s) on cpu x.
>
> If the whole thing is about doing the above then I don't understand why new
> prog type is needed. Can prog_test_run support existing BPF_PROG_TYPE_KPROBE?
> "enable many use cases" sounds vague. I don't think folks reading
> the patches can guess those "use cases".
> "Testing existing kprobe bpf progs" would sound more convincing to me.

Was just about to propose the same :) I wonder if generic test_run()
capability to trigger test programs of whatever supported type on a
specified CPU through IPI can be added. That way you can even use the
XDP program to do what Song seems to need.

TRACEPOINTs might also be a good fit here, given it seems simpler to
let users specify custom tracepoint data for test_run(). Having the
ability to unit-test KPROBE and TRACEPOINT, however rudimentary, is
already a big win.

> If the test_run framework can be extended to trigger kprobe with correct pt_regs.
> As part of it test_run would trigger on a given cpu with $ip pointing
> to some test fuction in test_run.c. For local test_run the stack trace
> would include bpf syscall chain. For IPI the stack trace would include
> the corresponding kernel pieces where top is our special test function.
> Sort of like pseudo kprobe where there is no actual kprobe logic,
> since kprobe prog doesn't care about mechanism. It needs correct
> pt_regs only as input context.
> The kprobe prog output (return value) has special meaning though,
> so may be kprobe prog type is not a good fit.

It does? I don't remember returning 1 from KPROBE changing anything. I
thought it's only the special bpf_override_return() that can influence
the kernel function return result.

> Something like fentry/fexit may be better, since verifier check_return_code()
> enforces 'return 0'. So their return value is effectively "void".
> Then prog_test_run would need to gain an ability to trigger
> fentry/fexit prog on a given cpu.