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 10:56:30 -0700 [thread overview]
Message-ID: <CAEf4BzYUTvjAJ4uvYxBbbO7Vjh+K++F0HJe8mJ09RdhOeLeZGQ@mail.gmail.com> (raw)
In-Reply-To: <20200805174552.56q6eauad7glyzgm@ast-mbp.dhcp.thefacebook.com>
On Wed, Aug 5, 2020 at 10:45 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Aug 05, 2020 at 10:27:28AM -0700, Andrii Nakryiko wrote:
> > 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.
>
> See comment in trace_call_bpf().
> And logic to handle it in kprobe_perf_func() for kprobes.
> and in perf_trace_run_bpf_submit() for tracepoints.
> It's historical and Song actually discovered an issue with such behavior.
> I don't remember whether we've concluded on the solution.
Oh, thanks for pointers. Never realized there is more going on with
those. I guess return 1; is not advised then, as it causes extra
overhead.
next prev parent reply other threads:[~2020-08-05 19:19 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-01 8:47 [PATCH bpf-next 0/5] introduce BPF_PROG_TYPE_USER Song Liu
2020-08-01 8:47 ` [PATCH bpf-next 1/5] bpf: " Song Liu
2020-08-01 13:58 ` kernel test robot
2020-08-01 15:21 ` kernel test robot
2020-08-06 18:18 ` kernel test robot
2020-08-06 18:18 ` [RFC PATCH] bpf: user_verifier_ops can be static kernel test robot
2020-08-01 8:47 ` [PATCH bpf-next 2/5] libbpf: support BPF_PROG_TYPE_USER programs Song Liu
2020-08-03 1:40 ` Andrii Nakryiko
2020-08-03 4:21 ` Song Liu
2020-08-03 5:05 ` Andrii Nakryiko
2020-08-04 1:18 ` Song Liu
2020-08-05 1:38 ` Andrii Nakryiko
2020-08-05 3:59 ` Song Liu
2020-08-05 5:32 ` Andrii Nakryiko
2020-08-05 6:26 ` Song Liu
2020-08-05 6:54 ` Andrii Nakryiko
2020-08-05 7:23 ` Song Liu
2020-08-05 17:44 ` Andrii Nakryiko
2020-08-01 8:47 ` [PATCH bpf-next 3/5] selftests/bpf: add selftest for BPF_PROG_TYPE_USER Song Liu
2020-08-03 1:43 ` Andrii Nakryiko
2020-08-03 4:33 ` Song Liu
2020-08-03 5:07 ` Andrii Nakryiko
2020-08-01 8:47 ` [PATCH bpf-next 4/5] selftests/bpf: move two functions to test_progs.c Song Liu
2020-08-03 1:46 ` Andrii Nakryiko
2020-08-03 4:34 ` Song Liu
2020-08-01 8:47 ` [PATCH bpf-next 5/5] selftests/bpf: add benchmark for uprobe vs. user_prog Song Liu
2020-08-03 1:51 ` Andrii Nakryiko
2020-08-03 4:47 ` Song Liu
2020-08-03 5:10 ` Andrii Nakryiko
2020-08-04 20:54 ` Song Liu
2020-08-05 1:52 ` Andrii Nakryiko
2020-08-05 4:47 ` Song Liu
2020-08-05 5:47 ` Andrii Nakryiko
2020-08-05 7:01 ` Song Liu
2020-08-05 17:39 ` Andrii Nakryiko
2020-08-05 18:41 ` Song Liu
2020-08-05 17:16 ` Alexei Starovoitov
2020-08-05 17:27 ` Andrii Nakryiko
2020-08-05 17:45 ` Alexei Starovoitov
2020-08-05 17:56 ` Andrii Nakryiko [this message]
2020-08-05 18:56 ` Song Liu
2020-08-05 22:50 ` Alexei Starovoitov
2020-08-05 23:50 ` Song Liu
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