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

[Alexei Starovoitov]

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@xxxxxxxxx> 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.