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

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Date:   Wed, 5 Aug 2020 18:56:26 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "john fastabend" <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>,
        Daniel Xu <dlxu@...com>
Subject: Re: [PATCH bpf-next 5/5] selftests/bpf: add benchmark for uprobe vs.
 user_prog



> On Aug 5, 2020, at 10:16 AM, Alexei Starovoitov <alexei.starovoitov@...il.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.

I was under the (probably wrong) impression that adding prog type is not
that big a deal. 

> Can prog_test_run support existing BPF_PROG_TYPE_KPROBE?

I haven't looked into all the details, but I bet this is possible.

> "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.
> 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.
> 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.

Maybe we add a new attach type for BPF_PROG_TYPE_TRACING, which is in 
parallel with BPF_TRACE_FENTRY and BPF_TRACE_EXIT? Say BPF_TRACE_USER? 
(Just realized I like this name :-D, it matches USDT...). Then we can 
enable test_run for most (if not all) tracing programs, including
fentry/fexit. 

Does this sound like a good plan?

Thanks,
Song