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

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Date:   Wed, 5 Aug 2020 10:16:39 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...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 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.
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.