linux-kernel - Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event

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Date:	Thu, 02 Jul 2015 11:37:51 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>, He Kuang <hekuang@...wei.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	rostedt@...dmis.org, masami.hiramatsu.pt@...achi.com,
	mingo@...hat.com, acme@...hat.com, jolsa@...nel.org,
	namhyung@...nel.org, linux-kernel@...r.kernel.org,
	pi3orama <pi3orama@....com>
Subject: Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event

On 7/2/15 2:24 AM, Wangnan (F) wrote:
> Yes, by using perf_trace_buf_prepare() + perf_trace_buf_submit() in
> helper function and let bpf program always returns 0 we can make data
> collected by BPF programs output into samples, if following problems
> are solved:
>
>   1. In bpf program there's no way to get 'struct perf_event' or 'struct
>      ftrace_event_call'. We have to deduce them through pt_regs:
>
>      pt_regs -> ip -> kprobe -> struct trace_kprobe -> struct
>       ftrace_event_call -> hlist_entry -> struct perf_event

yeah, going through hash table via get_kprobe() is not pretty.
How about using this_cpu_write(current_perf_event, ...) and using it
from the helper? bpf progs are non-preemptable and non-reentrable.
Also I think this helper would be more flexible if we can
allow passing sample_type into it.
Ideally from the program one could do something like:
bpf_event_output(buf, sizeof(buf), PERF_SAMPLE_CALLCHAIN);
which will prepare a sample with raw buf and callstack.
This way program can decide when and how send events to user space.

>   2. Even if we finally get 'struct perf_event', I'm not sure whether
>      user really concern on it. If we really concern on all information
>      output through perf_trace_buf_submit() like callstack and
>      register, why not make bpf program return non-zero instead? But then
>      we have to consider how to connect two samples together.

see my suggestion above. when sample_type was hard coded during event
creation it's a useful case on its own, but if we can make program to
provide this type dynamically, it will open whole new set of possibilities.

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