linux-kernel - Re: [PATCH 32/32] bpf: Introduce function for outputing data to perf event

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Message-ID: <55E10091.6020107@plumgrid.com>
Date:	Fri, 28 Aug 2015 17:45:05 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Wang Nan <wangnan0@...wei.com>, acme@...hat.com
Cc:	brendan.d.gregg@...il.com, daniel@...earbox.net, dsahern@...il.com,
	hekuang@...wei.com, jolsa@...nel.org, xiakaixu@...wei.com,
	masami.hiramatsu.pt@...achi.com, namhyung@...nel.org,
	a.p.zijlstra@...llo.nl, lizefan@...wei.com, pi3orama@....com,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 32/32] bpf: Introduce function for outputing data to perf
 event

On 8/28/15 12:06 AM, Wang Nan wrote:
> his patch adds a new trace event to establish infrastruction for bpf to
> output data to perf. Userspace perf tools can detect and use this event
> as using the existing tracepoint events.
>
> New bpf trace event entry in debugfs:
>
>       /sys/kernel/debug/tracing/events/bpf/bpf_output_data
>
> Userspace perf tools detect the new tracepoint event as:
>
>       bpf:bpf_output_data                          [Tracepoint event]
>
> Data in ring-buffer of perf events added to this event will be polled
> out, sample types and other attributes can be adjusted to those events
> directly without touching the original kprobe events.

Wang,
I have 2nd thoughts on this.
I've played with it, but global bpf:bpf_output_data event is limiting.
I'd like to use this bpf_output_trace_data() helper for tcp estats
gathering, but global collector will prevent other similar bpf programs
running in parallel.
So as a concept I think it's very useful, but we need a way to select
which ring-buffer to output data to.
proposal A:
Can we use ftrace:instances concept and make bpf_output_trace_data()
into that particular trace_pipe ?
proposal B:
bpf_perf_event_read() model is using nice concept of an array of
perf_events. Can we perf_event_open a 'new' event that can be mmaped
in user space and bpf_output_trace_data(idx, buf, buf_size) into it.
Where 'idx' will be an index of FD from perf_even_open of such
new event?

Thanks!

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