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

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Date:	Sat, 29 Aug 2015 10:15:25 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.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 2015/8/29 9:34, Alexei Starovoitov wrote:
> On 8/28/15 6:19 PM, Wangnan (F) wrote:
>> For me, I use bpf_output_trace_data() to output information like PMU 
>> count
>> value. Perf is the only receiver, so global collector is perfect. 
>> Could you
>> please describe your usecase in more detail?
>
> there is a special receiver in user space that only wants the data from
> the bpf program that it loaded. It shouldn't conflict with any other
> processes. Like when it's running, I still should be able to use perf
> for other performance analysis. There is no way to share single
> bpf:bpf_output_data event, since these user processes are completely
> independent.
>
I'd like to see whether it is possible to create dynamic tracepoints so
different receivers can listen on different tracepoints. For my side, maybe
I can encode format information into the new tracepoints so don't need
those LLVM patches.

For example:

# echo 'dynamic_tracepoint:mytracepoint <encode its format>' >> 
/sys/kernel/debug/tracing/dynamic_trace_events
# perf list
   ...
   dynamic_tracepoint:mytracepoint
   ...

In perf side we can encode the creation of dynamic tracepoint into 
bpf-loader
like what we currectly do for probing the kprobes.

This way reqires us to create a fresh new event source, in parallel with 
tracepoint. I'm not sure
how much work it needs. What do you think?

Thank you.

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