linux-kernel - Re: [RFC PATCH v4 10/29] bpf tools: Collect map definitions from 'maps' section

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Date:	Mon, 1 Jun 2015 15:03:11 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	paulus@...ba.org, a.p.zijlstra@...llo.nl, mingo@...hat.com,
	acme@...nel.org, jolsa@...nel.org, dsahern@...il.com,
	daniel@...earbox.net, brendan.d.gregg@...il.com,
	masami.hiramatsu.pt@...achi.com, lizefan@...wei.com,
	linux-kernel@...r.kernel.org, pi3orama@....com,
	xiakaixu 00238161 <xiakaixu@...wei.com>
Subject: Re: [RFC PATCH v4 10/29] bpf tools: Collect map definitions from
 'maps' section

On Mon, Jun 01, 2015 at 01:19:16PM +0800, Wangnan (F) wrote:
> 
> 
> On 2015/6/1 10:12, Namhyung Kim wrote:
> >Hi Alexei and Wang,
> >
> >On Thu, May 28, 2015 at 08:35:19PM -0700, Alexei Starovoitov wrote:
> >>On Thu, May 28, 2015 at 03:14:44PM +0800, Wangnan (F) wrote:
> >>>On 2015/5/28 14:09, Alexei Starovoitov wrote:
> >>As far as 'bpf_store_value' goes... I was thinking to expose perf ring_buffer
> >>to bpf programs, so that program can stream any data to perf that receives
> >>it via mmap. Then you don't need this '$outdata' hack.
> >Then we need to define and pass the format of such data so that perf
> >tools can read and process the data.  IIRC Masami suggested to have an
> >additional user event type for inserting/injecting non-perf events -
> >like PERF_RECORD_USER_DEFINED_TYPE?  And its contents is something
> >similar to tracepoint event format file so that we can reuse existing
> >code to parse the event definition.
> 
> Is it possible to expose such format through
> /sys/kernel/debug/tracing/events/*/*/format
> so we can avoid extra work on perf side and make it accessable by both perf
> and ftrace?

No, I mean export such format through an event in perf.data file.  It
still needs extra work on perf-tools side.  But by using user-defined
event types, there should be no kernel-side work.

Above is just a suggestion how to deal with external data/events in
perf.  But I'm seeing many people want such feature so we need a way
to handle it anyway. ;-)

> 
> Currently we do this by opening an internal PMU and adding a common
> field in trace_define_common_fields(). By reading that PMU in
> tracing_generic_entry_update() we are able to collect its value by both perf
> and ftrace, both
> kprobe events and tracepoints (the implementation is ugly. We have to
> hardwire the
> PMU because alerting common field dynamically is hard. If we want to trace
> multiple PMUs then
> recompiling is required). In serval usecase, we found that using ftrace
> should be better
> because the cost of perf is higher.
> 
> Although currently BPF programs can only get executed if it traced by perf,
> I think we can
> extend it to ftrace (but not sure how to do it now...).
> 
> Currently I'm still working on perf bpf stuffs. I think it has almost done.
> The next step
> should be solving arguments passing problem. After that we should enable
> eBPF programs to
> read hardware PMU. Outputting should be the final step. I'm glad to see many
> people are
> thinking on it. Please keep me in the loop if you have any new idea on this
> area.

Sure thing.  I really thank you to do this work!
Namhyung
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