linux-kernel - Re: Re: [RFC PATCH perf/core v2 00/16] perf-probe --cache and SDT support

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Date:	Fri, 17 Jul 2015 12:21:42 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Hemant Kumar <hemant@...ux.vnet.ibm.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Borislav Petkov <bp@...e.de>
Subject: Re: Re: [RFC PATCH perf/core v2 00/16] perf-probe --cache and
 SDT support

Hi Hemant,

On 2015/07/16 12:13, Hemant Kumar wrote:
> Hi Masami,
> 
> On 07/15/2015 02:43 PM, Masami Hiramatsu wrote:
>> Hi,
>>
>> Here is the 2nd version of the patchset for probe-cache and
>> initial SDT support which are going to be perf-cache finally.
> 
> Thanks for adding the SDT support.
> 
>> The perf-probe is useful for debugging, but it strongly depends
>> on the debuginfo. Without debuginfo, it is just a frontend of
>> ftrace's dynamic events. This can usually happen in server
>> farms or on cloud system, since no one wants to distribute
>> big debuginfo packages.
>>
>> To solve this issue, I had tried to make a pre-analyzed probes
>> ( https://lkml.org/lkml/2014/10/31/207 ) but it has a problm
>> that we can't ensure the probed binary is same as what we analyzed.
>> Arnaldo gave me an idea to reuse build-id cache for that perpose
>> and this series is the first prototype of that.
>>
>> At the same time, Hemant has started to support SDT probes which
>> also use the cache file of SDT info. So I decided to merge this
>> into the same build-id cache.
>> In this version, SDT support is still very limited, it works
>> as a part of probe-cache.
>>
>> In this version, perf probe supports --cache option which means
>> that perf probe manipulate probe caches, for example,
>>
>>    # perf probe --cache --add "probe-desc"
>>
>> does not only add probe events but also add "probe-desc" and
>> it's result on the cache. (Note that the cached entry is always
>> referred even without --cache)
>> The --list and --del commands also support --cache. Note that
>> both are only manipulate caches, not real events.
>>
>> To use SDT, we have to scan the target binary at first by using
>> perf-buildid-cache, e.g.
>>
>>    # perf buildid-cache --add /lib/libc-2.17.so
>>
>> And perf probe --cache --list shows what SDTs are scanned.
>>
>>    # perf probe --cache --list
>>    /usr/lib/libc-2.17.so (a6fb821bdf53660eb2c29f778757aef294d3d392):
>>    libc:setjmp=setjmp
>>    libc:longjmp=longjmp
>>    libc:longjmp_target=longjmp_target
>>    libc:memory_heap_new=memory_heap_new
>>    libc:memory_sbrk_less=memory_sbrk_less
>>    libc:memory_arena_reuse_free_list=memory_arena_reuse_free_list
>>    libc:memory_arena_reuse=memory_arena_reuse
>>    ...
>>
>> To use the SDT events, perf probe -x BIN %SDTEVENT allows you to
>> add a probe on SDTEVENT@....
>>
>>    # perf probe -x /lib/libc-2.17.so %memory_heap_new
>>
>> If you define a cached probe with event name, you can also reuse
>> it as same as SDT events.
>>
>>    # perf probe -x ./perf --cache -n 'myevent=dso__load $params'
>>
>> (Note that "-n" option only updates caches)
>> To use the above "myevent", you just have to add "%myevent".
>>
>>    # perf probe -x ./perf %myevent
>>
>>
>> TODOs:
>>   - Show available cached/SDT events by perf-list
>>   - Allow perf-record to use cached/SDT events directly
> 
> As I was already working on SDT events' recording
> https://lkml.org/lkml/2014/11/2/73,
> I can re-spin the patches on top of your patchset and make the
> required changes to implement the above TODOs.

Sounds great! :)
Note that you'll need to re-implement almost from scratch, since
now the SDT is implemented on buildid-cache. Maybe I have to work
on the buildid-cache one more to filter out binaries which are gone
or different version from current running one (e.g. old vmlinux).
It could help you to get available SDTs when showing it via perf-list.

> What would you suggest?

Now I'm thinking that we should avoid using %event syntax for perf-list
and perf-record to avoid confusion. For example, suppose that we have
"libfoo:bar" SDT event, when we just scanned the libfoo binary and
use it via perf-record, we'll run perf record -e "%libfoo:bar".
However, after we set the probe via perf-probe, we have to run
perf record -e "libfoo:bar". That difference looks no good.
So, I think in both case it should accept -e "libfoo:bar" syntax.

In this series I've introduced %event syntax only to recall cached event
setting explicitly, because perf-probe is a lower layer tool to set up
new event. IMO, perf-list and perf-record should be higher tools which
handle abstract events.

Thanks!


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@...achi.com
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