linux-kernel - Re: [RFC] perf-cache command interface design

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Date:	Sat, 08 Nov 2014 13:15:35 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Hemant Kumar <hemant@...ux.vnet.ibm.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org, srikar@...ux.vnet.ibm.com,
	peterz@...radead.org, oleg@...hat.com,
	hegdevasant@...ux.vnet.ibm.com, mingo@...hat.com,
	systemtap@...rceware.org, aravinda@...ux.vnet.ibm.com,
	penberg@....fi, brendan.d.gregg@...il.com,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>
Subject: Re: [RFC] perf-cache command interface design

(2014/11/07 19:51), Hemant Kumar wrote:
>>
>> File Format
>> ===========
>> All the cache files are placed under ~/.debug/ by default.
>> The paths of buildid cache of binary/symbols are not changed.
>>
>> The SDT/probe caches are placed under the ~/.debug/.probes/path/to/bin/bu/ildid
>> and that is linked to ~/.debug/.probes/.buildid/bu/ildid
>> # To avoid conflict with files under /probes/*, I picked up .probes/.
>>
>> This SDT/probe caches contain probe-definitions as following format.
>> ----
>> #buildid:BUILDID
>> #path:PATH
>> p:%PROVIDER/EVENT PATH:OFFSET [ARGS]
> 
> I think this format isn't accepted by the uprobe_events file (because of
> the '%') if we want to cat and grep into uprobe_events file directly.
> Although, in case of perf record, we can process it to remove '%' and
> then write into uprobe_events file. And, '%' is required as a flag for
> the SDT entries.

Ah, right...

> So, either we can modify the uprobe_events file to ignore '%' in the
> event group name (with an additional patch in the SDT series) or we can
> leave it to the user to write a script which can do some processing to
> remove '%' and then echo it to uprobe_events file.
> What would you think?

Hmm, ok. I think we can unify SDT and probe cache as just an cached
event. So replacing % with probe_ prefix is OK I think.

What I mean is if you add a file to cache, perf adds SDTs in the file,
and if you add a file with some expressions, like '* $params', perf
also adds probe caches. And you can use both events for not only
tracing but also profiling.

e.g.
 perf cache --add /bin/foo  # add foo's SDTs
 perf cache --add /bin/foo --probe 'foofunc $params' # add a probe cache for foofunc
 perf record -e %foo:sdtevent -e %foo:foofunc ... # we can record both SDT and cache

So, % prefix means "recall cached probe/sdt events to record if needed".
If you already set up the probe-events on those, you don't need % as below.

 perf probe --add "%foo:sdtevent" # add new probe_foo:sdtevent for %foo:sdtevent
 perf record -e probe_foo:sdtevent ...

Note that to avoid crash with existing event groups, we'll add "probe_" prefix
for cached events.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com

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