linux-kernel - Re: [PATCH 1/2] SDT markers listing by perf

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Date:	Wed, 25 Sep 2013 11:33:26 +0530
From:	Hemant <hkshaw@...ux.vnet.ibm.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
CC:	Namhyung Kim <namhyung@...nel.org>, linux-kernel@...r.kernel.org,
	srikar@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com,
	mingo@...hat.com, anton@...hat.com, systemtap@...rceware.org
Subject: Re: [PATCH 1/2] SDT markers listing by perf

On 09/25/2013 10:07 AM, Masami Hiramatsu wrote:
> (2013/09/15 20:28), Hemant wrote:
>> Hi Masami,
> Hi, and sorry for replying so late. I missed this in my mailbox.
>
>> On 09/04/2013 01:31 PM, Masami Hiramatsu wrote:
>>> (2013/09/04 15:42), Namhyung Kim wrote:
>>>> [SNIP]
>>>> You need to add it to Documentation/perf-probe.txt too.  In addition if
>>>> the --sdt option is only able to work with libelf, it should be wrapped
>>>> into the #ifdef LIBELF_SUPPORT pair.
>>>>
>>>> And I'm not sure that it's a good idea to have two behavior on a single
>>>> option (S) - show and probe (add).  Maybe it can be separated into two
>>>> or the S option can be used as a flag with existing --list and --add
>>>> option?
>>>>
>>> Good catch! :)
>>> No, that is really bad idea. All probes must be added by "--add" action.
>>> So we need a new probe syntax for specifying sdt marker.
>>>
>>> How about the below syntax?
>>>
>>> [EVENT=]%PROVIDER:MARKER [ARG ...]
>>>
>>> Of course, this will require to list up all markers with "%" prefix for
>>> continuity.
>>>
>>> And since --list option is to list up all existing(defined) probe events,
>>> I think --markers (as like as --funcs) is better for listing it up.
>>>
>>> Thank you!
>>>
>> I have one doubt here. Why do we need [ARG ...] in the syntax you
>> specified? I believe these args are to fetched from the sdt notes'
>> section of the elf of the executable/library. Or am I taking this in a
>> wrong way and this suggested syntax is actually for the uprobe_events
>> file in the tracing directory?
> Hm, indeed. Since all the arguments of the marker is defined in sdt notes,
> we actually don't need to specify each of them. However, other probe syntax
> has those arguments. I'd like to keep the same syntax style in the
> same command (action) for avoiding confusion.

Hmm, got it.

> I recommend this way; at the first step, we just find the marker address from
> sdt. And next, we will make the argument available. And eventually,
> it is better to introduce "$args" meta argument to fetch all the arguments
> of the marker.
>
> At this point, we can do
>
> perf probe %foo:bar $args

So, at first step (ignoring the arguments), we can go with :
perf probe %foo:bar

And, once, the argument support is enabled (all the arguments will be
fetched at the marker location), we can go with:
perf probe %foo:bar $args

>
> to trace full information from the marker foo:bar.
>
> Thank you,
>


-- 
Thanks
Hemant Kumar Shaw

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