linux-kernel - Re: [PATCH 0/6] perf/ftrace: Introduce hexadecimal type casting

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Message-Id: <20160819010143.73e406cfb2ecfaf4bebf5b2b@kernel.org>
Date:	Fri, 19 Aug 2016 01:01:43 +0900
From:	Masami Hiramatsu <mhiramat@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Naohiro Aota <naohiro.aota@...t.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Wang Nan <wangnan0@...wei.com>,
	Hemant Kumar <hemant@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/6] perf/ftrace: Introduce hexadecimal type casting

On Thu, 18 Aug 2016 11:14:42 -0300
Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> Em Thu, Aug 18, 2016 at 05:57:32PM +0900, Masami Hiramatsu escreveu:
> > Hi Arnaldo and Steven,
> > 
> > Here is an RFC series of hexadecimal type casting and
> > changing default type casting of perf and ftrace.
> > 
> > I've introduced x8,x16,x32,x64 according to previous
> > discussion on LKML.
> >   https://lkml.org/lkml/2016/8/10/339
> > 
> > This series includes not only adding hexadecimal types
> > (x8,x16,x32,x64), but also checking it is supported by
> > running kernel and keeping the backward compativility.
> > 
> > [1/6] Add hexadecimal type casting, but does not touch
> >    existing types like 'u8'.
> > [2/6] Show the supported types on README of ftrace so
> >    that user application (e.g. perf) can check that.
> > [3/6] Add a type availability check to perf-probe.
> > [4/6] Add hexadecimal prefix support to perf-probe if
> >    it is supported by the kernel. 
> > [5/6] Change the perf-probe default type casting for
> >    unsigned type to hexadecimal (for backward compatibility)
> > [6/6] Change ftrace's 'uNN' to show value in decimal
> >    and use 'xNN' by default (for backward compatibility)
> > 
> > This way, we can also add "octal" type, pointer type,
> > and "character" type etc. and perf can check whether
> > the kernel supports it or not. :)
> 
> But this requires a kernel update... If we do it all in the tooling
> side, no kernel changes are required _and_ newer tools will work with
> older kernels, as this is just a formatting issue, the value is there
> and from its format one can infer its value, it is not even necessary to
> look at its "type".
> 
> I understand this is necessary for ftrace, because the pretty printer is
> in the kernel, but I don't see why we would prevent tooling from doing
> this pretty printing work and make it support any kernel.
> 
> I.e. no need at all for checking if the kernel supports anything, just
> pretty print it.

That's should be handled by libtraceevent. And if you need just
a pretty printing, you can do it in python/perl script via perf-script.
Anyway, to see the event parameters via perf tools, we have to use
perf-script with/without scripts. As you know, perf-script without
scripts, the output format depends on libtraceevent. And it seems that
the libtraceevent output depends on ftrace event "format" file.

And also as you can see in Naohiro's report　( https://lkml.org/lkml/2016/8/5/191 )
he was using perf-probe with ftrace trace_pipe interface, which was my
expected usecase. In that case we have to change ftrace side to
support various pretty printing.

IOW, perf only has very strong pretty printing interface - script
based output - for trace event arguments. For lighter use cases,
ftrace provide the print format.

So, I think this is a better meeting point, wouldn't you?

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>