linux-kernel - Re: [PATCH 3/3] perf inject: Handle output file via perf_data

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Message-ID: <20131126124213.GA14838@ghostprotocols.net>
Date:	Tue, 26 Nov 2013 09:42:13 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>,
	David Ahern <dsahern@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 3/3] perf inject: Handle output file via perf_data_file
 object

Em Tue, Nov 26, 2013 at 11:03:24AM +0100, Jiri Olsa escreveu:
> On Mon, Nov 25, 2013 at 04:40:32PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Nov 22, 2013 at 03:24:28PM +0100, Jiri Olsa escreveu:
> > > Using the perf_data_file object to handle output
 
> SNIP
 
> > > +	if (!perf_data_file__is_pipe(&inject->output))
> > >  		return 0;

> > >  	return perf_event__repipe_synth(tool, event);
> > > @@ -351,10 +343,12 @@ static int __cmd_inject(struct perf_inject *inject)
> > >  {
> > >  	struct perf_session *session;
> > >  	int ret = -EINVAL;
> > > -	struct perf_data_file file = {
> > > +	struct perf_data_file file_in = {
> > 
> > Why don't leave it as 'file', and on a follow up patch _then_ rename it
> > to file_in? This way patch review gets easier, i.e. try avoiding doing
> > multiple things per patch.
> 
> the input file needed to be renamed, because new 'output' file was added

Why? Is 'file' going to be reused somehow?
 
> > 
> > >  		.path = inject->input_name,
> > >  		.mode = PERF_DATA_MODE_READ,
> > >  	};
> > > +	struct perf_data_file *file_out = &inject->output;
> > > +	int out_fd = perf_data_file__fd(file_out);
> > >  
> > >  	signal(SIGINT, sig_handler);
> > >  
> > > @@ -365,7 +359,7 @@ static int __cmd_inject(struct perf_inject *inject)
> > >  		inject->tool.tracing_data = perf_event__repipe_tracing_data;
> > >  	}
> > >  
> > > -	session = perf_session__new(&file, true, &inject->tool);
> > > +	session = perf_session__new(&file_in, true, &inject->tool);
> > 
> > This hunk, for example, wouldn't be here, the this patch would be
> > shorter, easier to review.
> > 
> > >  	if (session == NULL)
> > >  		return -ENOMEM;
> > >  
> > > @@ -391,14 +385,15 @@ static int __cmd_inject(struct perf_inject *inject)
> > >  		}
> > >  	}
> > >  
> > > -	if (!inject->pipe_output)
> > > -		lseek(inject->output, session->header.data_offset, SEEK_SET);
> > > +	if (!perf_data_file__is_pipe(file_out))
> > > +		lseek(out_fd, session->header.data_offset, SEEK_SET);
> > 
> > Couldn't this be left as:
> > 
> > -	if (!inject->pipe_output)
> > -		lseek(inject->output, session->header.data_offset, SEEK_SET);
> > +	if (!perf_data_file__is_pipe(file_out))
> > +		lseek(inject->output->fd, session->header.data_offset, SEEK_SET);
> > 
> > I.e. why wrap access to the fd like that?
> 
> well, inject->output->fd is used on 2 places within the function,
> so it seems logical to put it into variable and use it like that

What I'm trying to convey here is that for both this case and the other,
having looking at these two lines:

-                 inject->output
+                 inject->output->fd

Makes me instantaneously understand that inject->output now
encapsulates, among other things (probably), the file descriptor that
was called just inject->output, i.e. this patch probably isn't doing
anything more than using a new abstraction, the code flow probably
wasn't altered.

I.e. the smaller the patch, the better.

> > > -	if (!inject->pipe_output) {
> > > +	if (!perf_data_file__is_pipe(file_out)) {
> > >  		session->header.data_size = inject->bytes_written;
> > > -		perf_session__write_header(session, session->evlist, inject->output, true);
+		perf_session__write_header(session, session->evlist, inject->output->fd, true);
> > 
> > Why a line for 'true' all by itself?
> 
> line was crossing 80 chars limit

[1]+  Stopped                 mutt
[acme@zoo ~]$ 
[acme@zoo ~]$ echo $COLUMNS
173
[acme@zoo ~]$ 

I'm not really that strict with that old convention :-\ All in one line
would make it ~95 columns, not a big deal, even more since it _was_
already more than 80 columns.

I.e. your change was to replace 'inject->output' with 'out_fd', but you
did that _and_ reflowed, i.e. two changes into one. ;-)

Looking at this line makes me think: why do we have to pass 'session'
_and_ 'session->evlist', i.e. the 2nd parameter can be obtained from the
1st. Fixing that could get us more compact code _and_ a shorter line.

Will check that.

> > > -		OPT_STRING('o', "output", &output_name, "file",
> > > +		OPT_STRING('o', "output", &inject.output.path, "file",
> > 
> > see, here you directly access a perf_data_file member instead of having
> > another wrapper :-)
> 
> yes
> 
> I dont have strong opinions about wrappers, sometimes it seems
> appropriate, sometimes it does not.. tell me the guidance here
> and I'll kick the patch to fit ;-)

Well, a wrapper like perf_data_file__is_pipe(file_out) that maps to
file_out->is_pipe and will produce the same results at every call and
that we don't have the slightest intention of somehow hooking, I would
do away with it and use file_out->is_pipe directly.

- Arnaldo
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