linux-kernel - Re: [tip:perf/core] perf: Update perf_event

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Message-ID: <alpine.DEB.2.02.1307252313460.26062@pianoman.cluster.toy>
Date:	Thu, 25 Jul 2013 23:20:24 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Adrian Hunter <adrian.hunter@...el.com>
cc:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Update perf_event_type documentation


On Thu, 25 Jul 2013, Adrian Hunter wrote:

> >> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> >> index 0b1df41..00d8274 100644
> >> --- a/include/uapi/linux/perf_event.h
> >> +++ b/include/uapi/linux/perf_event.h
> >> @@ -478,6 +478,16 @@ enum perf_event_type {
> >>  	 * file will be supported by older perf tools, with these new optional
> >>  	 * fields being ignored.
> >>  	 *
> >> +	 * struct sample_id {
> >> +	 * 	{ u32			pid, tid; } && PERF_SAMPLE_TID
> >> +	 * 	{ u64			time;     } && PERF_SAMPLE_TIME
> >> +	 * 	{ u64			id;       } && PERF_SAMPLE_ID
> >> +	 * 	{ u64			stream_id;} && PERF_SAMPLE_STREAM_ID
> >> +	 * 	{ u32			cpu, res; } && PERF_SAMPLE_CPU
> >> +	 * } && perf_event_attr::sample_id_all
> >> +	 */
> >> +

a thing that personally bothers me are these imaginary struct definitions 
added as part of the documentation that aren't actually available in the 
public perf_event.h

I can see why it's done, but it can be confusing picking out in later 
definitions which struct fields are real and which ones are conceptual.

> >> @@ -498,6 +508,7 @@ enum perf_event_type {
> >>  	 *	struct perf_event_header	header;
> >>  	 *	u64				id;
> >>  	 *	u64				lost;
> >> +	 * 	struct sample_id		sample_id;
> >>  	 * };

This is what confused me; this documentation shows the sample_id
as always being there, but in reality that struct is only there
if perf_event_attr::sample_id_all is set.  

It might be clearer
if you stuck the perf_event_attr::sample_id_all qualifier each
place you add the sample_id field.

And it's nice that the header holds a size field which ensures backward 
compatability, I somehow had forgotten that with my initial complaint.

Vince
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