linux-kernel - Re: [RFC][PATCH] perf: sysfs type id

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Date:	Wed, 17 Nov 2010 20:57:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Lin Ming <ming.m.lin@...el.com>,
	"robert.richter" <robert.richter@....com>,
	fweisbec <fweisbec@...il.com>, paulus <paulus@...ba.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Kay Sievers <kay.sievers@...y.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC][PATCH] perf: sysfs type id

On Wed, 2010-11-17 at 11:47 -0800, Corey Ashford wrote:
> 
> This is an interesting approach, though for the IBM WSP (aka PowerEN) 
> chip, the config_format would have to be at a deeper level than the PMU, 
> because the modifiers that affect the event, vary from event to event. 
> Either that or you'd have to provide a complex union structure.

Eew.. how uhm creative ;-), yeah, not quite sure how to deal with that,
I wasn't planning to make a directory for each event, just a single file
with the u64 in it.

> However, above you say that you want to have "a few frequently 
> used/common events".  I thought that was the job of the perf "generic 
> events".  My understanding was that the sysfs tree was the solution for 
> all events, including arch-specific, and seldom-used events. 

My intent for the sysfs bits is to replace all the static stuff encoded
in PERF_TYPE_ and PERF_COUNT_ because it simply doesn't extend to a much
more dynamic world.

And GPUs/DSPs might have radically different common events than CPUs do.

Doing exhaustive lists in sysfs is simply going to waste tons of memory
for no real gain.

>  Ingo 
> pushed back on a user-space library solution (like libpfm4) because he 
> wanted event info in sysfs (or some other mechanism by which the kernel 
> could expose event info to user space).

I can't fully remember the details of that discussion.

> If there is going to be no place in sysfs for arch-specific events, I'll 
> want to start pushing for perf to be able to use a user space library again.
> 
> How about a compromise position: all of the arch-specific events are 
> exposed to user space via sysfs iff some CONFIG_* variable to set to 
> true.  Something like CONFIG_EXPOSE_ALL_HW_PERF_EVENTS_IN_SYSFS.
> This way you would only use all that memory when it's explicitly 
> configured in. 

Ingo, any opinions?
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