linux-kernel - Re: [PATCH 1/2] tools, perf: Add a precise event qualifier v2

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Message-ID: <alpine.DEB.2.02.1307241433380.8230@pianoman.cluster.toy>
Date:	Wed, 24 Jul 2013 14:47:03 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andi Kleen <ak@...ux.intel.com>, acme@...radead.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	torvalds@...ux-foundation.org, trinity@...r.kernel.org
Subject: Re: [PATCH 1/2] tools, perf: Add a precise event qualifier v2

On Wed, 24 Jul 2013, Andi Kleen wrote:

> On Tue, Jul 23, 2013 at 05:27:43PM -0400, Vince Weaver wrote:
> > 

> So format/precise would be 
> 
> flags:15-16
> 
> on little endian. Looks good?

I'd prefer if it were

  precise_ip:0-1

as the code you use to set the value looks like 
   attr->precise_ip=2;
I don't think you can even set bits 15-16 of flags directly, can you?

Althought having to have a parser being able to parse the names of all 50+
perf_event->attr arguments, then parse hex/decimal strings from another
file in sysfs, then trying to map that to binary fields in a complicated
structure that is then passed to a syscall gets a bit stupid at some 
point; this is one of the silliest interfaces I've had to deal with and 
I've done low-level X11 programming before.

Ideally we could do something sane, such as maybe having the event 
descriptions in a library in userspace where it belongs but it's a bit
late to close the barn door now.

> > I'm a bit grumpy about this because I just finished fixing the fallout 
> > from your last time breaking the ABI a few weeks ago when your broken code 
> > started dumping non-hex fields into the sysfs event strings.  I've learned 
> 
> Not sure what you're talking about?

I'm pretty sure it was one of your patches that added the new memory stuff 
which had "ldlat=1" in one of the events, and that was not a hex value 
prefixed by 0x as specified in the ABI documentation and thus broke
various parsers.

By the time I caught this the perf_event maintainers declared it was too 
late to do anything about it.

Vince
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