linux-kernel - Re: [PATCH] perfcounters: fix type/event

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Message-ID: <20090321095651.GA7201@elte.hu>
Date:	Sat, 21 Mar 2009 10:56:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perfcounters: fix type/event_id layout on big-endian
	systems


* Paul Mackerras <paulus@...ba.org> wrote:

> Ingo Molnar writes:
> 
> > hm, this ifdef really looks ugly. How about just changing event_id 
> > to 64 bits and having a separate u32 type field? The size impact is 
> > minimal, the cleanliness win is significant :-)
> 
> We wanted to get a complete and unique identifier for the event 
> into 64 bits so that we could put it into the ring buffer for 
> PERF_RECORD_GROUP and have it take up only one 8-byte slot and yet 
> identify uniquely which counter's value follows it.  I don't know 
> that that is absolutely necessary but it sounds like a nice 
> property.
> 
> We could easily go back to a 1-bit raw field and have the type be 
> either a 64-bit raw value or an 8-bit type plus 32 or 56-bit 
> event_id.
> 
> Or we could keep the current layout but use explicit shifts and 
> masks rather than bitfields.
> 
> I don't know the current C rules concerning unions very well, but 
> I have the impression that writing to one member of a union and 
> reading another is undefined behaviour, which is another strike 
> against the current code if true...
>
> Anyway, the point is that the current code doesn't compile on 
> powerpc and wouldn't work properly even if it did, so we need to 
> do something.

yeah - i pulled your fix, thanks Paul.

Maybe the best option is to get rid of the bitfields and use masks 
...

	Ingo
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