linux-kernel - Re: [RFC PATCH] perfcounters: provide expansion room in the ABI

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Date:	Thu, 5 Mar 2009 11:07:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC PATCH] perfcounters: provide expansion room in the ABI


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

> Ingo Molnar wrote:
> 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/paulus/perfcounters.git rfc
> > > 
> > >  include/linux/perf_counter.h |   12 +++++++++---
> > >  include/linux/syscalls.h     |    2 +-
> > >  kernel/perf_counter.c        |   10 +++++++---
> > >  3 files changed, 17 insertions(+), 7 deletions(-)
> > 
> > Pulled into tip:perfcounters/core, thanks Paul!
> 
> Actually, I goofed slightly.  In this bit of the patch:
> 
> @@ -80,10 +81,15 @@ struct perf_counter_hw_event {
>  				exclude_user   :  1, /* don't count user      */
>  				exclude_kernel :  1, /* ditto kernel          */
>  				exclude_hv     :  1, /* ditto hypervisor      */
> +				exclude_idle   :  1, /* don't count when idle */
>  
> -				__reserved_1 : 23;
> +				__reserved_1 : 55;
> +
> 
> the 55 should have been 54 if we want those bitfields to 
> occupy 64 bits.  With 55 there, __reserved_1 will go in the 
> next 64-bit quadword (x86-speak, or doubleword in 
> powerpc-speak).
> 
> Do you think that's worth changing?  If so I'll do a commit 
> changing the 55 to 54.

How about extending it to 118 instead? The creation of a counter 
is a slow path in any case.

	Ingo
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