linux-kernel - Re: [perfmon2] I.1 - System calls

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Message-Id: <200907131930.23840.arnd@arndb.de>
Date:	Mon, 13 Jul 2009 19:30:23 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	perfmon2-devel@...ts.sourceforge.net
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, eranian@...il.com,
	Philip Mucci <mucci@...s.utk.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Paul Mackerras <paulus@...ba.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [perfmon2] I.1 - System calls - ioctl

On Monday 13 July 2009, Peter Zijlstra wrote:
> On Mon, 2009-06-22 at 08:58 -0400, Christoph Hellwig wrote:
> > But talking about syscalls the sys_perf_counter_open prototype is
> > really ugly - it uses either the pid or cpu argument which is a pretty
> > clear indicator it should actually be two sys calls.
> 
> Would something like the below be any better?
> 
> It would allow us to later add something like PERF_TARGET_SOCKET and
> things like that.

I don't think it helps on the ugliness side. You basically make the
two arguments a union, but instead of adding another flag and directly
passing a union, you also add interface complexity.

A strong indication for the complexity is that you got it wrong ;-) :

> +struct perf_counter_target {
> +	__u32			id;
> +	__u64			val;
> +};

This structure is not compatible between 32 and 64 bit user space on x86,
because everything except i386 adds implicit padding between id and val.

Other than that, making it extensible sounds reasonable. How about just
using a '__u64 *target' and a bit in the 'flags' argument?

	Arnd <><
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