linux-kernel - Re: I.1 - System calls

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Message-ID: <20090622114931.GB24366@elte.hu>
Date:	Mon, 22 Jun 2009 13:49:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	eranian@...il.com
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: I.1 - System calls - ioctl

> I/ General API comments
>
> 1/ System calls
>
>   * ioctl()
>
>    You have defined 5 ioctls() so far to operate on an existing
>    event. I was under the impression that ioctl() should not be
>    used except for drivers.
>
> How do you justify your usage of ioctl() in this context?

We can certainly do a separate sys_perf_counter_ctrl() syscall - and
we will do that if people think the extra syscall slot is worth it
in this case.

The (mild) counter-argument so far was that the current ioctls are
very simple over "IO" attributes of counters:

 - enable
 - disable
 - reset
 - refresh
 - set-period

So they could be considered 'IO controls' in the classic sense and
act as a (mild) exception to the 'dont use ioctls' rule.

They are not some weird tacked-on syscall functionality - they
modify the IO properties of counters: on/off, value and rate. If
they go beyond that we'll put it all into a separate syscall and
deprecate the ioctl (which will have a relatively short half-time
due to the tools being hosted in the kernel repo).

This could happen right now in fact, if people think it's worth it.
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