linux-kernel - Re: [PATCH 3/5] perf_counter: rework ioctl()s

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Message-ID: <18951.32643.749495.538351@drongo.ozlabs.ibm.com>
Date:	Mon, 11 May 2009 11:29:39 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/5] perf_counter: rework ioctl()s

Peter Zijlstra writes:

> Corey noticed that ioctl()s on grouped counters didn't work on the whole group.
> This extends the ioctl() interface to take a second argument that is
> interpreted as a flags field. We then provide PERF_IOC_FLAG_GROUP to toggle
> the behaviour.
> 
> Having this flag gives the greatest flexibility, allowing you to individually
> enable/disable/reset counters in a group, or all together.

As far as enable/disable are concerned, I don't think this is really
necessary.  My intention was that if you want to enable/disable a
whole group you just enable/disable the leader and leave all its
siblings enabled, since if the leader is disabled the whole group
can't go on.

Corey's problem was that we have a bug where enabling the leader only
puts the leader on and not the enabled group members.  I meant to send
a patch to fix that ages ago but I got distracted.  I'll send out the
patch shortly.

Paul.
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