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Subject: Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org,
       stephane eranian <eranian@googlemail.com>
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	 <20090819092023.980284148@chello.nl>  <20090819105818.GB21150@elte.hu>
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Date: Wed, 19 Aug 2009 13:07:37 +0200
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On Wed, 2009-08-19 at 12:58 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > Provide the ability to configure a counter to send its output to
> > another (already existing) counter's output stream.
> > 
> > [ compile tested only ]
> 
> very nice!
> 
> two comments:
> 
> > Index: linux-2.6/include/linux/perf_counter.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/perf_counter.h
> > +++ linux-2.6/include/linux/perf_counter.h
> > @@ -216,6 +216,7 @@ struct perf_counter_attr {
> >  #define PERF_COUNTER_IOC_REFRESH	_IO ('$', 2)
> >  #define PERF_COUNTER_IOC_RESET		_IO ('$', 3)
> >  #define PERF_COUNTER_IOC_PERIOD		_IOW('$', 4, u64)
> > +#define PERF_COUNTER_IOC_SET_OUTPUT	_IO ('$', 5)
> 
> Time to add a new sys_perf_counter_chattr() syscall and deprecate 
> the ioctls?

Could do I guess.. in order to support things like refresh and reset we
need a few new offset fields in the attr struct, but that's doable.

> > @@ -415,6 +416,9 @@ enum perf_callchain_context {
> >  	PERF_CONTEXT_MAX		= (__u64)-4095,
> >  };
> >  
> > +#define PERF_FLAG_FD_NO_GROUP	(1U << 0)
> > +#define PERF_FLAG_FD_OUTPUT	(1U << 1)
> 
> Why not extend the size of perf_counter_attr and add an output_fd 
> parameter? Zero would mean no fd (and this is also backwards 
> compatible behavior).
> 
> FD_NO_GROUP is a bit unclean API as it aliases group_fd to two 
> purposes: the real group_fd and this new output_fd. I think we 
> should move output_fd to the attribute structure.

Yeah, the NO_GROUP thing is ugly..

I'll wait for some more comments before reworking this.