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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: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, Paul Mackerras <paulus@samba.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org,
       stephane eranian <eranian@googlemail.com>
In-Reply-To: <20090819161934.GD4972@nowhere>
References: <20090819091823.916851355@chello.nl>
	 <20090819092023.980284148@chello.nl>  <20090819161934.GD4972@nowhere>
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Date: Wed, 19 Aug 2009 18:24:28 +0200
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On Wed, 2009-08-19 at 18:19 +0200, Frederic Weisbecker wrote:
> On Wed, Aug 19, 2009 at 11:18:27AM +0200, Peter Zijlstra wrote:
> > Provide the ability to configure a counter to send its output to
> > another (already existing) counter's output stream.
> > 
> > [ compile tested only ]
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: stephane eranian <eranian@googlemail.com>
> > +int perf_counter_set_output(struct perf_counter *counter, int output_fd)
> > +{
> > +	struct perf_counter *output_counter = NULL;
> > +	struct file *output_file = NULL;
> > +	struct perf_counter *old_output;
> > +	int fput_needed = 0;
> > +	int ret = -EINVAL;
> > +
> > +	if (!output_fd)
> > +		goto set;
> > +
> > +	output_file = fget_light(output_fd, &fput_needed);
> > +	if (!output_file)
> > +		return -EBADF;
> > +
> > +	if (output_file->f_op != &perf_fops)
> > +		goto out;
> > +
> > +	output_counter = output_file->private_data;
> > +
> > +	/* Don't chain output fds */
> > +	if (output_counter->output)
> > +		goto out;
> 
> 
> 
> If you don't chain them, how do you propagate more than one
> path of output redirected?

you mean, fd2->fd1->fd0 ?

You don't, that sounds like a silly thing to do, do fd1->fd0, fd2->fd0
instead.