linux-kernel - Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels

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Message-ID: <20090819162711.GE4972@nowhere>
Date:	Wed, 19 Aug 2009 18:27:13 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	stephane eranian <eranian@...glemail.com>
Subject: Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels

On Wed, Aug 19, 2009 at 06:24:28PM +0200, Peter Zijlstra wrote:
> 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@...llo.nl>
> > > Cc: stephane eranian <eranian@...glemail.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.
> 


Ok.

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