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

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Date:	Wed, 19 Aug 2009 18:19:36 +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 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?

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