linux-kernel - Re: [patch 23/24] perfmon: kernel documentation

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Date:	Wed, 26 Nov 2008 13:21:07 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	eranian@...glemail.com
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org, andi@...stfloor.org,
	eranian@...il.com, sfr@...b.auug.org.au
Subject: Re: [patch 23/24] perfmon: kernel documentation

On Wed, Nov 26, 2008 at 12:43:00AM -0800, eranian@...glemail.com wrote:

I assume you'll be also submitting manpages with the same information?

> +
> +   A monitoring session is uniquely identified by a file descriptor obtained
> +   when the session is created. File sharing semantics apply to access the
> +   session inside a process. A session is never inherited across fork. The file
> +   descriptor can be used to receive counter overflow notifications or when the
> +   sampling buffer is full. It is possible to use poll/select on the descriptor
> +   to wait for notifications from multiple sessions. Similarly, the descriptor
> +   supports asynchronous notifications via SIGIO.

What happens when the fd is passed between processes using unix sockets fd 
passing?

> +
> +    We have released a simple monitoring tool to demonstrate the features of
> +    the interface. The tool is called pfmon and it comes with a simple helper
> +    library called libpfm. The library comes with a set of examples to show

I don't think "simple" is the right word to describe pfmon/libpfm @)

> +    There maybe other tools available for perfmon.

s/maybe/are/ ?

> +
> +   To destroy a session, the regular close() system call is used.

...

Some simple syscall examples would be nice. e.g. how to set up a counter
that it can be accessed using RDPMC on x86.

> +	/sys/kernel/perfmon/arg_mem_max(read-write):
> +
> +		Maximum size of vector arguments expressed in bytes.
> +		It can be modified but must be at least a page.
> +		Default: PAGE_SIZE

Is there any good reason ever to enlarge this beyond a page?

If it just depends on future hardware it would make more sense
to let a driver patch for that adjust it.

-Andi
-- 
ak@...ux.intel.com
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