Re: [PATCH] VFS: Add VFS event counter infrastructure

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, May 13, 2011 at 05:14:12PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> The recently discovered problem with RCU walks not working for absolute path
> motivated me to add some counters for these events to the VFS. Networking
> and VM has had such counters for a long time and they were always useful
> to diagnose performance problems. An advantage of counters over tracepoints
> is that they are always collected and are low enough overhead that
> they can be always enabled (unlike tracing)
> 
> This patch implements a simple per CPU counter framework for the VFS.
> The counters are per CPU and are very little overhead. The counters
> are output in debugfs (/sys/kernel/fs/vfsstat)

WTF?  Since when had debugfs become mandatory?

> +	fs = debugfs_create_dir("fs", NULL);
> +	if (fs)
> +		debugfs_create_file("vfsstat", 0444, fs, NULL, &vfsstat_fops);
> +	return 0;

Note that if that FPOS is configured away debugfs_create_dir() does _NOT_
return NULL - you get ERR_PTR(-ENODEV).