linux-kernel - Re: [patch 0/3] [Announcement] Performance Counters for Linux

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Message-ID: <49387463.5070100@zytor.com>
Date:	Thu, 04 Dec 2008 16:22:59 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux

Thomas Gleixner wrote:
> 
> We'd like to announce a brand new implementation of performance counter
> support for Linux. It is a very simple and extensible design that has the
> potential to implement the full range of features we would expect from such
> a subsystem.
> 

First of all, let me say I really like what I've seen so far.  The file
descriptor paradigm seems really elegant to me.

>  - Only one single new system call is needed: sys_perf_counter_open().
>    All performance-counter operations are implemented via standard
>    VFS APIs such as read() / fcntl() and poll().

As previously discussed, I think this should be a filesystem rather than
a system call.  There are a couple of advantages to doing it that way, IMO:

- Strings, rather than numbers, which means fewer constraints across
  architectures.
- The events available can be exported in the filesystem itself (via
  readdir) rather than via sysfs.
- Compatibility with existing tools, esp. non-C tools.

I'm thinking of something like:

/dev/perfctr/3/cache_misses/all/simple/300

i.e. /dev/perfctr/<cpu>/<event>/<pid>/<type>/<period>.  I am putting
<cpu> ahead of <event> in the hierarchy, so a readdir() on the <cpu>
directory can show the events available by name on that CPU.  Raw
hardware events can be accessed by something like
/dev/perfctr/<cpu>/0x4064/...

	-hpa
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