linux-kernel - Re: [RFC][PATCH v1 02/15] perf: export generic hardware events via sysfs

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Date:	Wed, 28 Jul 2010 12:02:17 +0200
From:	Robert Richter <robert.richter@....com>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
CC:	Lin Ming <ming.m.lin@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Johannes Berg <johannes@...solutions.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Greg KH <greg@...ah.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"eranian@...il.com" <eranian@...il.com>,
	"Gary.Mohr@...l.com" <Gary.Mohr@...l.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <carll@...ibm.com>,
	Kay Sievers <kay.sievers@...y.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH v1 02/15] perf: export generic hardware events via
 sysfs

On 27.07.10 13:51:55, Corey Ashford wrote:
> On 07/27/2010 01:27 AM, Robert Richter wrote:
> > On 26.07.10 22:18:23, Lin Ming wrote:
> >
> >>>> /sys/devices/system/cpu/cpu0/events
> >>>> |-- L1-dcache-load-misses
> >>>> |   |-- config
> >>>> |   `-- type
> >
> > vs.
> >
> >>>   |-- L1-dcache-load-misses  ===>  event name
> >>>   |   `-- id                 ===>  event id
> >
> >>> This is very simple and flexible and solves the original problem too.
> >>
> >> Yeah, this is flexible. I'll think about this closely.
> >
> > The thing is, if you start introducing the config/type i/f, we will
> > stick with it for a long time. I want to avoid this from the
> > beginning.
> >
> > Using an id only would work with your current implementation too, you
> > only need to maintain an id ->  config/type mapping, maybe in some
> > private data section, without exporting it to userspace.
> 
> Now that I understand that the sysfs id essentially points to a kernel 
> data structure, I like this idea a lot.  Before I was thinking that you 
> were trying to place a lot of info into the id itself.
> 
> The only downside I see, and maybe it's a very minor one, is that you'll 
> no longer have the ability to specify an event without specifying a 
> sysfs path... there's no other mechanism.

Right, the id is generated dynamically, it could be an incremented u64
value, a pointer to a kernel structure or something else. Its
implementation could even change from kernel to kernel without
breaking the i/f.

Note also, that you will still be able to setup current events using
the traditional way of filling in the syscall structure. Nothing will
change for this.

But yes, a new event could only support a sysfs id setup. And then,
you need sysfs. But you could also implement a non-sysfs setup in
addition.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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