linux-kernel - sysfs: Add an 'events' class. (was: Re: [RFC][PATCH] perf: sysfs type id)

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Date:	Wed, 10 Nov 2010 14:36:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Greg KH <gregkh@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Stephane Eranian <eranian@...gle.com>,
	"robert.richter" <robert.richter@....com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	fweisbec <fweisbec@...il.com>, paulus <paulus@...ba.org>,
	Kay Sievers <kay.sievers@...y.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: sysfs: Add an 'events' class. (was: Re: [RFC][PATCH] perf: sysfs
 type id)

* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2010-11-09 at 14:13 -0800, Greg KH wrote:
> 
> > You missed the embedded track at Plumbers where we talked about never
> > adding another class to the kernel.  Please use bus_id instead for this.
> 
> I did, it was early and I wasn't aware this all comes under the heading
> of embedded.
> 
> Anyway, anybody got a good example of bus_type I can 'borrow' ?
> 
> Also, it would be really nice if you (plural) could make this subsystem thing 
> happen, calling tihngs a bus that aren't a bus just makes me upset ;-)

Same here - calling events a 'bus' is like totally brain-dead IMHO. It implies 
something hardware, while many events are not related to any hardware component but 
are pure software abstractions: such as context-switches, or syscall entries, or VM 
events.

So i'd rather have 'events' or 'event_source' as a 'class' temporarily, than have it 
as a 'bus' temporarily - and we get the real fix whenever the sysfs unification 
happens.

I also have a question about this future plan mentioned in 
Documentation/sysfs-rules.txt:

 - Hierarchy in a single device tree
   There is only one valid place in sysfs where hierarchy can be examined
   and this is below: /sys/devices.
   It is planned that all device directories will end up in the tree
   below this directory.

So did i get it right, sysfs is going to convert from a VFS hiearchy/enumeration to 
a flat enumeration of entities, all listed in /dev/devices/?

Why is that done? I think it's quite nice that the actual topology is represented 
right now via the sysfs VFS structure - so that we have things like:

   /sys/devices/system/ioapic/ioapic0/

Where there's is a proper hierarchy showing that we have a 'system', which has an 
'ioapic', which has an ioapic numbered '0'. That entity could then grow 'events' and 
have:

   /sys/devices/system/ioapic/ioapic0/events/

And could show various IO-APIC events, such as (future, possible events):

   /sys/devices/system/ioapic/ioapic0/events/irq/
   /sys/devices/system/ioapic/ioapic0/events/register-read/
   /sys/devices/system/ioapic/ioapic0/events/register-write/
   /sys/devices/system/ioapic/ioapic0/events/affinity/
   [...]

So is the plan to get rid of such rich hiearchies and just use a flat store of 
everything in /sys/devices/?

My hope would be to _increase_ the depth of sysfs in the future, to express every 
meaningful hiearchy that exists in the system (be that hw hierarchy or some sw 
abstraction hierarchy). But maybe i got it all wrong so please advise.

Thanks,

	Ingo
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