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

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.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