linux-kernel - Re: [rfc] Describe events in a structured way via sysfs

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Date:	Thu, 24 Jun 2010 18:14:13 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Greg KH <greg@...ah.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.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] Describe events in a structured way via sysfs

On Thu, 2010-06-24 at 11:36 +0200, Ingo Molnar wrote:

> That's probably best achieved via a TRACE_EVENT() variant, by passing in the 
> sysfs location.
> 
> It might even make sense to make this a part of TRACE_EVENT() itself and make 
> 'NULL' the current default, non-sysfs-enumerated behavior. That way we can 
> gradually (and non-intrusively) find all the right sysfs places for events.

No, this doesn't work. A lot of events are multi-instance. Say you have
an event for each USB device. This event would have to show up in many
places in sysfs, and each trace_foo() invocation needs to get the struct
device pointer, not just the TRACE_EVENT() definition. Additionally, to
create/destroy the sysfs pieces we need something like
init_trace_foo(dev) and destroy_trace_foo(dev) be called when the sysfs
points for the device should be created/destroyed.

The TRACE_EVENT() just defines the template, but such multi-instance
events really should be standardised in terms of their struct device (or
maybe kobject).

I think that needs some TRACE_DEVICE_EVENT macro that creates the
required inlines etc, and including the init/destroy that are called
when the event should show up in sysfs.

There's no way you can have the event show up in sysfs at the right spot
with _just_ a TRACE_EVENT macro, since at define time in the header file
you don't even have a valid struct device pointer.

johannes

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