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

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Date:	Thu, 24 Jun 2010 11:36:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Johannes Berg <johannes@...solutions.net>
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

* Johannes Berg <johannes@...solutions.net> wrote:

> On Tue, 2010-06-22 at 15:22 +0800, Lin Ming wrote:
> 
> > > > net/wlan0/events/
> > > > net/waln1/events/
> > > > ....
> > > > net/walnN/events/
> > > 
> > > That's not appropriate either though since you may have multiple network 
> > > interfaces on the same hardware :)
> > 
> > Doesn't net/wlan0...wlanN mean multiple network interfaces on the same 
> > hardware?
> 
> Yes, but the trace points aren't per network interface but rather per 
> hardware piece.

Yeah - we generally want events to live at their 'natural' source in sysfs.

So if it's a per device hardware event, it should live with the hardware 
piece. If it's a higher level chipset event, it should live where the chipset 
driver is in sysfs. If it's a subsystem level event then it should live there.

I think what you mentioned in your other posting makes the most sense: give 
flexibility to tracepoint authors to place the event in the most sensible 
sysfs place. It is them who define the tracepoints and the events so any 
second guessing by a generic layer will probably get in the way. The generic 
tool layer will be content with having the event_source class in sysfs, to see 
'all' event sources ttheir topological structure.

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.

Thanks,

	Ingo
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