linux-kernel - Re: [RFC][PATCH v2 06/11] perf: core, export pmus via sysfs

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Date:	Tue, 18 May 2010 19:48:23 -0700
From:	Greg KH <greg@...ah.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	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>
Subject: Re: [RFC][PATCH v2 06/11] perf: core, export pmus via sysfs

On Wed, May 19, 2010 at 10:34:55AM +0800, Lin Ming wrote:
> On Wed, 2010-05-19 at 04:05 +0800, Greg KH wrote:
> > On Wed, May 19, 2010 at 01:46:42AM +0000, Lin Ming wrote:
> > > Now only exports cpu hardware events.
> > > 
> > > For each PMU, there are 2 sysfs dirs: event_source and events.
> > > 
> > > For example,
> > > /sys/devices/system/cpu/event_source/
> > > /sys/devices/system/cpu/events/
> > > 
> > > $ tree /sys/devices/system/cpu/event_source/
> > > /sys/devices/system/cpu/event_source/
> > > `-- id
> > > 
> > > $ tree /sys/devices/system/cpu/events/
> > > /sys/devices/system/cpu/events/
> > > |-- L1-dcache-load-misses
> > > |   |-- event_source -> ../../event_source
> > 
> > What is this symlink for?  Is it needed as they all seem to point to the
> > same thing.
> 
> The symlink is used to find the "event_source" of the "event".
> 
> /sys/devices/system/cpu/event_source/
> 
> /sys/devices/system/cpu/events/
> |-- L1-dcache-load-misses
> |   |-- event_source -> ../../event_source
> 
> For above example, "event_source" is the cpu pmu and the "event" is
> L1-dcache-load-misses.
> 
> Yes, they point to the same thing, because all the events
> under /sys/devices/system/cpu/events/* is monitored by the same event
> source, ie, the cpu pmu(/sys/devices/system/cpu/event_source/).

Then, why have a symlink if they are all going to be the same?

> > I'm still not really understanding what exactly you are trying to show
> > in the sysfs tree here anyway, perhaps the documentation update will
> > clear that up for me.
> 
> To support multiple pmus

What is a "pmu"?

> (or call them event source),

What is an "event source"?

> I want to show all
> the pmus and events in the sysfs tree.

Why do these things need to be in sysfs?  What benifit is it going to
have?

> For cpu pmu,
> /sys/devices/system/cpu/event_source/*
> /sys/devices/system/cpu/events/*

What is a "cpu pmu"?

> For node pmu,
> /sys/devices/system/node/event_source/*
> /sys/devices/system/node/events/*

What is a "node pmu"?

> For mce,
> /sys/kernel/events/mce/*

What?  No, don't create /sys/kernel/events/.  Is that in this patchset?
Ick, that's such a "general" name that it is going to get confusing very
quickly.  Heck, I still don't know what this "event" thing is yet :)

> and so on...the exact placement is not yet final.

Again, why do you need/want anything in sysfs in the first place?
What problem is it going to solve?  Who is going to benifit?  Why do
they care?  What is this whole thing about?

thanks,

greg k-h
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