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

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Date:	Wed, 19 May 2010 11:40:29 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Greg KH <greg@...ah.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, 2010-05-19 at 10:48 +0800, Greg KH wrote:
> 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?

So how about move the symlink to parent directory, like

/sys/devices/system/cpu/event_source/

/sys/devices/system/cpu/events/
|-- event_source -> ../event_source

> 
> > > 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"?

Performance Monitoring Unit

> 
> > (or call them event source),
> 
> What is an "event source"?

The source who generates the event.
The event includes hardware events, cache-misses, bus-cycles,
interrupts, error conditions, hotplug......
and software events, context-switches, page-faults, sched events......

> 
> > 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?

Kernel to provide the information of the event source and
hardware/software events via sysfs and user-space can get
this info from sysfs.

> 
> > For cpu pmu,
> > /sys/devices/system/cpu/event_source/*
> > /sys/devices/system/cpu/events/*
> 
> What is a "cpu pmu"?

The performance monitoring unit in cpu.

> 
> > For node pmu,
> > /sys/devices/system/node/event_source/*
> > /sys/devices/system/node/events/*
> 
> What is a "node pmu"?

The pmu shared by logical cpus in a package, for example, Nehalem uncore
pmu, PowerPc nest units.

> 
> > For mce,
> > /sys/kernel/events/mce/*
> 
> What?  No, don't create /sys/kernel/events/.  Is that in this patchset?

No, not 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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