linux-kernel - Re: [PATCH v2 1/6] perf: Add interface to add general events to sysfs

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Date:	Tue, 26 Jul 2011 09:06:07 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH v2 1/6] perf: Add interface to add general events to
 sysfs

On Mon, 2011-07-25 at 23:20 +0800, Greg KH wrote:
> On Mon, Jul 25, 2011 at 10:32:03AM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-07-25 at 16:11 +0800, Lin Ming wrote:
> > > It's strange that I didn't find a way to check if a directory is present
> > > in sysfs. Any hint?
> > 
> > sysfs is voodoo for me, best I can come up with is, ask the sysfs
> > folks ;-)
> > 
> > Kay, Greg?
> 
> What are you trying to do here specifically?

Now there are perf event source devices in sysfs.

$ tree /sys/bus/event_source/devices/
/sys/bus/event_source/devices/
|-- breakpoint
|-- cpu
|-- software
`-- tracepoint

I want to create a "events" directory to hold the most useful events
under the event_source.

$ tree /sys/bus/event_source/devices/cpu
| -- events

Then add one event each time to the "events" dir.

Add "event1"
$ tree /sys/bus/event_source/devices/cpu
| -- events
     | -- event1

Add "event2"
$ tree /sys/bus/event_source/devices/cpu
| -- events
     | -- event1
     | -- event2

So at the first time to add a event I need to check if the "events"
directory is present. If not, create it.

Like below pseudo code,

int perf_pmu_add_event(struct pmu *pmu, const char *name, ....)
{
	if there is no "events" dir under pmu->dev->kobj
		then create "events" dir

	add event "name" to "events"

	return ....
}

So the problem is, for example, how to check if there is a "events" dir
under /sys/bus/event_source/devices/cpu in kernel.

Thanks,
Lin Ming

> 
> sysfs is not ment to be walked from within the kernel, only from
> userspace.  Perhaps that is your problem here?
> 
> greg k-h


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