linux-kernel - Re: [RFC] [PATCH] perf: Attaching an event to a specific PMU

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Message-ID: <20110706171015.GA13869@elte.hu>
Date:	Wed, 6 Jul 2011 19:10:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Robert Richter <robert.richter@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH] perf: Attaching an event to a specific PMU


* Robert Richter <robert.richter@....com> wrote:

> On 05.07.11 05:12:52, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > > Overall, my approach improves the perf design. It adds a better 
> > > > and more intuitve access to perf from user space with clear and 
> > > > common methods and interfaces. Please let me know the concerns 
> > > > you have.
> > > 
> > > Its redundant, this interface ship has sailed, its not going to 
> > > happen.
> > 
> > Even if we had the choice, i don't see how a /dev based enumeration 
> > of PMUs is in any way better than a topologically attached set of 
> > PMUs in /sys.
> > 
> > This kind of structure is nice in principle:
> > 
> >  # ls -l /dev/pmu/
> >  total 0
> >  crw-rw---- 1 root root 254, 5 Jul  8  2011 breakpoint
> >  crw-rw---- 1 root root 254, 4 Jul  8  2011 cpu
> >  crw-rw---- 1 root root 254, 6 Jul  8  2011 proto
> >  crw-rw---- 1 root root 254, 1 Jul  8  2011 software
> >  crw-rw---- 1 root root 254, 2 Jul  8  2011 tracepoint
> > 
> > But it should be done in /sys/.
> 
> I have to learn yet why /dev is bad and /sys is good...

Because /sys is already there and already carries rather rich 
classification of various hardware components, devices and kernel 
subsystems.

/dev/ is mostly a flat registry of classical, unstructured devices.

> The system topology is always in /sys, also for device nodes. But 
> we can't get a device file descriptor from /sys. I doubt /sys is 
> capable to handle a device use count (need to be checked). We 
> actually must grab the pmu while attaching events to it. And, user 
> space implementation is must easier with /dev (see code in my 
> previous mail).

I think Peter suggested it that an open() done in /sys should give us 
a handle to a given event?

> My patch also includes code that creates a device class. It is also 
> visible in /sys/class/pmu/*.

But PMU is a very limited term: what we want is a higher level 
organization of 'event sources' and 'events'. Some may come from 
PMUs, many wont.

Thanks,

	Ingo
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