linux-kernel - Re: [RFC][PATCH 3/9] perf: export registerred pmus via sysfs

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Message-ID: <20100511093151.GA24284@elte.hu>
Date:	Tue, 11 May 2010 11:31:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"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>,
	Paul Mundt <lethal@...ux-sh.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <carll@...ibm.com>
Subject: Re: [RFC][PATCH 3/9] perf: export registerred pmus via sysfs


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2010-05-11 at 11:18 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > The new PMUs will use a dynamic range that starts at PERF_TYPE_MAX.
> > 
> > I dont think we should use a dynamic range of event sources - it's a 
> > completely useless indirection that has no meaning to humans.
> > 
> > As far as machine interfaces go a much cleaner approach would be to allow an 
> > open fd to a sysfs file to be passed to sys_perf_event_open() - this would 
> > identify the event source. This needs a small extension of the ABI but we 
> > could thus get rid of the 'type' enumeration altogether and express _all_ 
> > event sources via fds to sysfs files.
> 
> Whatever, that's almost identical. [...]

It's not identical: as we dont expose our mapping structure externally and 
dont have to have some dynamic type ID allocation layer/mechanism. Also, using 
fds is an elegant, Linuxish way of expressing some object's identity and 
passing it along.

( It also removes the possibility to intentionally or accidentally have type 
  IDs that are not reachable via the sysfs and vica verse. )

> [...] What we can do is reserve perf_event_attr::type with bit 31 set for 
> fd's and use the fd->file lookup instead of the type->pmu lookup.

Yeah, that sounds good.

	Ingo
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