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

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Date:	Thu, 13 May 2010 16:28:53 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	greg@...ah.com, Peter Zijlstra <peterz@...radead.org>,
	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

On Tue, 2010-05-11 at 17:18 +0800, 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.

I still don't understand this sys_fd -> pmu lookup, would you please
explain it more detail?

struct pmu {
	kobject kobj;
	...
};

What I can imagine is,

1. In userspace, sys_fd =
open("/sys/devices/system/cpu/event_source", ..), then sys_fd is passed
to sys_perf_event_open()

2. In kernel, sys_file = <find the sys file structure with sys_fd>

3. kobject = <retrieve the kobject from sys_file>

4. pmu = container_of(kobject, struct pmu, kobj)

If my understanding is correct, then step 3 above seems strange. It's
not the typical usage of sys file.

Thanks,
Lin Ming

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