linux-kernel - Re: [PATCH] perf_counter: extensible perf_counter

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Date:	Mon, 08 Jun 2009 14:18:09 -0700
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf_counter: extensible perf_counter_attr

Peter Zijlstra wrote:
> On Mon, 2009-06-08 at 12:02 -0700, Corey Ashford wrote:
>> Hi Peter,
>>
>> Peter Zijlstra wrote:
>>> Allow extending the perf_counter_attr structure by linking extended
>>> structures to it.
>>>
>>> Also, should we grow the directly reserved space in the structure a
>>> little more?
>>>
>>> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>>> ---
>>>  include/linux/perf_counter.h |    7 +++++++
>>>  kernel/perf_counter.c        |    1 +
>>>  2 files changed, 8 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
>>> index 3586df8..781d8ce 100644
>>> --- a/include/linux/perf_counter.h
>>> +++ b/include/linux/perf_counter.h
>>> @@ -175,6 +175,13 @@ struct perf_counter_attr {
>>>  	__u32			__reserved_3;
>>>
>>>  	__u64			__reserved_4;
>>> +
>>> +	struct perf_counter_attr_ext *ext_attrs;
>>> +};
>>> +
>>> +struct perf_counter_attr_ext {
>>> +	struct perf_counter_attr_ext 	*next;
>>> +	__u64				perf_attr_ext_type;
>>>  };
>> Let's say I want to extend the attributes by four 64-bit quantities... from the 
>> above definition, I'd need four additional records chained together, right?  How 
>> about something like this instead:
> 
> Ah, the idea was to do something like:
> 
> struct perf_counter_attr_feature {
> 	struct perf_counter_attr_ext	head;
> 	... more stuff ...
> };
> 
> and set head.perf_attr_ext_type = PERF_ATTR_EXT_FEATURE


So you'd do something like this then when assigning to the perf_counter_attr struct:

struct perf_counter_attr pca;
...
struct perf_counter_attr_feature feature;

...
pca.ext_attrs = (struct perf_counter_attr_ext *)&feature;
-or-
pca.ext_attrs = &feature.head; /* secretly know that there's data that lies past 
the attr struct header */

Am I understanding this correctly?

- Corey

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