linux-kernel - Re: [RFC] [Patch 4/4] lock contention tracking slimmed down

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Message-ID: <466D2498.3000105@de.ibm.com>
Date:	Mon, 11 Jun 2007 12:31:52 +0200
From:	Martin Peschke <mp3@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	linux-kernel@...r.kernel.org, jbaron@...hat.com,
	rostedt@...dmis.org, billh@...ppy.monkey.org, mingo@...e.hu,
	linux-s390@...r.kernel.org
Subject: Re: [RFC] [Patch 4/4] lock contention tracking slimmed down

Peter Zijlstra wrote:
> On Fri, 2007-06-08 at 19:37 +0200, Martin Peschke wrote:
>> Peter Zijlstra wrote:
>>> On Fri, 2007-06-08 at 19:18 +0200, Martin Peschke wrote:
>>>> Peter Zijlstra wrote:
>>>>> On Fri, 2007-06-08 at 19:00 +0200, Martin Peschke wrote:
>>>>>> Peter Zijlstra wrote:
>>>>>>> I'm confused as to where the class->stat objects are initialised? Is
>>>>>>> that done in lock_stat_init()? If so, then you have a bug.
>>>>>> static struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
>>>>>>
>>>>>> I assume this gets us class structures containing all zeros, doesn't it?
>>>>>> Then class->stat is zeros as well, which is handled by lib/statistics.
>>>>>> (In this case, data gathering hasn't been turned on yet, and statistic_inc()
>>>>>> and similar functions don't access other areas of struct statistic.)
>>>>> Who eventually calls percpu_alloc?
>>>> There is a small state machine calling percpu_alloc when users do
>>>>
>>>>     echo state=on > /debug/statistics/lockdep/definition
>>>>
>>>> So data gathering is off by default.
>>>>
>>>> It might make sense to allow "state=on" as a default. Then allocation would
>>>> be done in the context of statistic_attach().
>>> Right, the problem here is that you iterate over all_lock_classes once
>>> at init.
>>>
>>> Contrary to what the name might suggest, it are not all possible
>>> classes, just all active ones. So you'll only attach the classes which
>>> have been used up until the init point. All other classes used later
>>> will never be initialized.

One could work around this by iterating over the entire array...

Anyway, this is not good enough. The right solution was to teach my
code to deal with static (per-cpu) statistic counter arrays in addition
to buffers allocated with percpu_alloc(). Both use cases are valid.

Thanks for your feedback.

Martin

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