Message-ID: <48AA86ED.4060407@novell.com>
Date: Tue, 19 Aug 2008 04:40:13 -0400
From: Gregory Haskins <ghaskins@novell.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: mingo@elte.hu, paulmck@linux.vnet.ibm.com, tglx@linutronix.de,
       rostedt@goodmis.org, linux-kernel@vger.kernel.org,
       linux-rt-users@vger.kernel.org, gregory.haskins@gmail.com,
       David.Holmes@sun.com, jkacur@gmail.com
Subject: Re: [PATCH RT RFC v4 1/8] add generalized priority-inheritance	interface
References: <20080815202408.668.23736.stgit@dev.haskins.net>	 <20080815202823.668.26199.stgit@dev.haskins.net> <1218916561.10880.11.camel@lappy.programming.kicks-ass.net>
In-Reply-To: <1218916561.10880.11.camel@lappy.programming.kicks-ass.net>
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Hi Peter,

Peter Zijlstra wrote:
> On Fri, 2008-08-15 at 16:28 -0400, Gregory Haskins wrote:
>  =20
>> The kernel currently addresses priority-inversion through priority-
>> inheritence.  However, all of the priority-inheritence logic is
>> integrated into the Real-Time Mutex infrastructure.  This causes a few=

>> problems:
>>
>>  1) This tightly coupled relationship makes it difficult to extend to
>>     other areas of the kernel (for instance, pi-aware wait-queues may
>>     be desirable).
>>  2) Enhancing the rtmutex infrastructure becomes challenging because
>>     there is no seperation between the locking code, and the pi-code.
>>
>> This patch aims to rectify these shortcomings by designing a stand-alo=
ne
>> pi framework which can then be used to replace the rtmutex-specific
>> version.  The goal of this framework is to provide similar functionali=
ty
>> to the existing subsystem, but with sole focus on PI and the
>> relationships between objects that can boost priority, and the objects=

>> that get boosted.
>>
>> We introduce the concept of a "pi_source" and a "pi_sink", where, as t=
he
>> name suggests provides the basic relationship of a priority source, an=
d
>> its boosted target.  A pi_source acts as a reference to some arbitrary=

>> source of priority, and a pi_sink can be boosted (or deboosted) by
>> a pi_source.  For more details, please read the library documentation.=

>>
>> There are currently no users of this inteface.
>>    =20
>
> You should have started out by discussing your design - the document
> just rambles a bit about some implementation details - it doesn't talk
> about how it maps to the PI problem space.
>  =20

The doc is still a work-in-progress, but point taken ;)  I will address=20
this shortly.


> Anyway - from what I can make of the code, you managed to convert the p=
i
> graph walking code that used to be in rt_mutex_adjust_prio_chain() and
> was iterative, into a recursive function call.
>
> Not something you should do lightly..
>  =20

As we discussed on IRC yesterday, you are correct here.  I was thinking=20
that the graph couldn't get deeper than a few dozen entries, but I=20
forgot about userspace futex access.  But, this is precisely what the=20
"release early" policy is designed to catch ;)

I think I can make a slight adjustment to the model to return it to an=20
iterative design.  I will address this in v5.

Thanks for the review, Peter!

Regards,
-Greg


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