linux-kernel - Re: [PATCH RT RFC v4 1/8] add generalized priority-inheritance interface

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Message-ID: <48AA86ED.4060407@novell.com>
Date:	Tue, 19 Aug 2008 04:40:13 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...e.hu, paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org, gregory.haskins@...il.com,
	David.Holmes@....com, jkacur@...il.com
Subject: Re: [PATCH RT RFC v4 1/8] add generalized priority-inheritance	interface

Hi Peter,

Peter Zijlstra wrote:
> On Fri, 2008-08-15 at 16:28 -0400, Gregory Haskins wrote:
>   
>> 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-alone
>> pi framework which can then be used to replace the rtmutex-specific
>> version.  The goal of this framework is to provide similar functionality
>> 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 the
>> name suggests provides the basic relationship of a priority source, and
>> 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.
>>     
>
> 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.
>   

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


> Anyway - from what I can make of the code, you managed to convert the pi
> 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..
>   

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

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

Thanks for the review, Peter!

Regards,
-Greg





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