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

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Message-Id: <1218916561.10880.11.camel@lappy.programming.kicks-ass.net>
Date:	Sat, 16 Aug 2008 21:56:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
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

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.

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..

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