linux-kernel - Re: [PATCH 00/10] Latest numa/core release, v18

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Message-ID: <alpine.LFD.2.02.1212041825140.2701@ionos>
Date:	Tue, 4 Dec 2012 18:30:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mel Gorman <mgorman@...e.de>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 00/10] Latest numa/core release, v18

On Mon, 3 Dec 2012, Mel Gorman wrote:
> On Fri, Nov 30, 2012 at 12:37:49PM -0800, Linus Torvalds wrote:
> > So if this is a migration-specific scalability issue, then it might be
> > possible to solve by making the mutex be a rwsem instead, and have
> > migration only take it for reading.
> > 
> > Of course, I'm quite possibly wrong, and the code depends on full
> > mutual exclusion.
> > 
> > Just a thought, in case it makes somebody go "Hmm.."
> > 
> 
> Offhand, I cannot think of a reason why a rwsem would not work. This
> thing originally became a mutex because the RT people (Peter in
> particular) cared about being able to preempt faster. It'd be nice if
> they confirmed that rwsem is not be a problem for them.

rwsems are preemptable as well. So I don't think this was Peter's main
concern. If it works with an rwsem, then go ahead.

rwsems degrade on RT because we cannot do multiple reader boosting, so
they allow only a single reader which can take it recursive. But
that's an RT specific issue and nothing you should worry about.

Thanks,

	tglx
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