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

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Message-ID: <CA+55aFwjxm7OYuucHeE2WFr4p+jwr63t=kSdHndta_QkyFbyBQ@mail.gmail.com>
Date:	Fri, 30 Nov 2012 12:37:49 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 00/10] Latest numa/core release, v18

On Fri, Nov 30, 2012 at 11:58 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> When pushed hard enough via threaded workloads (for example via the
> numa02 test) then the upstream page migration code in mm/migration.c
> becomes unscalable, resulting in lot of scheduling on the anon vma
> mutex and a subsequent drop in performance.

Ugh.

I wonder if migration really needs that thing to be a mutex? I may be
wrong, but the anon_vma lock only protects the actual rmap chains, and
migration only ever changes the pte *contents*, not the actual chains
of pte's themselves, right?

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.."

               Linus
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