linux-kernel - Re: [PATCH] futex: eliminate cache miss from futex

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Message-ID: <20150910102220.GB19736@linux-q0g1.site>
Date:	Thu, 10 Sep 2015 03:22:21 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	kbuild test robot <fengguang.wu@...el.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] futex: eliminate cache miss from futex_hash()

On Wed, 09 Sep 2015, Rasmus Villemoes wrote:

>futex_hash() references two global variables: the base pointer
>futex_queues and the size of the array futex_hashsize. The latter is
>marked __read_mostly, while the former is not, so they are likely to
>end up very far from each other. This means that futex_hash() is
>likely to encounter two cache misses.
>
>We could mark futex_queues as __read_mostly as well, but that doesn't
>guarantee they'll end up next to each other (and even if they do, they
>may still end up in different cache lines). So put the two variables
>in a small singleton struct with sufficient alignment and mark that as
>__read_mostly.

This really doesn't have much practical effect -- not even on larger
boxes, where such things matter. For instance, I ran the patch on a
60-core IvyBridge with 'perf-bench futex', for which futex-hash
particularly benefits in good data layout (ie our current smp alignment).

http://linux-scalability.org/futex-__futex_data/

I think we should leave it as is.

Thanks,
Davidlohr
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