linux-kernel - Re: [PATCH 1/2] futex: eliminate cache miss from futex

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Message-ID: <87616fluvt.fsf@rasmusvillemoes.dk>
Date:	Mon, 22 Jun 2015 15:45:10 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Andy Lutomirski <luto@...capital.net>,
	Darren Hart <darren@...art.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] futex: eliminate cache miss from futex_hash()

On Mon, Jun 22 2015, Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Jun 22, 2015 at 03:16:18PM +0200, Rasmus Villemoes wrote:
>> +static struct {
>> +	struct futex_hash_bucket *queues;
>> +	unsigned long            hashsize;
>> +} __futex_data __read_mostly __aligned(16);
>
> Does: __aligned(sizeof(__futex_data)), work?
>

Unfortunately not:

kernel/futex.c:265:30: error: ‘__futex_data’ undeclared here (not in a function)
kernel/futex.c:265:1: error: requested alignment is not an integer constant

> Because 16 might waste 8 bytes on 32bit.

Yeah, wasting >= 48 bytes was the reason I didn't make it
____cacheline_aligned. If 8 bytes is also too much, I suppose one could
just give the struct a tag and then use sizeof(struct futex_data).

Rasmus
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