linux-kernel - Re: [PATCH v4 3/3] qspinlock, x86: Add x86 specific optimization for 2 contending tasks

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Message-ID: <20140221172811.GS9987@twins.programming.kicks-ass.net>
Date:	Fri, 21 Feb 2014 18:28:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 3/3] qspinlock, x86: Add x86 specific optimization for
 2 contending tasks

On Mon, Feb 17, 2014 at 03:41:24PM -0500, Waiman Long wrote:
> +	struct {
> +		u8  lock;	/* Lock bit	*/
> +		u8  wait;	/* Waiting bit	*/
> +		u16 qcode;	/* Queue code	*/
> +	};

16 bit code would result in 14 bits for the CPU number, that's only 16k,
I think SGI actually had a machine with that many CPUs in.

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