linux-kernel - Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

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Message-ID: <20140218073052.GV27965@twins.programming.kicks-ass.net>
Date:	Tue, 18 Feb 2014 08:30:52 +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 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On Mon, Feb 17, 2014 at 03:41:22PM -0500, Waiman Long wrote:
> +/*
> + * The queue node structure
> + *
> + * This structure is essentially the same as the mcs_spinlock structure
> + * in mcs_spinlock.h file. This structure is retained for future extension
> + * where new fields may be added.
> + */
> +struct qnode {
> +	u32		 wait;		/* Waiting flag		*/
> +	struct qnode	*next;		/* Next queue node addr */
> +};
> +
> +struct qnode_set {
> +	int		node_idx;	/* Current node to use */
> +	struct qnode	nodes[MAX_QNODES];
> +};
> +
> +/*
> + * Per-CPU queue node structures
> + */
> +static DEFINE_PER_CPU(struct qnode_set, qnset) ____cacheline_aligned
> +	= { 0 };

You really didn't pay attention did you.

That should be DEFINE_PER_CPU_ALIGNED()

Furthermore, your structure is bigger than 1 cacheline; your struct
qnode is 16 bytes, 4*16=64, and then you add that int.
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