linux-kernel - Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

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Date:	Fri, 21 Dec 2012 16:18:56 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com, walken@...gle.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay
 factor

On Fri, 2012-12-21 at 18:56 -0500, Rik van Riel wrote:
> Argh, the first one had a typo in it that did not influence
> performance with fewer threads running, but that made things
> worse with more than a dozen threads...

> +
> +		/*
> +		 * The lock is still busy, the delay was not long enough.
> +		 * Going through here 2.7 times will, on average, cancel
> +		 * out the decrement above. Using a non-integer number
> +		 * gets rid of performance artifacts and reduces oversleeping.
> +		 */
> +		if (delay < MAX_SPINLOCK_DELAY &&
> +				((inc.head & 3) == 0 || (inc.head & 7) == 1))
> +			delay++;

((inc.head & 3) == 0 || (inc.head & 7) == 1)) seems a strange condition
to me...


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