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 16:48:52 -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...
> 
> Please let me know if you can break these patches.
> ---8<---
> Subject: x86,smp: auto tune spinlock backoff delay factor

> +#define MIN_SPINLOCK_DELAY 1
> +#define MAX_SPINLOCK_DELAY 1000
> +DEFINE_PER_CPU(int, spinlock_delay) = { MIN_SPINLOCK_DELAY };

Using a single spinlock_delay per cpu assumes there is a single
contended spinlock on the machine, or that contended
spinlocks protect the same critical section.

Given that we probably know where the contended spinlocks are, couldnt
we use a real scalable implementation for them ?

A known contended one is the Qdisc lock in network layer. We added a
second lock (busylock) to lower a bit the pressure on a separate cache
line, but a scalable lock would be much better...

I guess there are patent issues...

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