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

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Message-ID: <50D52FF8.1050104@redhat.com>
Date:	Fri, 21 Dec 2012 22:58:48 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	David Daney <ddaney.cavm@...il.com>, 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] x86,smp: auto tune spinlock backoff delay factor

On 12/21/2012 10:49 PM, Steven Rostedt wrote:
> On Fri, Dec 21, 2012 at 09:51:35PM -0500, Rik van Riel wrote:

>> However, since spinlock contention should not be the
>> usual state, and all a scalable lock does is make sure
>> that N+1 CPUs does not perform worse than N CPUs, using
>> scalable locks is a stop-gap measure.
>>
>> I believe a stop-gap measure should be kept as simple as
>> we can. I am willing to consider moving to a per-lock
>> delay factor if we can figure out an easy way to do it,
>> but I would like to avoid too much extra complexity...
>
> Rik,
>
> I like your solution. It's rather simple and simple solutions tend to
> end up being the closest to optimal. The more complex a solution gets,
> the more it starts chasing fireflies.

> Anyway, I'd like to see this code tested, and more benchmarks run
> against it.

Absolutely.  I would love to see if this code actually
causes regressions anywhere.

It is simple enough that I suspect it will not, but there
really is only one way to find out.

The more people test this with different workloads on
different SMP systems, the better.

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