linux-kernel - Re: [RFC PATCH 3/5] x86,smp: auto tune spinlock backoff delay factor

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Date:	Fri, 04 Jan 2013 19:45:06 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	linux-kernel@...r.kernel.org, aquini@...hat.com,
	eric.dumazet@...il.com, lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>, knoel@...hat.com
Subject: Re: [RFC PATCH 3/5] x86,smp: auto tune spinlock backoff delay factor

On 01/03/2013 12:17 PM, Rik van Riel wrote:

>>> +               if (!(head % 7) && delay < MAX_SPINLOCK_DELAY)
>>> +                       delay++;
>>> +
>>> +               loops = delay * waiters_ahead;
>>
>> I don't like the head % 7 thing. I think using fixed point arithmetic
>> would be nicer:
>>
>> if (delay < MAX_SPINLOCK_DELAY)
>>    delay += 256/7; /* Or whatever constant we choose */
>>
>> loops = (delay * waiter_ahead) >> 8;
>
> I'll do that. That could get completely rid of any artifacts
> caused by incrementing sometimes, and not other times.
>
>> Also, we should probably skip the delay increment on the first loop
>> iteration - after all, we haven't waited yet, so we can't say that the
>> delay was too short.
>
> Good point. I will do that.

> I will build a kernel with the things you pointed out fixed,
> and will give it a spin this afternoon.
>
> Expect new patches soonish :)

After implementing all the ideas you came up with, which made
perfect sense to me, the code performs significantly worse
than before.

*sigh*

New patches will be coming ... later.

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