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

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Date:	Thu, 03 Jan 2013 09:05:53 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Rik van Riel" <riel@...hat.com>
Cc:	<eric.dumazet@...il.com>, <rostedt@...dmis.org>,
	<therbert@...gle.com>, <walken@...gle.com>, <jeremy@...p.org>,
	<tglx@...utronix.de>, <aquini@...hat.com>, <lwoodman@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff
 delay factor

>>> On 27.12.12 at 20:09, Rik van Riel <riel@...hat.com> wrote:
> On 12/27/2012 01:41 PM, Jan Beulich wrote:
>>>>> Rik van Riel <riel@...hat.com> 12/27/12 4:01 PM >>>
>>> On 12/27/2012 09:27 AM, Eric Dumazet wrote:
>>>> So the hash sounds good to me, because the hash key could mix both lock
>>>> address and caller IP ( __builtin_return_address(1) in
>>>> ticket_spin_lock_wait())
>>>
>>> The lock acquisition time depends on the holder of the lock,
>>> and what the CPUs ahead of us in line will do with the lock,
>>> not on the caller IP of the spinner.
>>
>> The lock holder could supply its __builtin_return_address(0) for use
>> in eventual hashing.
>>
>> Also, with all of this - did you evaluate the alternative of using
>> monitor/mwait instead?
> 
> How much bus traffic do monitor/mwait cause behind the scenes?

I would suppose that this just snoops the bus for writes, but the
amount of bus traffic involved in this isn't explicitly documented.

One downside of course is that unless a spin lock is made occupy
exactly a cache line, false wakeups are possible.

Jan

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