linux-kernel - Re: [PATCH 1/1] linux headers: header file(s) changes to enable spinlock use jumplabel

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Date:	Wed, 22 Feb 2012 16:18:09 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Tejun Heo <tj@...nel.org>,
	John Stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH 1/1] linux headers: header file(s) changes to enable spinlock
 use jumplabel

On 02/20/2012 03:03 PM, Gleb Natapov wrote:
> On Mon, Feb 20, 2012 at 11:44:25AM +0530, Raghavendra K T wrote:
>> On 02/20/2012 10:46 AM, Jeremy Fitzhardinge wrote:
[...]
>> But does pvlock have to use jump
>>>> label? I looked at the code and it is used like paravirt patching. Meaning
>>>> it is patched only once on a boot up when XEN is detected. May be use
>>>> paravirt patching instead of jump label? What if jump label will want
>>>> to use spinlock for some reason in the future (it uses mutex currently)?
>>>
>>> The point of the pv ticketlocks is to avoid any pvop calls on the
>>> lock/unlock fastpath, relegating them to only the slow path.
>>> Unfortunately, the pv unlock case can't be identical with the non-pv
>>> unlock, and jump_labels are lighter weight and more efficient than pvops.
>>>
>>> It doesn't matter if jump_labels start using spinlocks; all we need the
>>> jump_label machinery to do is patch the jump sites in the code so that
>>> one of two execution paths can be selected.  Since all the ticketlock
>>> jump_label patching happens before SMP is enabled, there's no problem
>>> with changing a lock while a cpu is executing the code.
>>>
>>
>> I also felt agreeing with Jeremy. seemed to me that latter is more
>> performance friendly. no?.
>>
>
> I thought not about pvop, but about alternative().  jump_labels is used
> by spinlock to patch out jump into nops It can be done via alternative()
> too I think.

I had remembered that this discussion already happened with Jeremy's V5
of ticketlock patches. pulling out link :

https://lkml.org/lkml/2011/10/13/384

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