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

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Date:	Mon, 20 Feb 2012 11:33:50 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.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 Mon, Feb 20, 2012 at 11:44:25AM +0530, Raghavendra K T wrote:
> On 02/20/2012 10:46 AM, Jeremy Fitzhardinge wrote:
> [...]
> >>>>  So we get following error when we try to include jump_label.h from
> >>>>arch/x86/include/asm/spinlock.h because of cyclic dependency
> >>>><spinlock.h>  ->  <jumplabe.h>  ->  <workque.h>  ->  ...<seqlock.h>  ->  <spinlock.h>
> >>>What about splitting the jump_label_key_deferred stuff into a separate
> >>>jump_label_deferred.h, and just include that where it's needed?
> >>>
> >>Andrew Jones did exactly that (CCed).
> 
> Sorry, did not get it. Tried to search the patch. Is it similar
> work or same work?. Could you please point. shall try both the way
> (current way and jump_label_deferred way). So whichever makes
> maintainer happy, let that go :)
> 
It was not CCed to any ML. I CCed Andrew so he can chime in.

> 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.

> (Hmm. Thinking..  By the way is it not that Jeremy's earlier version
> had implementation similar to what Gleb asked ?)

--
			Gleb.
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