linux-kernel - Re: Preemptable Ticket Spinlock

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Message-ID: <1366661294.6454.18.camel@laptop>
Date:	Mon, 22 Apr 2013 22:08:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Avi Kivity <avi.kivity@...il.com>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Chegu Vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Karen Noel <knoel@...hat.com>
Subject: Re: Preemptable Ticket Spinlock

On Mon, 2013-04-22 at 15:56 -0400, Rik van Riel wrote:
> On 04/22/2013 03:49 PM, Peter Zijlstra wrote:
> > On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
> 
> >> If the native spin_lock code has been called already at
> >> that time, the native code would still need to be modified
> >> to increment the ticket number by 2, so we end up with a
> >> compatible value in each spin lock's .tickets field, and
> >> prevent a deadlock after we switch over to the paravirt
> >> variant.
> >
> > I thought the stuff already made it upstream, but apparently not; the
> > lastest posting I'm aware of is here:
> >
> >    https://lkml.org/lkml/2012/5/2/105
> >
> > That stuff changes the normal ticket increment as well..
> 
> Jiannan,
> 
> It looks like the patch above could make a good patch
> 1 (or 2) in your patch series :)

I much prefer the entire series from Jeremy since it maintains the
ticket semantics and doesn't degrade the lock to unfair under
contention.

Now I suppose there's a reason its not been merged yet and I suspect
its !paravirt hotpath impact which wasn't rightly justified or somesuch
so maybe someone can work on that or so.. dunno.


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