linux-kernel - Re: Preemptable Ticket Spinlock

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Message-id: <CAJocwccDznaMGkBgWpZ9M5k3JUqf0pnvK2hDyRBR_JEseNrQ_g@mail.gmail.com>
Date:	Sun, 21 Apr 2013 19:07:51 -0400
From:	Jiannan Ouyang <ouyang@...pitt.edu>
To:	Rik van Riel <riel@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Sun, Apr 21, 2013 at 5:12 PM, Rik van Riel <riel@...hat.com> wrote:
> Your algorithm is very clever, and very promising.
>
> However, it does increase the size of the struct spinlock, and adds
> an additional atomic operation to spin_unlock, neither of which I
> suspect are necessary.
>
> If we always incremented the ticket number by 2 (instead of 1), then
> we could use the lower bit of the ticket number as the spinlock.
>
> If we do NOT run virtualized, we simply increment the ticket by 2
> in spin_unlock, and the code can remain otherwise the same.
>
> If we do run virtualized, we take that spinlock after acquiring
> the ticket (or timing out), just like in your code. In the
> virtualized spin_unlock, we can then release the spinlock and
> increment the ticket in one operation: by simply increasing the
> ticket by 1.
>
> In other words, we should be able to keep the overhead of this
> to an absolute minimum, and keep spin_unlock to be always the
> same cost it is today.
>
> --
> All rights reversed

Hi Rik,

Thanks for your feedback.

Yes I agree with you
- increase the size of struct spinlock is unnecessary
- your idea of utilize the lower bit and save one atomic operation
from unlock is cool!

I can come up with a updated patch soon.

--Jiannan
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