linux-kernel - Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

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Date:	Wed, 19 Feb 2014 12:39:29 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Waiman Long <waiman.long@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

On 02/19/2014 06:12 AM, Waiman Long wrote:
> On 02/18/2014 04:28 PM, Peter Zijlstra wrote:
>> On Tue, Feb 18, 2014 at 02:30:12PM -0500, Waiman Long wrote:
>>> I will start looking at how to make it work with paravirt. Hopefully, it
>>> won't take too long.
>> The cheap way out is to simply switch to the test-and-set spinlock on
>> whatever X86_FEATURE_ indicates a guest I suppose.
>
> I don't think there is X86_FEATURE flag that indicates running in a
> guest. In fact, a guest should never find out if it is running virtualized.
>
> After reading the current PV ticketlock implementation, I have a rough
> idea of what I need to do to implement PV support in qspinlock. A large
> portion of PV ticketlock code is find out the CPU number of the next one
> to get the lock. The current qspinlock implementation has already
> included CPU number of the previous member in the queue and it should be
> pretty easy to store CPU number of the next one in the queue node
> structure. These CPU numbers can then be supplied to the kick_cpu()
> function to schedule in the require the CPU to make sure that progress
> can be made.

That is correct.
Strict serialization of the lock is usually a headache for virtualized
guest (especially when overcommitted). I am eager to test the next
version.

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