From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in
	a real PV environment
Date: Thu, 27 Feb 2014 12:28:43 +0000
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References: <1393427668-60228-1-git-send-email-Waiman.Long@hp.com>
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To: Waiman Long <Waiman.Long@hp.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, Peter Zijlstra <peterz@infradead.org>, virtualization@lists.linux-foundation.org, Andi Kleen <andi@firstfloor.org>, "H.
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On 26/02/14 15:14, Waiman Long wrote:
> Locking is always an issue in a virtualized environment as the virtual
> CPU that is waiting on a lock may get scheduled out and hence block
> any progress in lock acquisition even when the lock has been freed.
> 
> One solution to this problem is to allow unfair lock in a
> para-virtualized environment. In this case, a new lock acquirer can
> come and steal the lock if the next-in-line CPU to get the lock is
> scheduled out. Unfair lock in a native environment is generally not a
> good idea as there is a possibility of lock starvation for a heavily
> contended lock.

I'm not sure I'm keen on losing the fairness in PV environment.  I'm
concerned that on an over-committed host, the lock starvation problem
will be particularly bad.

But I'll have to revist this once a non-broken PV qspinlock
implementation exists (or someone explains how the proposed one works).

David