WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support p

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support pv-ticketlock
From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
Date: Thu, 20 Jan 2011 17:29:58 +0530
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, Nick Piggin <npiggin@xxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Eric Dumazet <dada1@xxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, suzuki@xxxxxxxxxx, Avi Kivity <avi@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Américo Wang <xiyou.wangcong@xxxxxxxxx>, Linux Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Delivery-date: Thu, 20 Jan 2011 04:01:45 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4D373340.60608@xxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <cover.1289940821.git.jeremy.fitzhardinge@xxxxxxxxxx> <20110119164432.GA30669@xxxxxxxxxxxxxxxxxx> <20110119171239.GB726@xxxxxxxxxxxxxxxxxx> <1295457672.28776.144.camel@laptop> <4D373340.60608@xxxxxxxx>
Reply-to: vatsa@xxxxxxxxxxxxxxxxxx
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Jan 19, 2011 at 10:53:52AM -0800, Jeremy Fitzhardinge wrote:
> > I didn't really read the patch, and I totally forgot everything from
> > when I looked at the Xen series, but does the Xen/KVM hypercall
> > interface for this include the vcpu to await the kick from?
> >
> > My guess is not, since the ticket locks used don't know who the owner
> > is, which is of course, sad. There are FIFO spinlock implementations
> > that can do this though.. although I think they all have a bigger memory
> > footprint.
> 
> At least in the Xen code, a current owner isn't very useful, because we
> need the current owner to kick the *next* owner to life at release time,
> which we can't do without some structure recording which ticket belongs
> to which cpu.

If we had a yield-to [1] sort of interface _and_ information on which vcpu
owns a lock, then lock-spinners can yield-to the owning vcpu, while the
unlocking vcpu can yield-to the next-vcpu-in-waiting. The key here is not to
sleep when waiting for locks (as implemented by current patch-series, which can 
put other VMs at an advantage by giving them more time than they are entitled 
to) and also to ensure that lock-owner as well as the next-in-line lock-owner 
are not unduly made to wait for cpu. 

Is there a way we can dynamically expand the size of lock only upon contention
to include additional information like owning vcpu? Have the lock point to a
per-cpu area upon contention where additional details can be stored perhaps?

1. https://lkml.org/lkml/2011/1/14/44

- vatsa

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>