linux-kernel - Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support pv-ticketlock

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Date:	Fri, 21 Jan 2011 19:32:08 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	kvm@...r.kernel.org, suzuki@...ibm.com
Subject: Re: [PATCH 2/3] kvm hypervisor : Add hypercalls to support
 pv-ticketlock

On Thu, Jan 20, 2011 at 09:56:27AM -0800, Jeremy Fitzhardinge wrote:
> >  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)
> 
> Why?  If a VCPU can't make progress because its waiting for some
> resource, then why not schedule something else instead?

In the process, "something else" can get more share of cpu resource than its 
entitled to and that's where I was bit concerned. I guess one could
employ hard-limits to cap "something else's" bandwidth where it is of real 
concern (like clouds).

> Presumably when
> the VCPU does become runnable, the scheduler will credit its previous
> blocked state and let it run in preference to something else.

which may not be sufficient for it to gain back bandwidth lost while blocked
(speaking of mainline scheduler atleast).

> > 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?
> 
> As soon as you add a pointer to the lock, you're increasing its size. 

I didn't really mean to expand size statically. Rather have some bits of the 
lock word store pointer to a per-cpu area when there is contention (somewhat 
similar to how bits of rt_mutex.owner are used). I haven't thought thr' this in
detail to see if that is possible though.

- vatsa
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