Re: [Xen-devel] [PATCH RFC V6 0/11] Paravirtualized ticketlocks

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-04-16 at 16:44 +0100, Konrad Rzeszutek Wilk wrote:
> On Sat, Mar 31, 2012 at 09:37:45AM +0530, Srivatsa Vaddagiri wrote:
> > * Thomas Gleixner <tglx@xxxxxxxxxxxxx> [2012-03-31 00:07:58]:
> > 
> > > I know that Peter is going to go berserk on me, but if we are running
> > > a paravirt guest then it's simple to provide a mechanism which allows
> > > the host (aka hypervisor) to check that in the guest just by looking
> > > at some global state.
> > > 
> > > So if a guest exits due to an external event it's easy to inspect the
> > > state of that guest and avoid to schedule away when it was interrupted
> > > in a spinlock held section. That guest/host shared state needs to be
> > > modified to indicate the guest to invoke an exit when the last nested
> > > lock has been released.
> > 
> > I had attempted something like that long back:
> > 
> > http://lkml.org/lkml/2010/6/3/4
> > 
> > The issue is with ticketlocks though. VCPUs could go into a spin w/o
> > a lock being held by anybody. Say VCPUs 1-99 try to grab a lock in
> > that order (on a host with one cpu). VCPU1 wins (after VCPU0 releases it)
> > and releases the lock. VCPU1 is next eligible to take the lock. If 
> > that is not scheduled early enough by host, then remaining vcpus would keep 
> > spinning (even though lock is technically not held by anybody) w/o making 
> > forward progress.
> > 
> > In that situation, what we really need is for the guest to hint to host
> > scheduler to schedule VCPU1 early (via yield_to or something similar). 
> > 
> > The current pv-spinlock patches however does not track which vcpu is
> > spinning at what head of the ticketlock. I suppose we can consider 
> > that optimization in future and see how much benefit it provides (over
> > plain yield/sleep the way its done now).
> 
> Right. I think Jeremy played around with this some time?

5/11 "xen/pvticketlock: Xen implementation for PV ticket locks" tracks
which vcpus are waiting for a lock in "cpumask_t waiting_cpus" and
tracks which lock each is waiting for in per-cpu "lock_waiting". This is
used in xen_unlock_kick to kick the right CPU. There's a loop over only
the waiting cpus to figure out who to kick.

> > 
> > Do you see any issues if we take in what we have today and address the
> > finer-grained optimization as next step?
> 
> I think that is the proper course - these patches show
> that on baremetal we don't incur performance regressions and in
> virtualization case we benefit greatly. Since these are the basic
> building blocks of a kernel - taking it slow and just adding
> this set of patches for v3.5 is a good idea - and then building on top
> of that for further refinement.
> 
> > 
> > - vatsa 
> 
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