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 > > _______________________________________________ > Xen-devel mailing list > Xen-devel@xxxxxxxxxxxxx > http://lists.xen.org/xen-devel -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html