On Mon, Jul 26, 2010 at 10:18:58AM -0700, Jeremy Fitzhardinge wrote: > >I tried to refactor Xen's spinlock > >implementation to make it common for both Xen and KVM - but found that > >few differences between Xen and KVM (Xen has the ability to block on a > >particular event/irq for example) _and_ the fact that the guest kernel > >can be compiled to support both Xen and KVM hypervisors (CONFIG_XEN and > >CONFIG_KVM_GUEST can both be turned on) makes the "common" code a eye-sore. > >There will have to be: > > > > if (xen) { > > ... > > } else if (kvm) { > > .. > > } > > > >or possibly: > > > > alternative(NOP, some_xen_specific_call, ....) > > > >type of code in the common implementation. > > No, that doesn't look like a good approach. It suggests the > apparently commonality isn't really there. > > >For the time-being, I have made this KVM-specific only. At somepoint in future, > >I hope this can be made common between Xen/KVM. > > Did you see the patch series I posted a couple of weeks ago to > revamp pv spinlocks? Specifically, I dropped the notion of pv > spinlocks in which the entire spinlock implementation is replaced, > and added pv ticketlocks where the ticketlock algorithm is always > used for the fastpath, but it calls out to pvop calls for the > slowpath (a long spin, or unlocking a lock with waiters). It > significantly reduces the amount of hypervisor-specific code. Hmmm interesting - I will go thr' it in detail. > You can see the current patches in > > git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git > xen/pvticketlock-git [snip] > That's not actually the real problem. It's *a* problem, but > insignificant compared to the ticketlock-specific "next-in-line vcpu > scheduler bunfight" problem - lock holder preemption is a misnomer. > Fortunately the solutions to both are (nearly) the same. > > See Thomas Friebel's talk "Prevent Guests from Spinning Around > (http://www.xen.org/files/xensummitboston08/LHP.pdf). Yes I had seen Thomas's slides reporting huge degradation in performance with tick spinlock. > >b. Avoid preempting a lock-holder while its holding a (spin-) lock. > > > > In this scheme, guest OS can hint (set some flag in memory shared with > > hypervisor) whenever its holding a lock and hypervisor could defer preempting > > the guest vcpu when its holding a lock. With this scheme, we should never > > have a lock-acquiring vcpu spin on a preempted vcpu to release its lock. If > > ever it spins, its because somebody *currently running* is holding the lock - > > and hence it won't have to spin-wait too long. IOW we are pro-actively > > trying to prevent the LHP problem from occuring in the first place. This > > should improve job turnaround time for some workloads. [1] has some > > results based on this approach. > > This doesn't actually help the problem mentioned above, because it's > not a problem with the lock holder getting preempted, but what > happens once the lock has been released. Good point. I agree that the latter problem needs more attention, given a ticket-type implementation of spinlocks. Have you considered possible solutions for unmodified guests, which have similar ticket-type lock implementations? Not sure if that's important enough to investigate solutions like gang scheduling .. - vatsa -- 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