When running SMP virtual machines, it is possible for one VCPU to be spinning on a spinlock, while the VCPU that holds the spinlock is not currently running, because the host scheduler preempted it to run something else.
Both Intel and AMD CPUs have a feature that detects when a virtual CPU is spinning on a lock and will trap to the host. The current KVM code sleeps for a bit whenever that happens, which results in eg. a 64 VCPU Windows guest taking forever and a bit to boot up. This is because the VCPU holding the lock is actually running and not sleeping, so the pause is counter-productive. In other workloads a pause can also be counter-productive, with spinlock detection resulting in one guest giving up its CPU time to the others. Instead of spinning, it ends up simply not running much at all. This patch series aims to fix that, by having a VCPU that spins give the remainder of its timeslice to another VCPU in the same guest before yielding the CPU - one that is runnable but got preempted, hopefully the lock holder. v5: - fix the race condition Avi pointed out, by tracking vcpu->pid - also allows us to yield to vcpu tasks that got preempted while in qemu userspace v4: - change to newer version of Mike Galbraith's yield_to implementation - chainsaw out some code from Mike that looked like a great idea, but turned out to give weird interactions in practice v3: - more cleanups - change to Mike Galbraith's yield_to implementation - yield to spinning VCPUs, this seems to work better in some situations and has little downside potential v2: - make lots of cleanups and improvements suggested - do not implement timeslice scheduling or fairness stuff yet, since it is not entirely clear how to do that right (suggestions welcome) Benchmark "results": Two 4-CPU KVM guests are pinned to the same 4 physical CPUs. One guest runs the AMQP performance test, the other guest runs 0, 2 or 4 infinite loops, for CPU overcommit factors of 0, 1.5 and 4. The AMQP perftest is run 30 times, with 8 and 16 threads. 8thr no overcommit 1.5x overcommit 2x overcommit no PLE 198918 132625 90523.5 PLE 213904 127507 95098.5 16thr no overcommit 1.5x overcommit 2x overcommit no PLE 197526 127941 87187.8 PLE 210696 136874 87005.9 Note: there seems to be something wrong with CPU balancing, possibly related to cgroups. The AMQP guest only got about 80% CPU time (of 400% total) when running with 2x overcommit, as opposed to the expected 200%. Without PLE, the guest seems to get closer to 100% CPU time, which is still far below the expected. Without overcommit, the AMQP guest gets about 340-350% CPU time without the PLE code, and around 380% CPU time with the PLE code kicking the scheduler around. Unfortunately, it looks like this test ended up more as a demonstration of other scheduler issues, than as a performance test of the PLE code. -- All rights reversed. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
