From 58d5ec8f8ee318b26b29207874fbaee626973952 Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Tue, 8 Oct 2013 18:38:13 +0100 Subject: ARM: KVM: Yield CPU when vcpu executes a WFE On an (even slightly) oversubscribed system, spinlocks are quickly becoming a bottleneck, as some vcpus are spinning, waiting for a lock to be released, while the vcpu holding the lock may not be running at all. This creates contention, and the observed slowdown is 40x for hackbench. No, this isn't a typo. The solution is to trap blocking WFEs and tell KVM that we're now spinning. This ensures that other vpus will get a scheduling boost, allowing the lock to be released more quickly. Also, using CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance when the VM is severely overcommited. Quick test to estimate the performance: hackbench 1 process 1000 2xA15 host (baseline): 1.843s 2xA15 guest w/o patch: 2.083s 4xA15 guest w/o patch: 80.212s 8xA15 guest w/o patch: Could not be bothered to find out 2xA15 guest w/ patch: 2.102s 4xA15 guest w/ patch: 3.205s 8xA15 guest w/ patch: 6.887s So we go from a 40x degradation to 1.5x in the 2x overcommit case, which is vaguely more acceptable. Signed-off-by: Marc Zyngier Signed-off-by: Christoffer Dall --- arch/arm/kvm/Kconfig | 1 + 1 file changed, 1 insertion(+) (limited to 'arch/arm/kvm/Kconfig') diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig index ebf5015508b5..466bd299b1a8 100644 --- a/arch/arm/kvm/Kconfig +++ b/arch/arm/kvm/Kconfig @@ -20,6 +20,7 @@ config KVM bool "Kernel-based Virtual Machine (KVM) support" select PREEMPT_NOTIFIERS select ANON_INODES + select HAVE_KVM_CPU_RELAX_INTERCEPT select KVM_MMIO select KVM_ARM_HOST depends on ARM_VIRT_EXT && ARM_LPAE -- cgit