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 <marc.zyngier@xxxxxxx>
---
arch/arm/include/asm/kvm_arm.h | 4 +++-
arch/arm/kvm/Kconfig | 1 +
arch/arm/kvm/handle_exit.c | 6 +++++-
3 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/arch/arm/include/asm/kvm_arm.h b/arch/arm/include/asm/kvm_arm.h
index 64e9696..693d5b2 100644
--- a/arch/arm/include/asm/kvm_arm.h
+++ b/arch/arm/include/asm/kvm_arm.h
@@ -67,7 +67,7 @@
*/
#define HCR_GUEST_MASK (HCR_TSC | HCR_TSW | HCR_TWI | HCR_VM | HCR_BSU_IS | \
HCR_FB | HCR_TAC | HCR_AMO | HCR_IMO | HCR_FMO | \
- HCR_SWIO | HCR_TIDCP)
+ HCR_TWE | HCR_SWIO | HCR_TIDCP)
#define HCR_VIRT_EXCP_MASK (HCR_VA | HCR_VI | HCR_VF)
/* System Control Register (SCTLR) bits */
@@ -208,6 +208,8 @@
#define HSR_EC_DABT (0x24)
#define HSR_EC_DABT_HYP (0x25)
+#define HSR_WFI_IS_WFE (1U << 0)
+
#define HSR_HVC_IMM_MASK ((1UL << 16) - 1)
#define HSR_DABT_S1PTW (1U << 7)
diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
index ebf5015..466bd29 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
diff --git a/arch/arm/kvm/handle_exit.c b/arch/arm/kvm/handle_exit.c
index df4c82d..c4c496f 100644
--- a/arch/arm/kvm/handle_exit.c
+++ b/arch/arm/kvm/handle_exit.c
@@ -84,7 +84,11 @@ static int handle_dabt_hyp(struct kvm_vcpu *vcpu, struct kvm_run *run)
static int kvm_handle_wfi(struct kvm_vcpu *vcpu, struct kvm_run *run)
{
trace_kvm_wfi(*vcpu_pc(vcpu));
- kvm_vcpu_block(vcpu);
+ if (kvm_vcpu_get_hsr(vcpu) & HSR_WFI_IS_WFE)
+ kvm_vcpu_on_spin(vcpu);
+ else
+ kvm_vcpu_block(vcpu);
+
return 1;
}
--
1.8.2.3
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