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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 <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
This commit is contained in:
Marc Zyngier 2013-10-08 18:38:13 +01:00 committed by Christoffer Dall
parent d570142674
commit 58d5ec8f8e
3 changed files with 9 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
arch/arm/include/asm/kvm_arm.h
View File

@ -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)

1
arch/arm/kvm/Kconfig
View File

@ -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

6
arch/arm/kvm/handle_exit.c
View File

@ -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;
}