[PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus

Radim Krčmář
2017-08-21T20:35:22Z

We can add a variable instead of scanning all online VCPUs to know how
many are started. We can't trivially tell which VCPU is the last one,
though.

Suggested-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
arch/s390/include/asm/kvm_host.h | 1 +
arch/s390/kvm/kvm-s390.c | 39 +++++++++++++++------------------------
2 files changed, 16 insertions(+), 24 deletions(-)

diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index a409d5991934..be0d0bdf585b 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -735,6 +735,7 @@ struct kvm_arch{
struct mutex ipte_mutex;
struct ratelimit_state sthyi_limit;
spinlock_t start_stop_lock;
+ unsigned started_vcpus;
struct sie_page2 *sie_page2;
struct kvm_s390_cpu_model model;
struct kvm_s390_crypto crypto;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 9f23a9e81a91..1534778a3c66 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3414,25 +3414,17 @@ static void __enable_ibs_on_vcpu(struct kvm_vcpu *vcpu)

void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
{
- int i, online_vcpus, started_vcpus = 0;
-
if (!is_vcpu_stopped(vcpu))
return;

trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 1);
/* Only one cpu at a time may enter/leave the STOPPED state. */
spin_lock(&vcpu->kvm->arch.start_stop_lock);
- online_vcpus = atomic_read(&vcpu->kvm->online_vcpus);

- for (i = 0; i < online_vcpus; i++) {
- if (!is_vcpu_stopped(vcpu->kvm->vcpus[i]))
- started_vcpus++;
- }
-
- if (started_vcpus == 0) {
+ if (vcpu->kvm->arch.started_vcpus == 0) {
/* we're the only active VCPU -> speed it up */
__enable_ibs_on_vcpu(vcpu);
- } else if (started_vcpus == 1) {
+ } else if (vcpu->kvm->arch.started_vcpus == 1) {
/*
* As we are starting a second VCPU, we have to disable
* the IBS facility on all VCPUs to remove potentially
@@ -3441,6 +3433,8 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
__disable_ibs_on_all_vcpus(vcpu->kvm);
}

+ vcpu->kvm->arch.started_vcpus++;
+
atomic_andnot(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
/*
* Another VCPU might have used IBS while we were offline.
@@ -3453,16 +3447,12 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)

void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
{
- int i, online_vcpus, started_vcpus = 0;
- struct kvm_vcpu *started_vcpu = NULL;
-
if (is_vcpu_stopped(vcpu))
return;

trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 0);
/* Only one cpu at a time may enter/leave the STOPPED state. */
spin_lock(&vcpu->kvm->arch.start_stop_lock);
- online_vcpus = atomic_read(&vcpu->kvm->online_vcpus);

/* SIGP STOP and SIGP STOP AND STORE STATUS has been fully processed */
kvm_s390_clear_stop_irq(vcpu);
@@ -3470,19 +3460,20 @@ void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
atomic_or(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
__disable_ibs_on_vcpu(vcpu);

- for (i = 0; i < online_vcpus; i++) {
- if (!is_vcpu_stopped(vcpu->kvm->vcpus[i])) {
- started_vcpus++;
- started_vcpu = vcpu->kvm->vcpus[i];
- }
- }
+ vcpu->kvm->arch.started_vcpus--;
+
+ if (vcpu->kvm->arch.started_vcpus == 1) {
+ struct kvm_vcpu *started_vcpu;

- if (started_vcpus == 1) {
/*
- * As we only have one VCPU left, we want to enable the
- * IBS facility for that VCPU to speed it up.
+ * As we only have one VCPU left, we want to enable the IBS
+ * facility for that VCPU to speed it up.
*/
- __enable_ibs_on_vcpu(started_vcpu);
+ kvm_for_each_vcpu(i, started_vcpu, vcpu->kvm)
+ if (!is_vcpu_stopped(started_vcpu)) {
+ __enable_ibs_on_vcpu(started_vcpu);
+ break;
+ }
}

spin_unlock(&vcpu->kvm->arch.start_stop_lock);
--
2.13.3