[patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

Marcelo Tosatti
2015-04-08T23:33:24Z

[-- Attachment #1: kvm-use-simplewaitqueue-1 --]
[-- Type: text/plain, Size: 10348 bytes --]

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the
LAPIC path for a KVM guest.

The solution:

Allow waking up vcpu thread from hardirq context,
thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT
are sleepable locks. Therefore, waking up a waitqueue
waiter involves locking a sleeping lock, which
is not allowed from hard interrupt context.

cyclictest command line:
# cyclictest -m -n -q -p99 -l 1000000 -h60 -D 1m

This patch reduces the average latency in my tests from 14us to 11us.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---
arch/arm/kvm/arm.c | 4 ++--
arch/arm/kvm/psci.c | 4 ++--
arch/powerpc/include/asm/kvm_host.h | 4 ++--
arch/powerpc/kvm/book3s_hv.c | 20 ++++++++++----------
arch/s390/include/asm/kvm_host.h | 2 +-
arch/s390/kvm/interrupt.c | 8 ++++----
arch/x86/kvm/lapic.c | 6 +++---
include/linux/kvm_host.h | 4 ++--
virt/kvm/async_pf.c | 4 ++--
virt/kvm/kvm_main.c | 16 ++++++++--------
10 files changed, 36 insertions(+), 36 deletions(-)

Index: rt-linux/arch/arm/kvm/arm.c
===================================================================
--- rt-linux.orig/arch/arm/kvm/arm.c 2015-04-08 20:20:39.962649422 -0300
+++ rt-linux/arch/arm/kvm/arm.c 2015-04-08 20:20:41.966654408 -0300
@@ -441,9 +441,9 @@

static void vcpu_pause(struct kvm_vcpu *vcpu)
{
- wait_queue_head_t *wq = kvm_arch_vcpu_wq(vcpu);
+ struct swait_head *wq = kvm_arch_vcpu_wq(vcpu);

- wait_event_interruptible(*wq, !vcpu->arch.pause);
+ swait_event_interruptible(*wq, !vcpu->arch.pause);
}

static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
Index: rt-linux/arch/arm/kvm/psci.c
===================================================================
--- rt-linux.orig/arch/arm/kvm/psci.c 2015-04-08 20:20:39.962649422 -0300
+++ rt-linux/arch/arm/kvm/psci.c 2015-04-08 20:20:41.966654408 -0300
@@ -66,7 +66,7 @@
{
struct kvm *kvm = source_vcpu->kvm;
struct kvm_vcpu *vcpu = NULL, *tmp;
- wait_queue_head_t *wq;
+ struct swait_head *wq;
unsigned long cpu_id;
unsigned long context_id;
unsigned long mpidr;
@@ -123,7 +123,7 @@
smp_mb(); /* Make sure the above is visible */

wq = kvm_arch_vcpu_wq(vcpu);
- wake_up_interruptible(wq);
+ swait_wake_interruptible(wq);

return PSCI_RET_SUCCESS;
}
Index: rt-linux/arch/powerpc/include/asm/kvm_host.h
===================================================================
--- rt-linux.orig/arch/powerpc/include/asm/kvm_host.h 2015-04-08 20:20:39.963649425 -0300
+++ rt-linux/arch/powerpc/include/asm/kvm_host.h 2015-04-08 20:20:41.966654408 -0300
@@ -296,7 +296,7 @@
u8 in_guest;
struct list_head runnable_threads;
spinlock_t lock;
- wait_queue_head_t wq;
+ struct swait_head wq;
u64 stolen_tb;
u64 preempt_tb;
struct kvm_vcpu *runner;
@@ -618,7 +618,7 @@
u8 prodded;
u32 last_inst;

- wait_queue_head_t *wqp;
+ struct swait_head *wqp;
struct kvmppc_vcore *vcore;
int ret;
int trap;
Index: rt-linux/arch/powerpc/kvm/book3s_hv.c
===================================================================
--- rt-linux.orig/arch/powerpc/kvm/book3s_hv.c 2015-04-08 20:20:39.964649427 -0300
+++ rt-linux/arch/powerpc/kvm/book3s_hv.c 2015-04-08 20:20:41.966654408 -0300
@@ -84,11 +84,11 @@
{
int me;
int cpu = vcpu->cpu;
- wait_queue_head_t *wqp;
+ struct swait_head *wqp;

wqp = kvm_arch_vcpu_wq(vcpu);
- if (waitqueue_active(wqp)) {
- wake_up_interruptible(wqp);
+ if (swaitqueue_active(wqp)) {
+ swait_wake_interruptible(wqp);
++vcpu->stat.halt_wakeup;
}

@@ -639,8 +639,8 @@
tvcpu->arch.prodded = 1;
smp_mb();
if (vcpu->arch.ceded) {
- if (waitqueue_active(&vcpu->wq)) {
- wake_up_interruptible(&vcpu->wq);
+ if (swaitqueue_active(&vcpu->wq)) {
+ swait_wake_interruptible(&vcpu->wq);
vcpu->stat.halt_wakeup++;
}
}
@@ -1357,7 +1357,7 @@

INIT_LIST_HEAD(&vcore->runnable_threads);
spin_lock_init(&vcore->lock);
- init_waitqueue_head(&vcore->wq);
+ init_swait_head(&vcore->wq);
vcore->preempt_tb = TB_NIL;
vcore->lpcr = kvm->arch.lpcr;
vcore->first_vcpuid = core * threads_per_subcore;
@@ -1826,13 +1826,13 @@
*/
static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
{
- DEFINE_WAIT(wait);
+ DEFINE_SWAITER(wait);

- prepare_to_wait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
+ swait_prepare(&vc->wq, &wait, TASK_INTERRUPTIBLE);
vc->vcore_state = VCORE_SLEEPING;
spin_unlock(&vc->lock);
schedule();
- finish_wait(&vc->wq, &wait);
+ swait_finish(&vc->wq, &wait);
spin_lock(&vc->lock);
vc->vcore_state = VCORE_INACTIVE;
}
@@ -1873,7 +1873,7 @@
kvmppc_create_dtl_entry(vcpu, vc);
kvmppc_start_thread(vcpu);
} else if (vc->vcore_state == VCORE_SLEEPING) {
- wake_up(&vc->wq);
+ swait_wake(&vc->wq);
}

}
Index: rt-linux/arch/s390/include/asm/kvm_host.h
===================================================================
--- rt-linux.orig/arch/s390/include/asm/kvm_host.h 2015-04-08 20:20:39.964649427 -0300
+++ rt-linux/arch/s390/include/asm/kvm_host.h 2015-04-08 20:20:41.967654410 -0300
@@ -311,7 +311,7 @@
struct list_head list;
atomic_t active;
struct kvm_s390_float_interrupt *float_int;
- wait_queue_head_t *wq;
+ struct swait_head *wq;
atomic_t *cpuflags;
unsigned int action_bits;
};
Index: rt-linux/arch/s390/kvm/interrupt.c
===================================================================
--- rt-linux.orig/arch/s390/kvm/interrupt.c 2015-04-08 20:20:39.965649430 -0300
+++ rt-linux/arch/s390/kvm/interrupt.c 2015-04-08 20:20:41.967654410 -0300
@@ -619,13 +619,13 @@

void kvm_s390_vcpu_wakeup(struct kvm_vcpu *vcpu)
{
- if (waitqueue_active(&vcpu->wq)) {
+ if (swaitqueue_active(&vcpu->wq)) {
/*
* The vcpu gave up the cpu voluntarily, mark it as a good
* yield-candidate.
*/
vcpu->preempted = true;
- wake_up_interruptible(&vcpu->wq);
+ swait_wake_interruptible(&vcpu->wq);
vcpu->stat.halt_wakeup++;
}
}
@@ -736,7 +736,7 @@
spin_lock(&li->lock);
list_add(&inti->list, &li->list);
atomic_set(&li->active, 1);
- BUG_ON(waitqueue_active(li->wq));
+ BUG_ON(swaitqueue_active(li->wq));
spin_unlock(&li->lock);
return 0;
}
@@ -761,7 +761,7 @@
spin_lock(&li->lock);
list_add(&inti->list, &li->list);
atomic_set(&li->active, 1);
- BUG_ON(waitqueue_active(li->wq));
+ BUG_ON(swaitqueue_active(li->wq));
spin_unlock(&li->lock);
return 0;
}
Index: rt-linux/arch/x86/kvm/lapic.c
===================================================================
--- rt-linux.orig/arch/x86/kvm/lapic.c 2015-04-08 20:20:39.965649430 -0300
+++ rt-linux/arch/x86/kvm/lapic.c 2015-04-08 20:20:41.000000000 -0300
@@ -1539,7 +1539,7 @@
struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
struct kvm_lapic *apic = container_of(ktimer, struct kvm_lapic, lapic_timer);
struct kvm_vcpu *vcpu = apic->vcpu;
- wait_queue_head_t *q = &vcpu->wq;
+ struct swait_head *q = &vcpu->wq;

/*
* There is a race window between reading and incrementing, but we do
@@ -1553,8 +1553,8 @@
kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
}

- if (waitqueue_active(q))
- wake_up_interruptible(q);
+ if (swaitqueue_active(q))
+ swait_wake_interruptible(q);

if (lapic_is_periodic(apic)) {
hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);
Index: rt-linux/include/linux/kvm_host.h
===================================================================
--- rt-linux.orig/include/linux/kvm_host.h 2015-04-08 20:20:39.966649432 -0300
+++ rt-linux/include/linux/kvm_host.h 2015-04-08 20:20:41.967654410 -0300
@@ -244,7 +244,7 @@

int fpu_active;
int guest_fpu_loaded, guest_xcr0_loaded;
- wait_queue_head_t wq;
+ struct swait_head wq;
struct pid *pid;
int sigset_active;
sigset_t sigset;
@@ -687,7 +687,7 @@
}
#endif

-static inline wait_queue_head_t *kvm_arch_vcpu_wq(struct kvm_vcpu *vcpu)
+static inline struct swait_head *kvm_arch_vcpu_wq(struct kvm_vcpu *vcpu)
{
#ifdef __KVM_HAVE_ARCH_WQP
return vcpu->arch.wqp;
Index: rt-linux/virt/kvm/async_pf.c
===================================================================
--- rt-linux.orig/virt/kvm/async_pf.c 2015-04-08 20:20:39.966649432 -0300
+++ rt-linux/virt/kvm/async_pf.c 2015-04-08 20:20:41.968654413 -0300
@@ -94,8 +94,8 @@

trace_kvm_async_pf_completed(addr, gva);

- if (waitqueue_active(&vcpu->wq))
- wake_up_interruptible(&vcpu->wq);
+ if (swaitqueue_active(&vcpu->wq))
+ swait_wake_interruptible(&vcpu->wq);

mmput(mm);
kvm_put_kvm(vcpu->kvm);
Index: rt-linux/virt/kvm/kvm_main.c
===================================================================
--- rt-linux.orig/virt/kvm/kvm_main.c 2015-04-08 20:20:39.966649432 -0300
+++ rt-linux/virt/kvm/kvm_main.c 2015-04-08 20:20:41.968654413 -0300
@@ -221,7 +221,7 @@
vcpu->kvm = kvm;
vcpu->vcpu_id = id;
vcpu->pid = NULL;
- init_waitqueue_head(&vcpu->wq);
+ init_swait_head(&vcpu->wq);
kvm_async_pf_vcpu_init(vcpu);

page = alloc_page(GFP_KERNEL | __GFP_ZERO);
@@ -1740,10 +1740,10 @@
*/
void kvm_vcpu_block(struct kvm_vcpu *vcpu)
{
- DEFINE_WAIT(wait);
+ DEFINE_SWAITER(wait);

for (;;) {
- prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
+ swait_prepare(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);

if (kvm_arch_vcpu_runnable(vcpu)) {
kvm_make_request(KVM_REQ_UNHALT, vcpu);
@@ -1757,7 +1757,7 @@
schedule();
}

- finish_wait(&vcpu->wq, &wait);
+ swait_finish(&vcpu->wq, &wait);
}
EXPORT_SYMBOL_GPL(kvm_vcpu_block);

@@ -1769,11 +1769,11 @@
{
int me;
int cpu = vcpu->cpu;
- wait_queue_head_t *wqp;
+ struct swait_head *wqp;

wqp = kvm_arch_vcpu_wq(vcpu);
- if (waitqueue_active(wqp)) {
- wake_up_interruptible(wqp);
+ if (swaitqueue_active(wqp)) {
+ swait_wake_interruptible(wqp);
++vcpu->stat.halt_wakeup;
}

@@ -1878,7 +1878,7 @@
continue;
if (vcpu == me)
continue;
- if (waitqueue_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
+ if (swaitqueue_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
continue;
if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
continue;