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KVM: use yield_to instead of sleep in kvm_vcpu_on_spin 

Instead of sleeping in kvm_vcpu_on_spin, which can cause gigantic
slowdowns of certain workloads, we instead use yield_to to get
another VCPU in the same KVM guest to run sooner.

This seems to give a 10-15% speedup in certain workloads.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Rik van Riel 2011-02-01 09:53:28 -05:00 committed by Marcelo Tosatti
parent 34bb10b79d
commit 217ece6129
2 changed files with 48 additions and 10 deletions
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1
include/linux/kvm_host.h
View File

@ -235,6 +235,7 @@ struct kvm {
#endif
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
atomic_t online_vcpus;
int last_boosted_vcpu;
struct list_head vm_list;
struct mutex lock;
struct kvm_io_bus *buses[KVM_NR_BUSES];

57
virt/kvm/kvm_main.c
View File

@ -1484,18 +1484,55 @@ void kvm_resched(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_resched);
void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu)
void kvm_vcpu_on_spin(struct kvm_vcpu *me)
{
ktime_t expires;
DEFINE_WAIT(wait);
struct kvm *kvm = me->kvm;
struct kvm_vcpu *vcpu;
int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
int yielded = 0;
int pass;
int i;
prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
/* Sleep for 100 us, and hope lock-holder got scheduled */
expires = ktime_add_ns(ktime_get(), 100000UL);
schedule_hrtimeout(&expires, HRTIMER_MODE_ABS);
finish_wait(&vcpu->wq, &wait);
/*
* We boost the priority of a VCPU that is runnable but not
* currently running, because it got preempted by something
* else and called schedule in __vcpu_run. Hopefully that
* VCPU is holding the lock that we need and will release it.
* We approximate round-robin by starting at the last boosted VCPU.
*/
for (pass = 0; pass < 2 && !yielded; pass++) {
kvm_for_each_vcpu(i, vcpu, kvm) {
struct task_struct *task = NULL;
struct pid *pid;
if (!pass && i < last_boosted_vcpu) {
i = last_boosted_vcpu;
continue;
} else if (pass && i > last_boosted_vcpu)
break;
if (vcpu == me)
continue;
if (waitqueue_active(&vcpu->wq))
continue;
rcu_read_lock();
pid = rcu_dereference(vcpu->pid);
if (pid)
task = get_pid_task(vcpu->pid, PIDTYPE_PID);
rcu_read_unlock();
if (!task)
continue;
if (task->flags & PF_VCPU) {
put_task_struct(task);
continue;
}
if (yield_to(task, 1)) {
put_task_struct(task);
kvm->last_boosted_vcpu = i;
yielded = 1;
break;
}
put_task_struct(task);
}
}
}
EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);