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Subject[RFC -v5 PATCH 4/4] 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 hand
the rest of our timeslice to another vcpu in the same KVM guest.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9d56ed5..fab2250 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -187,6 +187,7 @@ 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];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 65e997a..a7c45c8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1292,18 +1292,52 @@ 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);
-
- 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);
+ 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;

- 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;
+ if (!pass && i < last_boosted_vcpu) {
+ i = last_boosted_vcpu;
+ continue;
+ } else if (pass && i > last_boosted_vcpu)
+ break;
+ if (vcpu == me)
+ continue;
+ if (!vcpu->pid)
+ continue;
+ if (waitqueue_active(&vcpu->wq))
+ continue;
+ task = get_pid_task(vcpu->pid, PIDTYPE_PID);
+ 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);


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