Commit 217ece61 authored by Rik van Riel's avatar Rik van Riel Committed by Marcelo Tosatti
Browse files

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: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent 34bb10b7

include/linux/kvm_host.h

+1 −0
Original line number Diff line number Diff line
@@ -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];

virt/kvm/kvm_main.c

+47 −10
Original line number Diff line number Diff line
@@ -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);



	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 = 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);

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