[PATCH/RFC] KVM: track pid for VCPU only on KVM_RUN ioctl

Christian Borntraeger
2014-08-05T14:43:59Z

We currently track the pid of the task that runs the VCPU in
vcpu_load. Since we call vcpu_load for all kind of ioctls on a
CPU, this causes hickups due to synchronize_rcu if one CPU is
modified by another CPU or the main thread (e.g. initialization,
reset). We track the pid only for the purpose of yielding, so
let's update the pid only in the KVM_RUN ioctl.

In addition, don't do a synchronize_rcu on startup (pid == 0).

This speeds up guest boot time on s390 noticably for some configs, e.g.
HZ=100, no full state tracking, 64 guest cpus 32 host cpus.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
CC: Rik van Riel <riel@redhat.com>
CC: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
CC: Michael Mueller <mimu@linux.vnet.ibm.com>
---
virt/kvm/kvm_main.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9ae9135..ebc8f54 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -124,14 +124,6 @@ int vcpu_load(struct kvm_vcpu *vcpu)

if (mutex_lock_killable(&vcpu->mutex))
return -EINTR;
- if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
- /* The thread running this VCPU changed. */
- struct pid *oldpid = vcpu->pid;
- struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
- rcu_assign_pointer(vcpu->pid, newpid);
- synchronize_rcu();
- put_pid(oldpid);
- }
cpu = get_cpu();
preempt_notifier_register(&vcpu->preempt_notifier);
kvm_arch_vcpu_load(vcpu, cpu);
@@ -1991,6 +1983,15 @@ static long kvm_vcpu_ioctl(struct file *filp,
r = -EINVAL;
if (arg)
goto out;
+ if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
+ /* The thread running this VCPU changed. */
+ struct pid *oldpid = vcpu->pid;
+ struct pid *newpid = get_task_pid(current, PIDTYPE_PID);
+ rcu_assign_pointer(vcpu->pid, newpid);
+ if (oldpid)
+ synchronize_rcu();
+ put_pid(oldpid);
+ }
r = kvm_arch_vcpu_ioctl_run(vcpu, vcpu->run);
trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
break;
--
1.8.4.2