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

Wanpeng Li
2014-08-18T05:01:53Z

Hi Christian,
On Tue, Aug 05, 2014 at 04:44:14PM +0200, Christian Borntraeger wrote:
>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;

One question:

>- if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {

When vcpu->pid and current->pids[PIDTYPE_PID].pid will be different?

Regards,
Wanpeng Li

>- /* 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
>
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