Merge tag 'kvm-3.9-1' of git://git.kernel.org/pub/scm/virt/kvm/kvm

Pull KVM updates from Marcelo Tosatti:
 "KVM updates for the 3.9 merge window, including x86 real mode
  emulation fixes, stronger memory slot interface restrictions, mmu_lock
  spinlock hold time reduction, improved handling of large page faults
  on shadow, initial APICv HW acceleration support, s390 channel IO
  based virtio, amongst others"

* tag 'kvm-3.9-1' of git://git.kernel.org/pub/scm/virt/kvm/kvm: (143 commits)
  Revert "KVM: MMU: lazily drop large spte"
  x86: pvclock kvm: align allocation size to page size
  KVM: nVMX: Remove redundant get_vmcs12 from nested_vmx_exit_handled_msr
  x86 emulator: fix parity calculation for AAD instruction
  KVM: PPC: BookE: Handle alignment interrupts
  booke: Added DBCR4 SPR number
  KVM: PPC: booke: Allow multiple exception types
  KVM: PPC: booke: use vcpu reference from thread_struct
  KVM: Remove user_alloc from struct kvm_memory_slot
  KVM: VMX: disable apicv by default
  KVM: s390: Fix handling of iscs.
  KVM: MMU: cleanup __direct_map
  KVM: MMU: remove pt_access in mmu_set_spte
  KVM: MMU: cleanup mapping-level
  KVM: MMU: lazily drop large spte
  KVM: VMX: cleanup vmx_set_cr0().
  KVM: VMX: add missing exit names to VMX_EXIT_REASONS array
  KVM: VMX: disable SMEP feature when guest is in non-paging mode
  KVM: Remove duplicate text in api.txt
  Revert "KVM: MMU: split kvm_mmu_free_page"
  ...

1 files changed, 19 insertions, 6 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 053dfd7692d1..f1bdecf09afb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4374,7 +4374,10 @@ EXPORT_SYMBOL(yield);
  * It's the caller's job to ensure that the target task struct
  * can't go away on us before we can do any checks.
  *
- * Returns true if we indeed boosted the target task.
+ * Returns:
+ *      true (>0) if we indeed boosted the target task.
+ *      false (0) if we failed to boost the target.
+ *      -ESRCH if there's no task to yield to.
  */
 bool __sched yield_to(struct task_struct *p, bool preempt)
 {
@@ -4388,6 +4391,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 
 again:
         p_rq = task_rq(p);
+        /*
+         * If we're the only runnable task on the rq and target rq also
+         * has only one task, there's absolutely no point in yielding.
+         */
+        if (rq->nr_running == 1 && p_rq->nr_running == 1) {
+                yielded = -ESRCH;
+                goto out_irq;
+        }
+
         double_rq_lock(rq, p_rq);
         while (task_rq(p) != p_rq) {
                 double_rq_unlock(rq, p_rq);
@@ -4395,13 +4407,13 @@ again:
         }
 
         if (!curr->sched_class->yield_to_task)
-                goto out;
+                goto out_unlock;
 
         if (curr->sched_class != p->sched_class)
-                goto out;
+                goto out_unlock;
 
         if (task_running(p_rq, p) || p->state)
-                goto out;
+                goto out_unlock;
 
         yielded = curr->sched_class->yield_to_task(rq, p, preempt);
         if (yielded) {
@@ -4414,11 +4426,12 @@ again:
                         resched_task(p_rq->curr);
         }
 
-out:
+out_unlock:
         double_rq_unlock(rq, p_rq);
+out_irq:
         local_irq_restore(flags);
 
-        if (yielded)
+        if (yielded > 0)
                 schedule();
 
         return yielded;