An over-committed guest with more vCPUs than pCPUs has a heavy overload
in osq_lock().

This is because if vCPU-A holds the osq lock and yields out, vCPU-B ends
up waiting for per_cpu node->locked to be set. IOW, vCPU-B waits for
vCPU-A to run and unlock the osq lock.

Use the new vcpu_is_preempted(cpu) interface to detect if a vCPU is
currently running or not, and break out of the spin-loop if so.

test case:

 $ perf record -a perf bench sched messaging -g 400 -p && perf report

 before patch:
 18.09%  sched-messaging  [kernel.vmlinux]  [k] osq_lock
 12.28%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
  5.27%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
  3.89%  sched-messaging  [kernel.vmlinux]  [k] wait_consider_task
  3.64%  sched-messaging  [kernel.vmlinux]  [k] _raw_write_lock_irq
  3.41%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner.is
  2.49%  sched-messaging  [kernel.vmlinux]  [k] system_call

 after patch:
 20.68%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner
  8.45%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
  4.12%  sched-messaging  [kernel.vmlinux]  [k] system_call
  3.01%  sched-messaging  [kernel.vmlinux]  [k] system_call_common
  2.83%  sched-messaging  [kernel.vmlinux]  [k] copypage_power7
  2.64%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
  2.00%  sched-messaging  [kernel.vmlinux]  [k] osq_lock

Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Tested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: David.Laight@ACULAB.COM
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: benh@kernel.crashing.org
Cc: bsingharora@gmail.com
Cc: dave@stgolabs.net
Cc: kernellwp@gmail.com
Cc: konrad.wilk@oracle.com
Cc: linuxppc-dev@lists.ozlabs.org
Cc: mpe@ellerman.id.au
Cc: paulmck@linux.vnet.ibm.com
Cc: paulus@samba.org
Cc: rkrcmar@redhat.com
Cc: virtualization@lists.linux-foundation.org
Cc: will.deacon@arm.com
Cc: xen-devel-request@lists.xenproject.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1478077718-37424-3-git-send-email-xinhui.pan@linux.vnet.ibm.com


[ Translated to English. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>