This patch allows one attempt for the lock waiter to steal the lock
when entering the PV slowpath. To prevent lock starvation, the pending
bit will be set by the queue head vCPU when it is in the active lock
spinning loop to disable any lock stealing attempt.  This helps to
reduce the performance penalty caused by lock waiter preemption while
not having much of the downsides of a real unfair lock.

The pv_wait_head() function was renamed as pv_wait_head_or_lock()
as it was modified to acquire the lock before returning. This is
necessary because of possible lock stealing attempts from other tasks.

Linux kernel builds were run in KVM guest on an 8-socket, 4
cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
Haswell-EX system. Both systems are configured to have 32 physical
CPUs. The kernel build times before and after the patch were:

                    Westmere                    Haswell
  Patch         32 vCPUs    48 vCPUs    32 vCPUs    48 vCPUs
  -----         --------    --------    --------    --------
  Before patch   3m15.6s    10m56.1s     1m44.1s     5m29.1s
  After patch    3m02.3s     5m00.2s     1m43.7s     3m03.5s

For the overcommited case (48 vCPUs), this patch is able to reduce
kernel build time by more than 54% for Westmere and 44% for Haswell.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447190336-53317-1-git-send-email-Waiman.Long@hpe.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>