[PATCH 0/6 v2] locking/qspinlock: Enhance pvqspinlock performance

[Waiman Long <Waiman.Long@hp.com>]

v1->v2:
 - Take out the queued unfair lock patches
 - Add a patch to simplify the PV unlock code
 - Move pending bit and statistics collection patches to the front
 - Keep vCPU kicking in pv_kick_node(), but defer it to unlock time
   when appropriate.
 - Change the wait-early patch to use adaptive spinning to better
   balance the difference effect on normal and over-committed guests.
 - Add patch-to-patch performance changes in the patch commit logs.

This patchset tries to improve the performance of both normal and
over-commmitted VM guests. The kick-ahead and adaptive spinning
patches are inspired by the "Do Virtual Machines Really Scale?" blog
from Sanidhya Kashyap.

Patch 1 simplifies the unlock code by doing unconditional vCPU kick
when _Q_SLOW_VAL is set as the chance of spurious wakeup showing
up in the statistical data that I collected was very low (1 or 2
occasionally).

Patch 2 adds pending bit support to pvqspinlock improving performance
at light load.

Patch 3 allows the collection of various count data that are useful
to see what is happening in the system. They do add a bit of overhead
when enabled. This will slow the system a little bit.

Patch 4 enables multiple vCPU kicks at unlock time, outside of the
critical section. Coupled with patch 5 which defers kicking to unlock
time, this will improve system performance in overcommitted guests
and sometime even in normal guests.

Patch 6 enables adaptive spinning in the queue nodes. This patch can lead
to pretty big performance increase in over-committed guest at the expense
of a slight performance hit in normal guests.

Waiman Long (6):
  locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL
  locking/pvqspinlock: Add pending bit support
  locking/pvqspinlock: Collect slowpath lock statistics
  locking/pvqspinlock: Allow vCPUs kick-ahead
  locking/pvqspinlock: Opportunistically defer kicking to unlock time
  locking/pvqspinlock: Queue node adaptive spinning

 arch/x86/Kconfig                    |    7 +
 kernel/locking/qspinlock.c          |   38 +++-
 kernel/locking/qspinlock_paravirt.h |  452 +++++++++++++++++++++++++++++++++--
 3 files changed, 473 insertions(+), 24 deletions(-)