powerpc/qspinlock: provide accounting and options for sleepy locks

Finding the owner or a queued waiter on a lock with a preempted vcpu is
indicative of an oversubscribed guest causing the lock to get into
trouble. Provide some options to detect this situation and have new CPUs
avoid queueing for a longer time (more steal iterations) to minimise the
problems caused by vcpu preemption on the queue.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221126095932.1234527-17-npiggin@gmail.com

arch/powerpc/include/asm/qspinlock_types.h

@@ -30,7 +30,7 @@ typedef struct qspinlock {
  *
  *     0: locked bit
  *  1-14: lock holder cpu
- *    15: unused bit
+ *    15: lock owner or queuer vcpus observed to be preempted bit
  *    16: must queue bit
  * 17-31: tail cpu (+1)
  */
@@ -50,6 +50,11 @@ typedef struct qspinlock {
 #error "qspinlock does not support such large CONFIG_NR_CPUS"
 #endif
 
+/* 0x00008000 */
+#define _Q_SLEEPY_OFFSET	15
+#define _Q_SLEEPY_BITS		1
+#define _Q_SLEEPY_VAL		(1U << _Q_SLEEPY_OFFSET)
+
 /* 0x00010000 */
 #define _Q_MUST_Q_OFFSET	16
 #define _Q_MUST_Q_BITS		1