s390/spinlock: introduce spinlock wait queueing

The queued spinlock code for s390 follows the principles of the common
code qspinlock implementation but with a few notable differences.

The format of the spinlock_t locking word differs, s390 needs to store
the logical CPU number of the lock holder in the spinlock_t to be able
to use the diagnose 9c directed yield hypervisor call.

The inline code sequences for spin_lock and spin_unlock are nice and
short. The inline portion of a spin_lock now typically looks like this:

	lhi	%r0,0			# 0 indicates an empty lock
	l	%r1,0x3a0		# CPU number + 1 from lowcore
	cs	%r0,%r1,<some_lock>	# lock operation
	jnz	call_wait		# on failure call wait function
locked:
	...
call_wait:
	la	%r2,<some_lock>
	brasl	%r14,arch_spin_lock_wait
	j	locked

A spin_unlock is as simple as before:

	lhi	%r0,0
	sth	%r0,2(%r2)		# unlock operation

After a CPU has queued itself it may not enable interrupts again for the
arch_spin_lock_flags() variant. The arch_spin_lock_wait_flags wait function
is removed.

To improve performance the code implements opportunistic lock stealing.
If the wait function finds a spinlock_t that indicates that the lock is
free but there are queued waiters, the CPU may steal the lock up to three
times without queueing itself. The lock stealing update the steal counter
in the lock word to prevent more than 3 steals. The counter is reset at
the time the CPU next in the queue successfully takes the lock.

While the queued spinlocks improve performance in a system with dedicated
CPUs, in a virtualized environment with continuously overcommitted CPUs
the queued spinlocks can have a negative effect on performance. This
is due to the fact that a queued CPU that is preempted by the hypervisor
will block the queue at some point even without holding the lock. With
the classic spinlock it does not matter if a CPU is preempted that waits
for the lock. Therefore use the queued spinlock code only if the system
runs with dedicated CPUs and fall back to classic spinlocks when running
with shared CPUs.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 7 insertions, 11 deletions

diff --git a/arch/s390/include/asm/spinlock.h b/arch/s390/include/asm/spinlock.h
index 6727cc30d59b..2da4a6d13f54 100644
--- a/arch/s390/include/asm/spinlock.h
+++ b/arch/s390/include/asm/spinlock.h
@@ -36,15 +36,11 @@ bool arch_vcpu_is_preempted(int cpu);
  */
 
 void arch_lock_relax(int cpu);
+void arch_spin_relax(arch_spinlock_t *lock);
 
 void arch_spin_lock_wait(arch_spinlock_t *);
 int arch_spin_trylock_retry(arch_spinlock_t *);
-void arch_spin_lock_wait_flags(arch_spinlock_t *, unsigned long flags);
-
-static inline void arch_spin_relax(arch_spinlock_t *lock)
-{
-	arch_lock_relax(lock->lock);
-}
+void arch_spin_lock_setup(int cpu);
 
 static inline u32 arch_spin_lockval(int cpu)
 {
@@ -64,8 +60,7 @@ static inline int arch_spin_is_locked(arch_spinlock_t *lp)
 static inline int arch_spin_trylock_once(arch_spinlock_t *lp)
 {
 	barrier();
-	return likely(arch_spin_value_unlocked(*lp) &&
-		      __atomic_cmpxchg_bool(&lp->lock, 0, SPINLOCK_LOCKVAL));
+	return likely(__atomic_cmpxchg_bool(&lp->lock, 0, SPINLOCK_LOCKVAL));
 }
 
 static inline void arch_spin_lock(arch_spinlock_t *lp)
@@ -78,7 +73,7 @@ static inline void arch_spin_lock_flags(arch_spinlock_t *lp,
 					unsigned long flags)
 {
 	if (!arch_spin_trylock_once(lp))
-		arch_spin_lock_wait_flags(lp, flags);
+		arch_spin_lock_wait(lp);
 }
 
 static inline int arch_spin_trylock(arch_spinlock_t *lp)
@@ -95,8 +90,9 @@ static inline void arch_spin_unlock(arch_spinlock_t *lp)
 #ifdef CONFIG_HAVE_MARCH_ZEC12_FEATURES
 		"	.long	0xb2fa0070\n"	/* NIAI 7 */
 #endif
-		"	st	%1,%0\n"
-		: "=Q" (lp->lock) : "d" (0) : "cc", "memory");
+		"	sth	%1,%0\n"
+		: "=Q" (((unsigned short *) &lp->lock)[1])
+		: "d" (0) : "cc", "memory");
 }
 
 /*