[Xen-devel] [PATCH 6/8] x86/ticketlocks: when paravirtualizing ticket locks, increment by 2

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

Increment ticket head/tails by 2 rather than 1 to leave the LSB free
to store a "is in slowpath state" bit.  This halves the number
of possible CPUs for a given ticket size, but this shouldn't matter
in practice - kernels built for 32k+ CPU systems are probably
specially built for the hardware rather than a generic distro
kernel.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h       |   16 ++++++++--------
 arch/x86/include/asm/spinlock_types.h |   10 +++++++++-
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index d1a3970..7a1c0c4 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -82,7 +82,7 @@ static __always_inline void __ticket_unlock_kick(struct 
arch_spinlock *lock, __t
  */
 static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
 {
-       register struct __raw_tickets inc = { .tail = 1 };
+       register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
 
        inc = xadd(&lock->tickets, inc);
 
@@ -108,7 +108,7 @@ static __always_inline int 
arch_spin_trylock(arch_spinlock_t *lock)
        if (old.tickets.head != old.tickets.tail)
                return 0;
 
-       new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
+       new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
 
        /* cmpxchg is a full barrier, so nothing can move before it */
        return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == 
old.head_tail;
@@ -117,24 +117,24 @@ static __always_inline int 
arch_spin_trylock(arch_spinlock_t *lock)
 #if (NR_CPUS < 256)
 static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
 {
-       asm volatile(UNLOCK_LOCK_PREFIX "incb %0"
+       asm volatile(UNLOCK_LOCK_PREFIX "addb %1, %0"
                     : "+m" (lock->head_tail)
-                    :
+                    : "i" (TICKET_LOCK_INC)
                     : "memory", "cc");
 }
 #else
 static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
 {
-       asm volatile(UNLOCK_LOCK_PREFIX "incw %0"
+       asm volatile(UNLOCK_LOCK_PREFIX "addw %1, %0"
                     : "+m" (lock->head_tail)
-                    :
+                    : "i" (TICKET_LOCK_INC)
                     : "memory", "cc");
 }
 #endif
 
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
-       __ticket_t next = lock->tickets.head + 1;
+       __ticket_t next = lock->tickets.head + TICKET_LOCK_INC;
 
        __ticket_unlock_release(lock);
        __ticket_unlock_kick(lock, next);
@@ -151,7 +151,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t 
*lock)
 {
        struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
-       return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
+       return ((tmp.tail - tmp.head) & TICKET_MASK) > TICKET_LOCK_INC;
 }
 #define arch_spin_is_contended arch_spin_is_contended
 
diff --git a/arch/x86/include/asm/spinlock_types.h 
b/arch/x86/include/asm/spinlock_types.h
index dbe223d..aa9a205 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -3,7 +3,13 @@
 
 #include <linux/types.h>
 
-#if (CONFIG_NR_CPUS < 256)
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define __TICKET_LOCK_INC      2
+#else
+#define __TICKET_LOCK_INC      1
+#endif
+
+#if (CONFIG_NR_CPUS < (256 / __TICKET_LOCK_INC))
 typedef u8  __ticket_t;
 typedef u16 __ticketpair_t;
 #else
@@ -11,6 +17,8 @@ typedef u16 __ticket_t;
 typedef u32 __ticketpair_t;
 #endif
 
+#define TICKET_LOCK_INC        ((__ticket_t)__TICKET_LOCK_INC)
+
 #define TICKET_SHIFT   (sizeof(__ticket_t) * 8)
 #define TICKET_MASK    ((__ticket_t)((1 << TICKET_SHIFT) - 1))
 
-- 
1.7.6


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