[Xen-devel] [PATCH RFC V8 8/17] x86/pvticketlock: When paravirtualizing ticket locks, increment by 2

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

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>
Tested-by: Attilio Rao <attilio.rao@xxxxxxxxxx> 
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h       |   10 +++++-----
 arch/x86/include/asm/spinlock_types.h |   10 +++++++++-
 2 files changed, 14 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index ee4bbd4..60b7e83 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -81,7 +81,7 @@ static __always_inline void __ticket_unlock_kick(struct 
arch_spinlock *lock,
  */
 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);
 
@@ -107,7 +107,7 @@ 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;
@@ -115,9 +115,9 @@ static __always_inline int 
arch_spin_trylock(arch_spinlock_t *lock)
 
 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;
 
-       __add(&lock->tickets.head, 1, UNLOCK_LOCK_PREFIX);
+       __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
        __ticket_unlock_kick(lock, next);
 }
 
@@ -132,7 +132,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t 
*lock)
 {
        struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
-       return (__ticket_t)(tmp.tail - tmp.head) > 1;
+       return (__ticket_t)(tmp.tail - tmp.head) > 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 83fd3c7..e96fcbd 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)
 
 typedef struct arch_spinlock {


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