x86/pvticketlock: When paravirtualizing ticket locks, increment by 2
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 7442410..04a5cd5 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -78,7 +78,7 @@ 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);
@@ -104,7 +104,7 @@ 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;
@@ -112,9 +112,9 @@ 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);
}
@@ -129,7 +129,7 @@ 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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