From 69f9cae90907e09af95fb991ed384670cef8dd32 Mon Sep 17 00:00:00 2001 From: "Peter Zijlstra (Intel)" Date: Fri, 24 Apr 2015 14:56:34 -0400 Subject: locking/qspinlock: Optimize for smaller NR_CPUS When we allow for a max NR_CPUS < 2^14 we can optimize the pending wait-acquire and the xchg_tail() operations. By growing the pending bit to a byte, we reduce the tail to 16bit. This means we can use xchg16 for the tail part and do away with all the repeated compxchg() operations. This in turn allows us to unconditionally acquire; the locked state as observed by the wait loops cannot change. And because both locked and pending are now a full byte we can use simple stores for the state transition, obviating one atomic operation entirely. This optimization is needed to make the qspinlock achieve performance parity with ticket spinlock at light load. All this is horribly broken on Alpha pre EV56 (and any other arch that cannot do single-copy atomic byte stores). Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Waiman Long Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Boris Ostrovsky Cc: Borislav Petkov Cc: Daniel J Blueman Cc: David Vrabel Cc: Douglas Hatch Cc: H. Peter Anvin Cc: Konrad Rzeszutek Wilk Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Paolo Bonzini Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Raghavendra K T Cc: Rik van Riel Cc: Scott J Norton Cc: Thomas Gleixner Cc: virtualization@lists.linux-foundation.org Cc: xen-devel@lists.xenproject.org Link: http://lkml.kernel.org/r/1429901803-29771-6-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/qspinlock.c | 69 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 68 insertions(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 82bb4a9e9009..e17efe7b8d4d 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -24,6 +24,7 @@ #include #include #include +#include #include /* @@ -56,6 +57,10 @@ * node; whereby avoiding the need to carry a node from lock to unlock, and * preserving existing lock API. This also makes the unlock code simpler and * faster. + * + * N.B. The current implementation only supports architectures that allow + * atomic operations on smaller 8-bit and 16-bit data types. + * */ #include "mcs_spinlock.h" @@ -96,6 +101,62 @@ static inline struct mcs_spinlock *decode_tail(u32 tail) #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK) +/* + * By using the whole 2nd least significant byte for the pending bit, we + * can allow better optimization of the lock acquisition for the pending + * bit holder. + */ +#if _Q_PENDING_BITS == 8 + +struct __qspinlock { + union { + atomic_t val; + struct { +#ifdef __LITTLE_ENDIAN + u16 locked_pending; + u16 tail; +#else + u16 tail; + u16 locked_pending; +#endif + }; + }; +}; + +/** + * clear_pending_set_locked - take ownership and clear the pending bit. + * @lock: Pointer to queued spinlock structure + * + * *,1,0 -> *,0,1 + * + * Lock stealing is not allowed if this function is used. + */ +static __always_inline void clear_pending_set_locked(struct qspinlock *lock) +{ + struct __qspinlock *l = (void *)lock; + + WRITE_ONCE(l->locked_pending, _Q_LOCKED_VAL); +} + +/* + * xchg_tail - Put in the new queue tail code word & retrieve previous one + * @lock : Pointer to queued spinlock structure + * @tail : The new queue tail code word + * Return: The previous queue tail code word + * + * xchg(lock, tail) + * + * p,*,* -> n,*,* ; prev = xchg(lock, node) + */ +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) +{ + struct __qspinlock *l = (void *)lock; + + return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET; +} + +#else /* _Q_PENDING_BITS == 8 */ + /** * clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure @@ -131,6 +192,7 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) } return old; } +#endif /* _Q_PENDING_BITS == 8 */ /** * queued_spin_lock_slowpath - acquire the queued spinlock @@ -205,8 +267,13 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) * we're pending, wait for the owner to go away. * * *,1,1 -> *,1,0 + * + * this wait loop must be a load-acquire such that we match the + * store-release that clears the locked bit and create lock + * sequentiality; this is because not all clear_pending_set_locked() + * implementations imply full barriers. */ - while ((val = atomic_read(&lock->val)) & _Q_LOCKED_MASK) + while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK) cpu_relax(); /* -- cgit v1.2.3-59-g8ed1b