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locking/pvqspinlock: Relax cmpxchg's to improve performance on some architectures

All the locking related cmpxchg's in the following functions are replaced with the _acquire variants: - pv_queued_spin_steal_lock() - trylock_clear_pending() This change should help performance on architectures that use LL/SC. The cmpxchg in pv_kick_node() is replaced with a relaxed version with explicit memory barrier to make sure that it is fully ordered in the writing of next->lock and the reading of pn->state whether the cmpxchg is a success or failure without affecting performance in non-LL/SC architectures. On a 2-socket 12-core 96-thread Power8 system with pvqspinlock explicitly enabled, the performance of a locking microbenchmark with and without this patch on a 4.13-rc4 kernel with Xinhui's PPC qspinlock patch were as follows: # of thread w/o patch with patch % Change ----------- --------- ---------- -------- 8 5054.8 Mop/s 5209.4 Mop/s +3.1% 16 3985.0 Mop/s 4015.0 Mop/s +0.8% 32 2378.2 Mop/s 2396.0 Mop/s +0.7%

Waiman Long committed 9 years ago

Commit 34d54f31 parent 966a9671 file · +17 −7Browse files

Showing the changes to kernel/locking (1 of 1 file). Show all

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kernel/locking/qspinlock_paravirt.h+17−7
@@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock) · 71 unmodified lines
72 struct __qspinlock *l = (void *)lock;72 struct __qspinlock *l = (void *)lock;
73 73
74 if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&74 if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
75 (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {75 (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
76 qstat_inc(qstat_pv_lock_stealing, true);76 qstat_inc(qstat_pv_lock_stealing, true);
77 return true;77 return true;
78 }78 }
@@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct qspinlock *lock) · 22 unmodified lines
101 101
102/*102/*
103 * The pending bit check in pv_queued_spin_steal_lock() isn't a memory103 * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
104 * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock104 * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
105 * just to be sure that it will get it.105 * lock just to be sure that it will get it.
106 */106 */
107static __always_inline int trylock_clear_pending(struct qspinlock *lock)107static __always_inline int trylock_clear_pending(struct qspinlock *lock)
108{108{
109 struct __qspinlock *l = (void *)lock;109 struct __qspinlock *l = (void *)lock;
110 110
111 return !READ_ONCE(l->locked) &&111 return !READ_ONCE(l->locked) &&
112 (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)112 (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
113 == _Q_PENDING_VAL);113 _Q_LOCKED_VAL) == _Q_PENDING_VAL);
114}114}
115#else /* _Q_PENDING_BITS == 8 */115#else /* _Q_PENDING_BITS == 8 */
116static __always_inline void set_pending(struct qspinlock *lock)116static __always_inline void set_pending(struct qspinlock *lock)
@@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock) · 21 unmodified lines
138 */138 */
139 old = val;139 old = val;
140 new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;140 new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
141 val = atomic_cmpxchg(&lock->val, old, new);141 val = atomic_cmpxchg_acquire(&lock->val, old, new);
142 142
143 if (val == old)143 if (val == old)
144 return 1;144 return 1;
@@ -362,8 +362,18 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node) · 217 unmodified lines
362 * observe its next->locked value and advance itself.362 * observe its next->locked value and advance itself.
363 *363 *
364 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()364 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
365 *
366 * The write to next->locked in arch_mcs_spin_unlock_contended()
367 * must be ordered before the read of pn->state in the cmpxchg()
368 * below for the code to work correctly. To guarantee full ordering
369 * irrespective of the success or failure of the cmpxchg(),
370 * a relaxed version with explicit barrier is used. The control
371 * dependency will order the reading of pn->state before any
372 * subsequent writes.
365 */373 */
366 if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)374 smp_mb__before_atomic();
375 if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed)
376 != vcpu_halted)
367 return;377 return;
368 378
369 /*379 /*