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Message-ID: <20161226055002.GA8717@tardis.cn.ibm.com>
Date: Mon, 26 Dec 2016 13:50:02 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Waiman Long <longman@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
Pan Xinhui <xinhui@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve
performance on some archs
Hi Wainman,
On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
> A number of cmpxchg calls in qspinlock_paravirt.h were replaced by more
> relaxed versions to improve performance on architectures that use LL/SC.
>
> All the locking related cmpxchg's are replaced with the _acquire
> variants:
> - pv_queued_spin_steal_lock()
> - trylock_clear_pending()
>
> The cmpxchg's related to hashing are replaced by either by the _release
> or the _relaxed variants. See the inline comment for details.
>
> Signed-off-by: Waiman Long <longman@...hat.com>
>
> v1->v2:
> - Add comments in changelog and code for the rationale of the change.
>
> ---
> kernel/locking/qspinlock_paravirt.h | 50 ++++++++++++++++++++++++-------------
> 1 file changed, 33 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index e3b5520..c31d1ab 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock)
> struct __qspinlock *l = (void *)lock;
>
> if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
> - (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
> + (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
> qstat_inc(qstat_pv_lock_stealing, true);
> return true;
> }
> @@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct qspinlock *lock)
>
> /*
> * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
> - * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock
> - * just to be sure that it will get it.
> + * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
> + * lock to provide the proper memory barrier.
> */
> static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> {
> struct __qspinlock *l = (void *)lock;
>
> return !READ_ONCE(l->locked) &&
> - (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)
> - == _Q_PENDING_VAL);
> + (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
> + _Q_LOCKED_VAL) == _Q_PENDING_VAL);
> }
> #else /* _Q_PENDING_BITS == 8 */
> static __always_inline void set_pending(struct qspinlock *lock)
> @@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> */
> old = val;
> new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> - val = atomic_cmpxchg(&lock->val, old, new);
> + val = atomic_cmpxchg_acquire(&lock->val, old, new);
>
> if (val == old)
> return 1;
> @@ -209,9 +209,15 @@ static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
> struct pv_hash_entry *he;
> int hopcnt = 0;
>
> + /*
> + * Synchronizing with the node state variable will control who does
> + * the hashing - the lock holder or lock waiter. The control
> + * dependency will ensure that node value is written after the lock
> + * value. So we don't need other ordering guarantee.
> + */
By this comment, you mean that
cmpxchg_relaxed(&he->lock, NULL, lock);
r1 = ll he->lock;
<compare part>
sc he->lock, lock // successed
if (r1)
WRITE_ONCE(he->node, node);
the sc and WRITE_ONCE() can not be reordered because of the control
dependency? I dont think this is true. Yes the sc must execute before
the WRITE_ONCE(), but the memory/cache effects may be reordered. IOW,
the following may happen
CPU 0 CPU 1
=================== =======================
{x = 0, y = 0} if (!cmpxchg_relaxed(&y, 0, 1))
WRITE_ONCE(x, 1);
r1 = READ_ONCE(x);
smp_rmb();
r2 = READ_ONCE(y);
The following result is possible:
y = 1 && r1 = 1 && r2 = 0
Or I'm missing your point here? ;-)
Regards,
Boqun
> for_each_hash_entry(he, offset, hash) {
> hopcnt++;
> - if (!cmpxchg(&he->lock, NULL, lock)) {
> + if (!cmpxchg_relaxed(&he->lock, NULL, lock)) {
> WRITE_ONCE(he->node, node);
> qstat_hop(hopcnt);
> return &he->lock;
> @@ -309,7 +315,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> * MB MB
> * [L] pn->locked [RmW] pn->state = vcpu_hashed
> *
> - * Matches the cmpxchg() from pv_kick_node().
> + * Matches the cmpxchg_release() from pv_kick_node().
> */
> smp_store_mb(pn->state, vcpu_halted);
>
> @@ -323,8 +329,14 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> * If pv_kick_node() changed us to vcpu_hashed, retain that
> * value so that pv_wait_head_or_lock() knows to not also try
> * to hash this lock.
> + *
> + * The smp_store_mb() and control dependency above will ensure
> + * that state change won't happen before that. Synchronizing
> + * with pv_kick_node() wrt hashing by this waiter or by the
> + * lock holder is done solely by the state variable. There is
> + * no other ordering requirement.
> */
> - cmpxchg(&pn->state, vcpu_halted, vcpu_running);
> + cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_running);
>
> /*
> * If the locked flag is still not set after wakeup, it is a
> @@ -360,9 +372,12 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
> * pv_wait_node(). If OTOH this fails, the vCPU was running and will
> * observe its next->locked value and advance itself.
> *
> - * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
> + * Matches with smp_store_mb() and cmpxchg_relaxed() in pv_wait_node().
> + * A release barrier is used here to ensure that node->locked is
> + * always set before changing the state. See comment in pv_wait_node().
> */
> - if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
> + if (cmpxchg_release(&pn->state, vcpu_halted, vcpu_hashed)
> + != vcpu_halted)
> return;
>
> /*
> @@ -461,8 +476,8 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
> }
>
> /*
> - * The cmpxchg() or xchg() call before coming here provides the
> - * acquire semantics for locking. The dummy ORing of _Q_LOCKED_VAL
> + * The cmpxchg_acquire() or xchg() call before coming here provides
> + * the acquire semantics for locking. The dummy ORing of _Q_LOCKED_VAL
> * here is to indicate to the compiler that the value will always
> * be nozero to enable better code optimization.
> */
> @@ -488,11 +503,12 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
> }
>
> /*
> - * A failed cmpxchg doesn't provide any memory-ordering guarantees,
> - * so we need a barrier to order the read of the node data in
> - * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
> + * A failed cmpxchg_release doesn't provide any memory-ordering
> + * guarantees, so we need a barrier to order the read of the node
> + * data in pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
> *
> - * Matches the cmpxchg() in pv_wait_head_or_lock() setting _Q_SLOW_VAL.
> + * Matches the cmpxchg_acquire() in pv_wait_head_or_lock() setting
> + * _Q_SLOW_VAL.
> */
> smp_rmb();
>
> --
> 1.8.3.1
>
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