On 06/12/2013 11:40 PM, Paul E. McKenney wrote:
> Breaking up locks is better than implementing high-contention locks, but
> if we must have high-contention locks, why not make them automatically
> switch between light-weight ticket locks at low contention and queued
> locks at high contention? After all, this would remove the need for
> the developer to predict which locks will be highly contended.
>
> This commit allows ticket locks to automatically switch between pure
> ticketlock and queued-lock operation as needed. If too many CPUs are
> spinning on a given ticket lock, a queue structure will be allocated
> and the lock will switch to queued-lock operation. When the lock becomes
> free, it will switch back into ticketlock operation. The low-order bit
> of the head counter is used to indicate that the lock is in queued mode,
> which forces an unconditional mismatch between the head and tail counters.
> This approach means that the common-case code path under conditions of
> low contention is very nearly that of a plain ticket lock.
>
> A fixed number of queueing structures is statically allocated in an
> array. The ticket-lock address is used to hash into an initial element,
> but if that element is already in use, it moves to the next element. If
> the entire array is already in use, continue to spin in ticket mode.
>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> [ paulmck: Eliminate duplicate code and update comments (Steven Rostedt). ]
> [ paulmck: Address Eric Dumazet review feedback. ]
> [ paulmck: Use Lai Jiangshan idea to eliminate smp_mb(). ]
> [ paulmck: Expand ->head_tkt from s32 to s64 (Waiman Long). ]
> [ paulmck: Move cpu_relax() to main spin loop (Steven Rostedt). ]
> [ paulmck: Reduce queue-switch contention (Waiman Long). ]
> [ paulmck: __TKT_SPIN_INC for __ticket_spin_trylock() (Steffen Persvold). ]
> [ paulmck: Type safety fixes (Steven Rostedt). ]
> [ paulmck: Pre-check cmpxchg() value (Waiman Long). ]
> [ paulmck: smp_mb() downgrade to smp_wmb() (Lai Jiangshan). ]
Hi, Paul,
I simplify the code and remove the search by encoding the index of struct
tkt_q_head
into lock->tickets.head.
A) lock->tickets.head(when queued-lock):
---------------------------------
index of struct tkt_q_head |0|1|
---------------------------------
The bit0 = 1 for queued, and the bit1 = 0 is reserved for
__ticket_spin_unlock(),
thus __ticket_spin_unlock() will not change the higher bits of
lock->tickets.head.
B) tqhp->head is for the real value of lock->tickets.head.
if the last bit of tqhp->head is 1, it means it is the head ticket when started
queuing.
Thanks,
Lai
kernel/tktqlock.c | 51 +++++++++++++--------------------------------------
1 files changed, 13 insertions(+), 38 deletions(-)
diff --git a/kernel/tktqlock.c b/kernel/tktqlock.c
index 912817c..1329d0f 100644
--- a/kernel/tktqlock.c
+++ b/kernel/tktqlock.c
@@ -33,7 +33,7 @@ struct tkt_q {
struct tkt_q_head {
arch_spinlock_t *ref; /* Pointer to spinlock if in use. */
- s64 head_tkt; /* Head ticket when started queuing. */
+ __ticket_t head; /* Real head when queued. */
struct tkt_q *spin; /* Head of queue. */
struct tkt_q **spin_tail; /* Tail of queue. */
};
@@ -77,15 +77,8 @@ static unsigned long tkt_q_hash(arch_spinlock_t *lock)
*/
static struct tkt_q_head *tkt_q_find_head(arch_spinlock_t *lock)
{
- int i;
- int start;
-
- start = i = tkt_q_hash(lock);
- do
- if (ACCESS_ONCE(tkt_q_heads[i].ref) == lock)
- return &tkt_q_heads[i];
- while ((i = tkt_q_next_slot(i)) != start);
- return NULL;
+ BUILD_BUG_ON(TKT_Q_NQUEUES > (1 << (TICKET_SHIFT - 2)));
+ return &tkt_q_heads[ACCESS_ONCE(lock->tickets.head) >> 2];
}
/*
@@ -101,11 +94,11 @@ static bool tkt_q_try_unqueue(arch_spinlock_t *lock,
struct tkt_q_head *tqhp)
/* Pick up the ticket values. */
asold = ACCESS_ONCE(*lock);
- if ((asold.tickets.head & ~0x1) == asold.tickets.tail) {
+ if (tqhp->head == asold.tickets.tail) {
/* Attempt to mark the lock as not having a queue. */
asnew = asold;
- asnew.tickets.head &= ~0x1;
+ asnew.tickets.head = tqhp->head;
if (cmpxchg(&lock->head_tail,
asold.head_tail,
asnew.head_tail) == asold.head_tail) {
@@ -128,12 +121,9 @@ void tkt_q_do_wake(arch_spinlock_t *lock)
struct tkt_q_head *tqhp;
struct tkt_q *tqp;
- /*
- * If the queue is still being set up, wait for it. Note that
- * the caller's xadd() provides the needed memory ordering.
- */
- while ((tqhp = tkt_q_find_head(lock)) == NULL)
- cpu_relax();
+ tqhp = tkt_q_find_head(lock);
+ ACCESS_ONCE(lock->tickets.head) -= __TKT_SPIN_INC;
+ ACCESS_ONCE(tqhp->head) = (tqhp->head & ~0x1) + __TKT_SPIN_INC;
for (;;) {
@@ -145,9 +135,7 @@ void tkt_q_do_wake(arch_spinlock_t *lock)
return; /* No element, successfully removed queue. */
cpu_relax();
}
- if (ACCESS_ONCE(tqhp->head_tkt) != -1)
- ACCESS_ONCE(tqhp->head_tkt) = -1;
- smp_mb(); /* Order pointer fetch and assignment against handoff. */
+ smp_mb(); /* Order modification, pointer fetch and assignment against
handoff. */
ACCESS_ONCE(tqp->cpu) = -1;
}
EXPORT_SYMBOL(tkt_q_do_wake);
@@ -169,10 +157,7 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct
__raw_tickets inc)
*/
smp_mb(); /* See above block comment. */
- /* If there no longer is a queue, leave. */
tqhp = tkt_q_find_head(lock);
- if (tqhp == NULL)
- return false;
/* Initialize our queue element. */
tq.cpu = raw_smp_processor_id();
@@ -180,9 +165,8 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct
__raw_tickets inc)
tq.next = NULL;
/* Check to see if we already hold the lock. */
- if (ACCESS_ONCE(tqhp->head_tkt) == inc.tail) {
+ if (ACCESS_ONCE(tqhp->head) == (inc.tail | 0x1)) {
/* The last holder left before queue formed, we hold lock. */
- tqhp->head_tkt = -1;
return true;
}
@@ -290,16 +274,14 @@ tkt_q_init_contend(int i, arch_spinlock_t *lock, struct
__raw_tickets inc)
* Record the head counter in case one of the spinning
* CPUs already holds the lock but doesn't realize it yet.
*/
- tqhp->head_tkt = asold.tickets.head;
+ tqhp->head = asold.tickets.head | 0x1;
/* The low-order bit in the head counter says "queued". */
- asnew.tickets.head |= 0x1;
+ asnew.tickets.head = (i << 2) + 0x1;
} while (cmpxchg(&lock->head_tail,
asold.head_tail,
asnew.head_tail) != asold.head_tail);
- /* Point the queue at the lock and go spin on it. */
- ACCESS_ONCE(tqhp->ref) = lock;
return tkt_q_do_spin(lock, inc);
}
@@ -321,15 +303,8 @@ bool tkt_q_start_contend(arch_spinlock_t *lock, struct
__raw_tickets inc)
* the lock with the corresponding queue.
*/
do {
- /*
- * Use 0x1 to mark the queue in use, but also avoiding
- * any spinners trying to use it before we get it all
- * initialized.
- */
if (!tkt_q_heads[i].ref &&
- cmpxchg(&tkt_q_heads[i].ref,
- NULL,
- (arch_spinlock_t *)0x1) == NULL) {
+ cmpxchg(&tkt_q_heads[i].ref, NULL, lock) == NULL) {
/* Succeeded, now go initialize it. */
return tkt_q_init_contend(i, lock, inc);
--
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