Re: [PATCH RFC ticketlock] Auto-queued ticketlock

[Lai Jiangshan]

On Mon, Jun 10, 2013 at 3:36 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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?
>
> This commit therefore 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.
>
> This has been only lightly tested in the kernel, though a userspace
> implementation has survived substantial testing.
>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 33692ea..b4a91b0 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -34,6 +34,8 @@
>  # define UNLOCK_LOCK_PREFIX
>  #endif
>
> +#ifndef CONFIG_TICKET_LOCK_QUEUED
> +
>  /*
>   * Ticket locks are conceptually two parts, one indicating the current head of
>   * the queue, and the other indicating the current tail. The lock is acquired
> @@ -62,6 +64,25 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>         barrier();              /* make sure nothing creeps before the lock is taken */
>  }
>
> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +
> +bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc);
> +
> +static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
> +{
> +       register struct __raw_tickets inc = { .tail = 2 };
> +
> +       inc = xadd(&lock->tickets, inc);
> +       for (;;) {
> +               if (inc.head == inc.tail || tkt_spin_pass(lock, inc))
> +                       break;
> +               inc.head = ACCESS_ONCE(lock->tickets.head);
> +       }
> +       barrier(); /* smp_mb() on Power or ARM. */
> +}
> +
> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +
>  static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>  {
>         arch_spinlock_t old, new;
> @@ -70,17 +91,37 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
>         if (old.tickets.head != old.tickets.tail)
>                 return 0;
>
> +#ifndef CONFIG_TICKET_LOCK_QUEUED
>         new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +       new.head_tail = old.head_tail + (2 << TICKET_SHIFT);
> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
>
>         /* 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;
>  }
>
> +#ifndef CONFIG_TICKET_LOCK_QUEUED
> +
>  static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
>  {
>         __add(&lock->tickets.head, 1, UNLOCK_LOCK_PREFIX);
>  }
>
> +#else /* #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +
> +extern void tkt_q_do_wake(arch_spinlock_t *asp);
> +
> +static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
> +{
> +       __ticket_t head = 2;
> +
> +       head = xadd(&lock->tickets.head, 2);
> +       if (head & 0x1)
> +               tkt_q_do_wake(lock);
> +}
> +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
> +
>  static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
>  {
>         struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
> diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
> index ad0ad07..cdaefdd 100644
> --- a/arch/x86/include/asm/spinlock_types.h
> +++ b/arch/x86/include/asm/spinlock_types.h
> @@ -7,12 +7,18 @@
>
>  #include <linux/types.h>
>
> -#if (CONFIG_NR_CPUS < 256)
> +#if (CONFIG_NR_CPUS < 128)
>  typedef u8  __ticket_t;
>  typedef u16 __ticketpair_t;
> -#else
> +#define TICKET_T_CMP_GE(a, b) (UCHAR_MAX / 2 >= (unsigned char)((a) - (b)))
> +#elif (CONFIG_NR_CPUS < 32768)
>  typedef u16 __ticket_t;
>  typedef u32 __ticketpair_t;
> +#define TICKET_T_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> +#else
> +typedef u32 __ticket_t;
> +typedef u64 __ticketpair_t;
> +#define TICKET_T_CMP_GE(a, b) (UINT_MAX / 2 >= (unsigned int)((a) - (b)))
>  #endif
>
>  #define TICKET_SHIFT   (sizeof(__ticket_t) * 8)
> @@ -21,7 +27,11 @@ typedef struct arch_spinlock {
>         union {
>                 __ticketpair_t head_tail;
>                 struct __raw_tickets {
> +#ifdef __BIG_ENDIAN__
> +                       __ticket_t tail, head;
> +#else /* #ifdef __BIG_ENDIAN__ */
>                         __ticket_t head, tail;
> +#endif /* #else #ifdef __BIG_ENDIAN__ */
>                 } tickets;
>         };
>  } arch_spinlock_t;
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index e9ef6d6..816a87c 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -15,6 +15,7 @@
>  #include <asm/byteorder.h>
>  #include <uapi/linux/kernel.h>
>
> +#define UCHAR_MAX      ((u8)(~0U))
>  #define USHRT_MAX      ((u16)(~0U))
>  #define SHRT_MAX       ((s16)(USHRT_MAX>>1))
>  #define SHRT_MIN       ((s16)(-SHRT_MAX - 1))
> diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
> index 44511d1..ad9c67c 100644
> --- a/kernel/Kconfig.locks
> +++ b/kernel/Kconfig.locks
> @@ -223,3 +223,21 @@ endif
>  config MUTEX_SPIN_ON_OWNER
>         def_bool y
>         depends on SMP && !DEBUG_MUTEXES
> +
> +config TICKET_LOCK_QUEUED
> +       bool "Dynamically switch between ticket and queued locking"
> +       default n
> +       ---help---
> +         Enable dynamic switching between ticketlock and queued locking
> +         on a per-lock basis.  This option will slow down low-contention
> +         acquisition and release very slightly (additional conditional
> +         in release path), but will provide more efficient operation at
> +         high levels of lock contention.  High-contention operation will
> +         not be quite as efficient as would be a pure queued lock, but
> +         this dynamic approach consumes less memory than queud locks
> +         and also runs faster at low levels of contention.
> +
> +         Say "Y" if you are running on a large system with a workload
> +         that is likely to result in high levels of contention.
> +
> +         Say "N" if you are unsure.
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 271fd31..70a91f7 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -51,6 +51,7 @@ endif
>  obj-$(CONFIG_SMP) += spinlock.o
>  obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
>  obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
> +obj-$(CONFIG_TICKET_LOCK_QUEUED) += tktqlock.o
>  obj-$(CONFIG_UID16) += uid16.o
>  obj-$(CONFIG_MODULES) += module.o
>  obj-$(CONFIG_MODULE_SIG) += module_signing.o modsign_pubkey.o modsign_certificate.o
> diff --git a/kernel/tktqlock.c b/kernel/tktqlock.c
> new file mode 100644
> index 0000000..f01b760
> --- /dev/null
> +++ b/kernel/tktqlock.c
> @@ -0,0 +1,333 @@
> +/*
> + * Queued ticket spinlocks.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright IBM Corporation, 2013
> + *
> + * Authors: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> + */
> +#include <linux/types.h>
> +#include <linux/kernel.h>
> +#include <linux/spinlock.h>
> +#include <linux/smp.h>
> +#include <linux/percpu.h>
> +
> +struct tkt_q {
> +       int cpu;
> +       __ticket_t tail;
> +       struct tkt_q *next;
> +};
> +
> +struct tkt_q_head {
> +       arch_spinlock_t *ref;           /* Pointer to spinlock if in use. */
> +       s32 head_tkt;                   /* Head ticket when started queuing. */
> +       struct tkt_q *spin;             /* Head of queue. */
> +       struct tkt_q **spin_tail;       /* Tail of queue. */
> +};
> +
> +/*
> + * TKT_Q_SWITCH is twice the number of CPUs that must be spinning on a
> + * given ticket lock to motivate switching to spinning on a queue.
> + * The reason that it is twice the number is because the bottom bit of
> + * the ticket is reserved for the bit that indicates that a queue is
> + * associated with the lock.
> + */
> +#define TKT_Q_SWITCH  (16 * 2)
> +
> +/*
> + * TKT_Q_NQUEUES is the number of queues to maintain.  Large systems
> + * might have multiple highly contended locks, so provide more queues for
> + * systems with larger numbers of CPUs.
> + */
> +#define TKT_Q_NQUEUES (DIV_ROUND_UP(NR_CPUS + TKT_Q_SWITCH - 1, TKT_Q_SWITCH) * 2)
> +
> +/* The queues themselves. */
> +struct tkt_q_head tkt_q_heads[TKT_Q_NQUEUES];
> +
> +/* Advance to the next queue slot, wrapping around to the beginning. */
> +static int tkt_q_next_slot(int i)
> +{
> +       return (++i < TKT_Q_NQUEUES) ? i : 0;
> +}
> +
> +/* Very crude hash from lock address to queue slot number. */
> +static unsigned long tkt_q_hash(arch_spinlock_t *asp)
> +{
> +       return (((unsigned long)asp) >> 8) % TKT_Q_NQUEUES;
> +}
> +
> +/*
> + * Return a pointer to the queue header associated with the specified lock,
> + * or return NULL if there is no queue for the lock or if the lock's queue
> + * is in transition.
> + */
> +static struct tkt_q_head *tkt_q_find_head(arch_spinlock_t *asp)
> +{
> +       int i;
> +       int start;
> +
> +       start = i = tkt_q_hash(asp);
> +       do
> +               if (tkt_q_heads[i].ref == asp)
> +                       return &tkt_q_heads[i];
> +       while ((i = tkt_q_next_slot(i)) != start);
> +       return NULL;
> +}
> +
> +/*
> + * Try to stop queuing, reverting back to normal ticket-lock operation.
> + * We can only stop queuing when the queue is empty, which means that
> + * we need to correctly handle races where someone shows up in the queue
> + * just as we are trying to dispense with the queue.  They win, we lose.
> + */
> +static bool tkt_q_try_unqueue(arch_spinlock_t *asp, struct tkt_q_head *tqhp)
> +{
> +       arch_spinlock_t asold;
> +       arch_spinlock_t asnew;
> +
> +       /* Pick up the ticket values. */
> +       asold = ACCESS_ONCE(*asp);
> +       if ((asold.tickets.head & ~0x1) == asold.tickets.tail) {
> +
> +               /* Attempt to mark the lock as not having a queue. */
> +               asnew = asold;
> +               asnew.tickets.head &= ~0x1;
> +               if (cmpxchg(&asp->head_tail,
> +                           asold.head_tail,
> +                           asnew.head_tail) == asold.head_tail) {
> +
> +                       /* Succeeded, mark the queue as unused. */
> +                       ACCESS_ONCE(tqhp->ref) = NULL;
> +                       return true;
> +               }
> +       }
> +
> +       /* Failed, tell the caller there is still a queue to pass off to. */
> +       return false;
> +}
> +
> +/*
> + * Hand the lock off to the first CPU on the queue.
> + */
> +void tkt_q_do_wake(arch_spinlock_t *asp)
> +{
> +       struct tkt_q_head *tqhp;
> +       struct tkt_q *tqp;
> +
> +       /* If the queue is still being set up, wait for it. */
> +       while ((tqhp = tkt_q_find_head(asp)) == NULL)
> +               cpu_relax();
> +
> +       for (;;) {
> +
> +               /* Find the first queue element. */
> +               tqp = ACCESS_ONCE(tqhp->spin);
> +               if (tqp != NULL)
> +                       break;  /* Element exists, hand off lock. */
> +               if (tkt_q_try_unqueue(asp, tqhp))
> +                       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. */
> +       ACCESS_ONCE(tqp->cpu) = -1;
> +}
> +
> +/*
> + * Given a lock that already has a queue associated with it, spin on
> + * that queue.  Return false if there was no queue (which means we do not
> + * hold the lock) and true otherwise (meaning we -do- hold the lock).
> + */
> +bool tkt_q_do_spin(arch_spinlock_t *asp, struct __raw_tickets inc)
> +{
> +       struct tkt_q **oldtail;
> +       struct tkt_q tq;
> +       struct tkt_q_head *tqhp;
> +
> +       /*
> +        * Ensure that accesses to queue header happen after sensing
> +        * the lock's have-queue bit.
> +        */
> +       smp_mb();  /* See above block comment. */
> +
> +       /* If there no longer is a queue, leave. */
> +       tqhp = tkt_q_find_head(asp);
> +       if (tqhp == NULL)
> +               return false;
> +
> +       /* Initialize our queue element. */
> +       tq.cpu = raw_smp_processor_id();
> +       tq.tail = inc.tail;
> +       tq.next = NULL;
> +
> +       /* Check to see if we already hold the lock. */
> +       if (ACCESS_ONCE(tqhp->head_tkt) == inc.tail) {
> +               /* The last holder left before queue formed, we hold lock. */
> +               tqhp->head_tkt = -1;
> +               return true;
> +       }
> +
> +       /* Add our element to the tail of the queue. */
> +       oldtail = xchg(&tqhp->spin_tail, &tq.next);
> +       ACCESS_ONCE(*oldtail) = &tq;
> +
> +       /* Spin until handoff. */
> +       while (ACCESS_ONCE(tq.cpu) != -1)
> +               cpu_relax();
> +
> +       /*
> +        * Remove our element from the queue.  If the queue is now empty,
> +        * update carefully so that the next acquisition will queue itself
> +        * at the head of the list.
> +        */
> +       if (tq.next == NULL) {
> +
> +               /* Mark the queue empty. */
> +               tqhp->spin = NULL;
> +
> +               /* Try to point the tail back at the head. */
> +               if (cmpxchg(&tqhp->spin_tail,
> +                           &tq.next,
> +                           &tqhp->spin) == &tq.next)
> +                       return true; /* Succeeded, queue is now empty. */
> +
> +               /* Failed, if needed, wait for the enqueue to complete. */
> +               while (tq.next == NULL)
> +                       cpu_relax();
> +
> +               /* The following code will repair the head. */
> +       }
> +       smp_mb(); /* Force ordering between handoff and critical section. */
> +
> +       /* Advance list-head pointer. */
> +       ACCESS_ONCE(tqhp->spin) = tq.next;
> +       return true;
> +}
> +
> +/*
> + * Given a lock that does not have a queue, attempt to associate the
> + * i-th queue with it, returning true if successful (meaning we hold
> + * the lock) or false otherwise (meaning we do -not- hold the lock).
> + * Note that the caller has already filled in ->ref with 0x1, so we
> + * own the queue.
> + */
> +static bool
> +tkt_q_init_contend(int i, arch_spinlock_t *asp, struct __raw_tickets inc)
> +{
> +       arch_spinlock_t asold;
> +       arch_spinlock_t asnew;
> +       struct tkt_q_head *tqhp;
> +
> +       /* Initialize the i-th queue header. */
> +       tqhp = &tkt_q_heads[i];
> +       tqhp->spin = NULL;
> +       tqhp->spin_tail = &tqhp->spin;
> +
> +       /* Each pass through this loop attempts to mark the lock as queued. */
> +       do {
> +               asold.head_tail = ACCESS_ONCE(asp->head_tail);
> +               asnew = asold;
> +               if (asnew.tickets.head & 0x1) {
> +
> +                       /* Someone beat us to it, back out. */
> +                       smp_mb();
> +                       ACCESS_ONCE(tqhp->ref) = NULL;
> +
> +                       /* Spin on the queue element they set up. */
> +                       return tkt_q_do_spin(asp, inc);
> +               }
> +
> +               /* The low-order bit in the head counter says "queued". */
> +               asnew.tickets.head |= 0x1;
> +       } while (cmpxchg(&asp->head_tail,
> +                asold.head_tail,
> +                asnew.head_tail) != asold.head_tail);
> +
> +       /* Point the queue at the lock and go spin on it. */
> +       tqhp->head_tkt = asold.tickets.head;
> +       smp_mb(); /* Ensure head_tkt is set prior to queuers seeing tqhp. */
> +       ACCESS_ONCE(tqhp->ref) = asp;
> +       return tkt_q_do_spin(asp, inc);
> +}

Just small revise.

I just move " tqhp->head_tkt = asold.tickets.head;" into the loop, so
we can use "asp->tickets.head & 0x1" to
indicates that queued spinlock is prepared instead of by "tqhp->ref == asp".

See the append diff.
(And I guess, after it you can force only the CPUs which
"inc.tail - tqhp->head_tkt > TKT_Q_SWITCH"
do queued spin to remove the thundering herd)

diff --git a/kernel/tktqlock.c b/kernel/tktqlock.c
index f01b760..4ea409b 100644
--- a/kernel/tktqlock.c
+++ b/kernel/tktqlock.c
@@ -27,7 +27,6 @@

 struct tkt_q {
 	int cpu;
-	__ticket_t tail;
 	struct tkt_q *next;
 };

@@ -127,9 +126,8 @@ void tkt_q_do_wake(arch_spinlock_t *asp)
 	struct tkt_q_head *tqhp;
 	struct tkt_q *tqp;

-	/* If the queue is still being set up, wait for it. */
-	while ((tqhp = tkt_q_find_head(asp)) == NULL)
-		cpu_relax();
+	tqhp = tkt_q_find_head(asp);
+	BUG_ON(!tqhp);

 	for (;;) {

@@ -141,8 +139,6 @@ void tkt_q_do_wake(arch_spinlock_t *asp)
 			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. */
 	ACCESS_ONCE(tqp->cpu) = -1;
 }
@@ -164,20 +160,16 @@ bool tkt_q_do_spin(arch_spinlock_t *asp, struct
__raw_tickets inc)
 	 */
 	smp_mb();  /* See above block comment. */

-	/* If there no longer is a queue, leave. */
 	tqhp = tkt_q_find_head(asp);
-	if (tqhp == NULL)
-		return false;
+	BUG_ON(!tqhp);

 	/* Initialize our queue element. */
 	tq.cpu = raw_smp_processor_id();
-	tq.tail = inc.tail;
 	tq.next = NULL;

 	/* Check to see if we already hold the lock. */
 	if (ACCESS_ONCE(tqhp->head_tkt) == inc.tail) {
 		/* The last holder left before queue formed, we hold lock. */
-		tqhp->head_tkt = -1;
 		return true;
 	}

@@ -251,16 +243,14 @@ tkt_q_init_contend(int i, arch_spinlock_t *asp,
struct __raw_tickets inc)
 			return tkt_q_do_spin(asp, inc);
 		}

+		/* Point the queue at the lock and go spin on it. */
+		tqhp->head_tkt = asold.tickets.head;
 		/* The low-order bit in the head counter says "queued". */
 		asnew.tickets.head |= 0x1;
 	} while (cmpxchg(&asp->head_tail,
 		 asold.head_tail,
 		 asnew.head_tail) != asold.head_tail);

-	/* Point the queue at the lock and go spin on it. */
-	tqhp->head_tkt = asold.tickets.head;
-	smp_mb(); /* Ensure head_tkt is set prior to queuers seeing tqhp. */
-	ACCESS_ONCE(tqhp->ref) = asp;
 	return tkt_q_do_spin(asp, inc);
 }

@@ -282,14 +272,9 @@ bool tkt_q_start_contend(arch_spinlock_t *asp,
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 (cmpxchg(&tkt_q_heads[i].ref,
 			    NULL,
-			    (arch_spinlock_t *)0x1) == NULL) {
+			    asp) == NULL) {

 			/* Succeeded, now go initialize it. */
 			return tkt_q_init_contend(i, asp, inc);
--
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