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Message-ID: <CACvQF51K80e3CbTqqwb=WyTVUqndVKP_-fGNnpoh9Gfoyzv5ig@mail.gmail.com>
Date: Tue, 11 Jun 2013 22:48:17 +0800
From: Lai Jiangshan <eag0628@...il.com>
To: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
josh@...htriplett.org, niv@...ibm.com,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
David Howells <dhowells@...hat.com>,
Eric Dumazet <edumazet@...gle.com>, darren@...art.com,
Frédéric Weisbecker <fweisbec@...il.com>,
Silas Boyd-Wickizer <sbw@....edu>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock
On Mon, Jun 10, 2013 at 3:36 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com>
>
> 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@...ux.vnet.ibm.com>
> + */
> +#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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