Re: [PATCH RFC ticketlock] v3 Auto-queued ticketlock
From: Paul E. McKenney
Date: Fri Jun 14 2013 - 19:47:42 EST
On Fri, Jun 14, 2013 at 03:12:43PM +0800, Lai Jiangshan wrote:
> On 06/14/2013 07:57 AM, Paul E. McKenney wrote:
> > On Fri, Jun 14, 2013 at 07:25:57AM +0800, Lai Jiangshan wrote:
> >> On Thu, Jun 13, 2013 at 11:22 PM, Paul E. McKenney
> >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>> On Thu, Jun 13, 2013 at 10:55:41AM +0800, Lai Jiangshan wrote:
> >>>> 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 <paulmck@xxxxxxxxxxxxxxxxxx>
> >>>>> [ 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|
> >>>> ---------------------------------
> >>>
> >>> Interesting approach! It might reduce queued-mode overhead a bit in
> >>> some cases, though I bet that in the common case the first queue element
> >>> examined is the right one. More on this below.
> >>>
> >>>> 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.
> >>>
> >>> But don't you also need the xadd() in __ticket_spin_unlock() to become
> >>> a cmpxchg() for this to work? Or is your patch missing your changes to
> >>> arch/x86/include/asm/spinlock.h? Either way, this is likely to increase
> >>> the no-contention overhead, which might be counterproductive. Wouldn't
> >>> hurt to get measurements, though.
> >>
> >> No, don't need to change __ticket_spin_unlock() in my idea.
> >> bit1 in the tickets.head is reserved for __ticket_spin_unlock(),
> >> __ticket_spin_unlock() only changes the bit1, it will not change
> >> the higher bits. tkt_q_do_wake() will restore the tickets.head.
> >>
> >> This approach avoids cmpxchg in __ticket_spin_unlock().
> >
> > Ah, I did miss that. But doesn't the adjustment in __ticket_spin_lock()
> > need to be atomic in order to handle concurrent invocations of
> > __ticket_spin_lock()?
> >
> > Either way, it would be good to see the performance effects of this.
> >
> > Thanx, Paul
> >
> >>> Given the results that Davidlohr posted, I believe that the following
> >>> optimizations would also provide some improvement:
> >>>
> >>> 1. Move the call to tkt_spin_pass() from __ticket_spin_lock()
> >>> to a separate linker section in order to reduce the icache
> >>> penalty exacted by the spinloop. This is likely to be causing
> >>> some of the performance reductions in the cases where ticket
> >>> locks are not highly contended.
> >>>
> >>> 2. Limit the number of elements searched for in the array of
> >>> queues. However, this would help only if a number of ticket
> >>> locks were in queued mode at the same time.
> >>>
> >>> 3. Dynamically allocate the queue array at boot. This might
> >>> also reduce cache pressure, again, at least in cases where
> >>> there are a number of ticket locks in queued mode at the
> >>> same time.
> >>>
> >>> Frederic just reminded me that I owe him some energy-efficiency improvements
> >>> for adaptive ticks, so I won't get to these very quickly. Please feel free
> >>> to take these on -- the patch clearly does well under high contention, so
> >>> reducing the no-contention penalty could really help.
> >>>
> >>> Thanx, Paul
> >>>
> >>>> 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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> >>> the body of a message to majordomo@xxxxxxxxxxxxxxx
> >>> More majordomo info at http://vger.kernel.org/majordomo-info.html
> >>> Please read the FAQ at http://www.tux.org/lkml/
>
> Hi, Paul.
>
> More possible improvement. Again, this is untested.
>
> This improvement removes slow path from unlock() by:
> 1) Instead of forcing all competitor to spin on its queued-node,
> this improvement selects one and only one competitor and force it still spinning on the lock.
> 2) The selected competitor is the leader of the queue.
> 3) the queue is used for passing the leadership instead of passing the lock holder.
>
> Implemented on top of my previous improvement.
> Would you merge them all as one patch to get more reviews if you agree my improvement?
This one does have the advantage of reducing the non-queued overhead by
allowing the original __ticket_spin_unlock() to be used as-is, which is
good. After all, the biggest weakness in my earliers versions was not the
handoff, but rather the performance regressions when in non-queued mode.
Of course, the performance regressions would need to be negligible for
this to have any chance of inclusion. My guess is that this patch gets
things half-way there, but that the ____ticket_spin_lock() path needs
some help (for example, moving the queued-lock handling out of line)
is required to get something that has a chance.
If you test them both for correct function, I will post them as separate
commits on a qlock branch in my -rcu tree. I do have a user-mode test
harness that might help, though of course in-kernel testing is needed.
> Thanks,
> Lai
>
> PS:
>
> After this, we can shrink the size of struct tkt_q_head.
> Is this size important?
Not hugely, but every bit helps.
> struct tkt_q_head {
> __ticket_t head; /* Real head when queued. */
> struct tkt_q **spin_tail; /* Tail of queue. */
> };
>
> And "__ticket_t head;" can be also removed.
>
>
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 5aa0177..01c3bdd 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -90,27 +90,11 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
> 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 *lock);
> -
> -static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
> -{
> - __ticket_t head = 2;
> -
> - head = xadd(&lock->tickets.head, head);
> - 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/kernel/tktqlock.c b/kernel/tktqlock.c
> index 1329d0f..b658fae 100644
> --- a/kernel/tktqlock.c
> +++ b/kernel/tktqlock.c
> @@ -94,7 +94,7 @@ 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 (tqhp->head == asold.tickets.tail) {
> + if (tqhp->head + __TKT_SPIN_INC == asold.tickets.tail) {
>
> /* Attempt to mark the lock as not having a queue. */
> asnew = asold;
> @@ -114,33 +114,6 @@ static bool tkt_q_try_unqueue(arch_spinlock_t *lock, struct tkt_q_head *tqhp)
> }
>
> /*
> - * Hand the lock off to the first CPU on the queue.
> - */
> -void tkt_q_do_wake(arch_spinlock_t *lock)
> -{
> - struct tkt_q_head *tqhp;
> - struct tkt_q *tqp;
> -
> - 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 (;;) {
> -
> - /* Find the first queue element. */
> - tqp = ACCESS_ONCE(tqhp->spin);
> - if (tqp != NULL)
> - break; /* Element exists, hand off lock. */
> - if (tkt_q_try_unqueue(lock, tqhp))
> - return; /* No element, successfully removed queue. */
> - cpu_relax();
> - }
> - smp_mb(); /* Order modification, pointer fetch and assignment against handoff. */
> - ACCESS_ONCE(tqp->cpu) = -1;
> -}
> -EXPORT_SYMBOL(tkt_q_do_wake);
> -
> -/*
> * 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).
> @@ -150,6 +123,7 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
> struct tkt_q **oldtail;
> struct tkt_q tq;
> struct tkt_q_head *tqhp;
> + int index;
>
> /*
> * Ensure that accesses to queue header happen after sensing
> @@ -157,6 +131,7 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
> */
> smp_mb(); /* See above block comment. */
>
> + index = ACCESS_ONCE(lock->tickets.head) >> 2;
> tqhp = tkt_q_find_head(lock);
>
> /* Initialize our queue element. */
> @@ -178,10 +153,29 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
> oldtail = xchg(&tqhp->spin_tail, &tq.next);
> ACCESS_ONCE(*oldtail) = &tq;
>
> - /* Spin until handoff. */
> - while (ACCESS_ONCE(tq.cpu) != -1)
> + if (oldtail != &tqhp->spin) {
> + /* Spin until get the queue leadership */
> + while (ACCESS_ONCE(tq.cpu) != -1)
> + cpu_relax();
> + smp_mb(); /* Force ordering between get leadership and access lock->tickets.head */
> + }
> +
> + /*
> + * Spin until hold the lock. if the next smp_mb() doesn't help,
> + * it should be implemented arch-depended
> + */
> + inc.head = index * __TKT_SPIN_INC * 2 + 1;
> + while (ACCESS_ONCE(lock->tickets.head) != inc.head + __TKT_SPIN_INC)
> cpu_relax();
So this detects the atomic increment and then undoes it below, correct?
> + smp_mb(); /* Force ordering between (prev C.S. & lock->tickets.head)
> + and (current C.S. & tqhp->head & hand off) */
> +
> + /* store queued-lock tickets head */
> + ACCESS_ONCE(lock->tickets.head) = inc.head;
> + /* update real tickets head */
> + ACCESS_ONCE(tqhp->head) = (tqhp->head & ~0x1) + __TKT_SPIN_INC;
> +
> /*
> * Remove our element from the queue. If the queue is now empty,
> * update carefully so that the next acquisition will enqueue itself
> @@ -217,8 +211,10 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
> /* Try to point the tail back at the head. */
> if (cmpxchg(&tqhp->spin_tail,
> &tq.next,
> - &tqhp->spin) == &tq.next)
> + &tqhp->spin) == &tq.next) {
> + tkt_q_try_unqueue(lock, tqhp);
> return true; /* Succeeded, queue is now empty. */
> + }
>
> /* Failed, if needed, wait for the enqueue to complete. */
> while (tq.next == NULL)
> @@ -226,14 +222,13 @@ bool tkt_q_do_spin(arch_spinlock_t *lock, struct __raw_tickets inc)
>
> /* The following code will repair the head. */
> }
> - smp_mb(); /* Force ordering between handoff and critical section. */
>
> /*
> - * Advance list-head pointer. This same task will be the next to
> - * access this when releasing the lock, so no need for a memory
> - * barrier after the following assignment.
> + * Advance list-head pointer. tqhp->spin is useless, it can be removed.
> */
> ACCESS_ONCE(tqhp->spin) = tq.next;
> + ACCESS_ONCE(tq.next->cpu) = -1; /* hand off queue leadership */
> +
> return true;
> }
>
> @@ -277,7 +272,7 @@ tkt_q_init_contend(int i, arch_spinlock_t *lock, struct __raw_tickets inc)
> tqhp->head = asold.tickets.head | 0x1;
>
> /* The low-order bit in the head counter says "queued". */
> - asnew.tickets.head = (i << 2) + 0x1;
> + asnew.tickets.head = i * __TKT_SPIN_INC * 2 + 0x1;
> } while (cmpxchg(&lock->head_tail,
> asold.head_tail,
> asnew.head_tail) != asold.head_tail);
>
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