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    SubjectRe: [PATCH RFC ticketlock] Auto-queued ticketlock
    On Mon, Jun 10, 2013 at 02:35:06PM -0700, Eric Dumazet wrote:
    > On Sun, 2013-06-09 at 12:36 -0700, 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?
    > >
    > > 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@linux.vnet.ibm.com>
    >
    > This looks a great idea ;)

    Glad you like it! Hopefully workloads like it as well. ;-)

    > > +
    > > +static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
    > > +{
    > > + __ticket_t head = 2;
    > > +
    > > + head = xadd(&lock->tickets.head, 2);
    >
    > head = xadd(&lock->tickets.head, head);

    Yikes! Good catch, fixed.

    > > + if (head & 0x1)
    > > + tkt_q_do_wake(lock);
    > > +}
    > > +#endif /* #else #ifndef CONFIG_TICKET_LOCK_QUEUED */
    >
    > > + */
    > > +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;
    > > +}
    >
    > EXPORT_SYMBOL(tkt_q_do_wake) ?

    Good point, just in case we want to use spinlocks in modules. ;-)
    Same for tkt_spin_pass(), I guess.

    > Hmm, unfortunately I lack time this week to fully read the patch !

    I suspect that there is very little danger of this patch going in this
    week, so you should have some additional time. ;-)

    Thanx, Paul



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