On Mon, 2019-01-28 at 12:14 -0600, Gage Eads wrote:
> 64-bit head and tail index widths greatly increases the time it takes for
> them to wrap-around (with current CPU speeds, it won't happen within the
> author's lifetime). This is fundamental to avoiding the ABA problem -- in
> which a thread mistakes reading the same tail index in two accesses to mean
> that the ring was not modified in the intervening time -- in the upcoming
> non-blocking ring implementation. Using a 64-bit index makes the
> possibility of this occurring effectively zero.
Just an observation.
The following invariant holds (using ring_size instead of mask):
∀ index: ring[index % ring_size].index % ring_size == index % ring_size
i.e. the N (N=log2 ring size) lsb of ring[].index will always be the same (for a
specific slot) so serve no purpose.
This means we don't have to store the whole index in each slot, it is enough to
store "index / ring_size" (which I call the lap counter). This could be useful
for an implementation for 32-bit platforms which support 64-bit CAS (to write
the slot ptr & index (lap counter) atomically) and uses 64-bit head & tail
indexes (to avoid the quick wrap around you would have with 32-bit ring
indexes).
So
ring[index % ring_size].lap = index / ring_size;
An implementation could of course use bitwise-and instead of modulo and bitwise-
right shift instead of division. The 2-logaritm of ring_size should also be pre-
calcucated and stored in the ring buffer metadata.
-- Ola
>
> This commit places the new producer and consumer structures in the same
> location in struct rte_ring as their 32-bit counterparts. Since the 32-bit
> versions are padded out to a cache line, there is space for the new
> structure without affecting the layout of struct rte_ring. Thus, the ABI is
> preserved.
>
> Signed-off-by: Gage Eads <gage.e...@intel.com>
> ---
> lib/librte_ring/rte_ring.h | 23 +++++-
> lib/librte_ring/rte_ring_c11_mem.h | 153
> +++++++++++++++++++++++++++++++++++++
> lib/librte_ring/rte_ring_generic.h | 139 +++++++++++++++++++++++++++++++++
> 3 files changed, 312 insertions(+), 3 deletions(-)
>
> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> index af5444a9f..00dfb5b85 100644
> --- a/lib/librte_ring/rte_ring.h
> +++ b/lib/librte_ring/rte_ring.h
> @@ -1,6 +1,6 @@
> /* SPDX-License-Identifier: BSD-3-Clause
> *
> - * Copyright (c) 2010-2017 Intel Corporation
> + * Copyright (c) 2010-2019 Intel Corporation
> * Copyright (c) 2007-2009 Kip Macy km...@freebsd.org
> * All rights reserved.
> * Derived from FreeBSD's bufring.h
> @@ -70,6 +70,15 @@ struct rte_ring_headtail {
> uint32_t single; /**< True if single prod/cons */
> };
>
> +/* 64-bit version of rte_ring_headtail, for use by rings that need to avoid
> + * head/tail wrap-around.
> + */
> +struct rte_ring_headtail_64 {
> + volatile uint64_t head; /**< Prod/consumer head. */
> + volatile uint64_t tail; /**< Prod/consumer tail. */
> + uint32_t single; /**< True if single prod/cons */
> +};
> +
> /**
> * An RTE ring structure.
> *
> @@ -97,11 +106,19 @@ struct rte_ring {
> char pad0 __rte_cache_aligned; /**< empty cache line */
>
> /** Ring producer status. */
> - struct rte_ring_headtail prod __rte_cache_aligned;
> + RTE_STD_C11
> + union {
> + struct rte_ring_headtail prod __rte_cache_aligned;
> + struct rte_ring_headtail_64 prod_64 __rte_cache_aligned;
> + };
> char pad1 __rte_cache_aligned; /**< empty cache line */
>
> /** Ring consumer status. */
> - struct rte_ring_headtail cons __rte_cache_aligned;
> + RTE_STD_C11
> + union {
> + struct rte_ring_headtail cons __rte_cache_aligned;
> + struct rte_ring_headtail_64 cons_64 __rte_cache_aligned;
> + };
> char pad2 __rte_cache_aligned; /**< empty cache line */
> };
>
> diff --git a/lib/librte_ring/rte_ring_c11_mem.h
> b/lib/librte_ring/rte_ring_c11_mem.h
> index 0fb73a337..47acd4c7c 100644
> --- a/lib/librte_ring/rte_ring_c11_mem.h
> +++ b/lib/librte_ring/rte_ring_c11_mem.h
> @@ -178,4 +178,157 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
> return n;
> }
>
> +/**
> + * @internal This function updates the producer head for enqueue using
> + * 64-bit head/tail values.
> + *
> + * @param r
> + * A pointer to the ring structure
> + * @param is_sp
> + * Indicates whether multi-producer path is needed or not
> + * @param n
> + * The number of elements we will want to enqueue, i.e. how far should the
> + * head be moved
> + * @param behavior
> + * RTE_RING_QUEUE_FIXED: Enqueue a fixed number of items from a ring
> + * RTE_RING_QUEUE_VARIABLE: Enqueue as many items as possible from ring
> + * @param old_head
> + * Returns head value as it was before the move, i.e. where enqueue starts
> + * @param new_head
> + * Returns the current/new head value i.e. where enqueue finishes
> + * @param free_entries
> + * Returns the amount of free space in the ring BEFORE head was moved
> + * @return
> + * Actual number of objects enqueued.
> + * If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
> + */
> +static __rte_always_inline unsigned int
> +__rte_ring_move_prod_head_64(struct rte_ring *r, unsigned int is_sp,
> + unsigned int n, enum rte_ring_queue_behavior behavior,
> + uint64_t *old_head, uint64_t *new_head,
> + uint32_t *free_entries)
> +{
> + const uint32_t capacity = r->capacity;
> + uint64_t cons_tail;
> + unsigned int max = n;
> + int success;
> +
> + *old_head = __atomic_load_n(&r->prod_64.head, __ATOMIC_RELAXED);
> + do {
> + /* Reset n to the initial burst count */
> + n = max;
> +
> + /* Ensure the head is read before tail */
> + __atomic_thread_fence(__ATOMIC_ACQUIRE);
> +
> + /* load-acquire synchronize with store-release of ht->tail
> + * in update_tail.
> + */
> + cons_tail = __atomic_load_n(&r->cons_64.tail,
> + __ATOMIC_ACQUIRE);
> +
> + /* The subtraction is done between two unsigned 32bits value
> + * (the result is always modulo 32 bits even if we have
> + * *old_head > cons_tail). So 'free_entries' is always
> between 0
> + * and capacity (which is < size).
> + */
> + *free_entries = (capacity + cons_tail - *old_head);
> +
> + /* check that we have enough room in ring */
> + if (unlikely(n > *free_entries))
> + n = (behavior == RTE_RING_QUEUE_FIXED) ?
> + 0 : *free_entries;
> +
> + if (n == 0)
> + return 0;
> +
> + *new_head = *old_head + n;
> + if (is_sp)
> + r->prod_64.head = *new_head, success = 1;
> + else
> + /* on failure, *old_head is updated */
> + success = __atomic_compare_exchange_n(&r-
> >prod_64.head,
> + old_head, *new_head,
> + 0, __ATOMIC_RELAXED,
> + __ATOMIC_RELAXED);
> + } while (unlikely(success == 0));
> + return n;
> +}
> +
> +/**
> + * @internal This function updates the consumer head for dequeue using
> + * 64-bit head/tail values.
> + *
> + * @param r
> + * A pointer to the ring structure
> + * @param is_sc
> + * Indicates whether multi-consumer path is needed or not
> + * @param n
> + * The number of elements we will want to enqueue, i.e. how far should the
> + * head be moved
> + * @param behavior
> + * RTE_RING_QUEUE_FIXED: Dequeue a fixed number of items from a ring
> + * RTE_RING_QUEUE_VARIABLE: Dequeue as many items as possible from ring
> + * @param old_head
> + * Returns head value as it was before the move, i.e. where dequeue starts
> + * @param new_head
> + * Returns the current/new head value i.e. where dequeue finishes
> + * @param entries
> + * Returns the number of entries in the ring BEFORE head was moved
> + * @return
> + * - Actual number of objects dequeued.
> + * If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
> + */
> +static __rte_always_inline unsigned int
> +__rte_ring_move_cons_head_64(struct rte_ring *r, unsigned int is_sc,
> + unsigned int n, enum rte_ring_queue_behavior behavior,
> + uint64_t *old_head, uint64_t *new_head,
> + uint32_t *entries)
> +{
> + unsigned int max = n;
> + uint64_t prod_tail;
> + int success;
> +
> + /* move cons.head atomically */
> + *old_head = __atomic_load_n(&r->cons_64.head, __ATOMIC_RELAXED);
> + do {
> + /* Restore n as it may change every loop */
> + n = max;
> +
> + /* Ensure the head is read before tail */
> + __atomic_thread_fence(__ATOMIC_ACQUIRE);
> +
> + /* this load-acquire synchronize with store-release of ht-
> >tail
> + * in update_tail.
> + */
> + prod_tail = __atomic_load_n(&r->prod_64.tail,
> + __ATOMIC_ACQUIRE);
> +
> + /* The subtraction is done between two unsigned 32bits value
> + * (the result is always modulo 32 bits even if we have
> + * cons_head > prod_tail). So 'entries' is always between 0
> + * and size(ring)-1.
> + */
> + *entries = (prod_tail - *old_head);
> +
> + /* Set the actual entries for dequeue */
> + if (n > *entries)
> + n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 :
> *entries;
> +
> + if (unlikely(n == 0))
> + return 0;
> +
> + *new_head = *old_head + n;
> + if (is_sc)
> + r->cons_64.head = *new_head, success = 1;
> + else
> + /* on failure, *old_head will be updated */
> + success = __atomic_compare_exchange_n(&r-
> >cons_64.head,
> + old_head, *new_head,
> + 0, __ATOMIC_RELAXED,
> + __ATOMIC_RELAXED);
> + } while (unlikely(success == 0));
> + return n;
> +}
> +
> #endif /* _RTE_RING_C11_MEM_H_ */
> diff --git a/lib/librte_ring/rte_ring_generic.h
> b/lib/librte_ring/rte_ring_generic.h
> index ea7dbe5b9..2158e092a 100644
> --- a/lib/librte_ring/rte_ring_generic.h
> +++ b/lib/librte_ring/rte_ring_generic.h
> @@ -167,4 +167,143 @@ __rte_ring_move_cons_head(struct rte_ring *r, unsigned
> int is_sc,
> return n;
> }
>
> +/**
> + * @internal This function updates the producer head for enqueue using
> + * 64-bit head/tail values.
> + *
> + * @param r
> + * A pointer to the ring structure
> + * @param is_sp
> + * Indicates whether multi-producer path is needed or not
> + * @param n
> + * The number of elements we will want to enqueue, i.e. how far should the
> + * head be moved
> + * @param behavior
> + * RTE_RING_QUEUE_FIXED: Enqueue a fixed number of items from a ring
> + * RTE_RING_QUEUE_VARIABLE: Enqueue as many items as possible from ring
> + * @param old_head
> + * Returns head value as it was before the move, i.e. where enqueue starts
> + * @param new_head
> + * Returns the current/new head value i.e. where enqueue finishes
> + * @param free_entries
> + * Returns the amount of free space in the ring BEFORE head was moved
> + * @return
> + * Actual number of objects enqueued.
> + * If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
> + */
> +static __rte_always_inline unsigned int
> +__rte_ring_move_prod_head_64(struct rte_ring *r, unsigned int is_sp,
> + unsigned int n, enum rte_ring_queue_behavior behavior,
> + uint64_t *old_head, uint64_t *new_head,
> + uint32_t *free_entries)
> +{
> + const uint32_t capacity = r->capacity;
> + unsigned int max = n;
> + int success;
> +
> + do {
> + /* Reset n to the initial burst count */
> + n = max;
> +
> + *old_head = r->prod_64.head;
> +
> + /* add rmb barrier to avoid load/load reorder in weak
> + * memory model. It is noop on x86
> + */
> + rte_smp_rmb();
> +
> + /*
> + * The subtraction is done between two unsigned 64bits value
> + * (the result is always modulo 64 bits even if we have
> + * *old_head > cons_tail). So 'free_entries' is always
> between 0
> + * and capacity (which is < size).
> + */
> + *free_entries = (capacity + r->cons_64.tail - *old_head);
> +
> + /* check that we have enough room in ring */
> + if (unlikely(n > *free_entries))
> + n = (behavior == RTE_RING_QUEUE_FIXED) ?
> + 0 : *free_entries;
> +
> + if (n == 0)
> + return 0;
> +
> + *new_head = *old_head + n;
> + if (is_sp)
> + r->prod_64.head = *new_head, success = 1;
> + else
> + success = rte_atomic64_cmpset(&r->prod_64.head,
> + *old_head, *new_head);
> + } while (unlikely(success == 0));
> + return n;
> +}
> +
> +/**
> + * @internal This function updates the consumer head for dequeue using
> + * 64-bit head/tail values.
> + *
> + * @param r
> + * A pointer to the ring structure
> + * @param is_sc
> + * Indicates whether multi-consumer path is needed or not
> + * @param n
> + * The number of elements we will want to enqueue, i.e. how far should the
> + * head be moved
> + * @param behavior
> + * RTE_RING_QUEUE_FIXED: Dequeue a fixed number of items from a ring
> + * RTE_RING_QUEUE_VARIABLE: Dequeue as many items as possible from ring
> + * @param old_head
> + * Returns head value as it was before the move, i.e. where dequeue starts
> + * @param new_head
> + * Returns the current/new head value i.e. where dequeue finishes
> + * @param entries
> + * Returns the number of entries in the ring BEFORE head was moved
> + * @return
> + * - Actual number of objects dequeued.
> + * If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
> + */
> +static __rte_always_inline unsigned int
> +__rte_ring_move_cons_head_64(struct rte_ring *r, unsigned int is_sc,
> + unsigned int n, enum rte_ring_queue_behavior behavior,
> + uint64_t *old_head, uint64_t *new_head,
> + uint32_t *entries)
> +{
> + unsigned int max = n;
> + int success;
> +
> + do {
> + /* Restore n as it may change every loop */
> + n = max;
> +
> + *old_head = r->cons_64.head;
> +
> + /* add rmb barrier to avoid load/load reorder in weak
> + * memory model. It is noop on x86
> + */
> + rte_smp_rmb();
> +
> + /* The subtraction is done between two unsigned 64bits value
> + * (the result is always modulo 64 bits even if we have
> + * cons_head > prod_tail). So 'entries' is always between 0
> + * and size(ring)-1.
> + */
> + *entries = (r->prod_64.tail - *old_head);
> +
> + /* Set the actual entries for dequeue */
> + if (n > *entries)
> + n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 :
> *entries;
> +
> + if (unlikely(n == 0))
> + return 0;
> +
> + *new_head = *old_head + n;
> + if (is_sc)
> + r->cons_64.head = *new_head, success = 1;
> + else
> + success = rte_atomic64_cmpset(&r->cons_64.head,
> + *old_head, *new_head);
> + } while (unlikely(success == 0));
> + return n;
> +}
> +
> #endif /* _RTE_RING_GENERIC_H_ */
--
Ola Liljedahl, Networking System Architect, Arm
Phone +46706866373, Skype ola.liljedahl
