Re: [dpdk-dev] [PATCH v3 9/9] ring: add C11 memory model for new sync modes

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>
Hi Konstantin,
	It would be good to blend this commit with the other commits. Few comments inline.

> Subject: [PATCH v3 9/9] ring: add C11 memory model for new sync modes
> 
> Add C11 atomics based implementation for RTS and HTS head/tail update
> primitivies.
> 
> Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> ---
>  lib/librte_ring/Makefile               |   4 +-
>  lib/librte_ring/meson.build            |   2 +
>  lib/librte_ring/rte_ring_hts.h         |   4 +
>  lib/librte_ring/rte_ring_hts_c11_mem.h | 222 +++++++++++++++++++++++++
>  lib/librte_ring/rte_ring_hts_elem.h    |   4 +
>  lib/librte_ring/rte_ring_rts.h         |   4 +
>  lib/librte_ring/rte_ring_rts_c11_mem.h | 198 ++++++++++++++++++++++
>  lib/librte_ring/rte_ring_rts_elem.h    |   4 +
>  8 files changed, 441 insertions(+), 1 deletion(-)  create mode 100644
> lib/librte_ring/rte_ring_hts_c11_mem.h
>  create mode 100644 lib/librte_ring/rte_ring_rts_c11_mem.h
> 
> diff --git a/lib/librte_ring/Makefile b/lib/librte_ring/Makefile index
> 5f8662737..927d105bf 100644
> --- a/lib/librte_ring/Makefile
> +++ b/lib/librte_ring/Makefile
> @@ -22,9 +22,11 @@ SYMLINK-$(CONFIG_RTE_LIBRTE_RING)-include :=
> rte_ring.h \
>  					rte_ring_hts.h \
>  					rte_ring_hts_elem.h \
>  					rte_ring_hts_generic.h \
> +					rte_ring_hts_c11_mem.h \
>  					rte_ring_peek.h \
>  					rte_ring_rts.h \
>  					rte_ring_rts_elem.h \
> -					rte_ring_rts_generic.h
> +					rte_ring_rts_generic.h \
> +					rte_ring_rts_c11_mem.h
> 
>  include $(RTE_SDK)/mk/rte.lib.mk
> diff --git a/lib/librte_ring/meson.build b/lib/librte_ring/meson.build index
> f5f84dc6e..f2e37a8e4 100644
> --- a/lib/librte_ring/meson.build
> +++ b/lib/librte_ring/meson.build
> @@ -7,10 +7,12 @@ headers = files('rte_ring.h',
>  		'rte_ring_c11_mem.h',
>  		'rte_ring_generic.h',
>  		'rte_ring_hts.h',
> +		'rte_ring_hts_c11_mem.h',
>  		'rte_ring_hts_elem.h',
>  		'rte_ring_hts_generic.h',
>  		'rte_ring_peek.h',
>  		'rte_ring_rts.h',
> +		'rte_ring_rts_c11_mem.h',
>  		'rte_ring_rts_elem.h',
>  		'rte_ring_rts_generic.h')
> 
> diff --git a/lib/librte_ring/rte_ring_hts.h b/lib/librte_ring/rte_ring_hts.h index
> 062d7be6c..ddaa47ff1 100644
> --- a/lib/librte_ring/rte_ring_hts.h
> +++ b/lib/librte_ring/rte_ring_hts.h
> @@ -29,7 +29,11 @@
>  extern "C" {
>  #endif
> 
> +#ifdef RTE_USE_C11_MEM_MODEL
> +#include <rte_ring_hts_c11_mem.h>
> +#else
>  #include <rte_ring_hts_generic.h>
> +#endif
> 
>  /**
>   * @internal Enqueue several objects on the HTS ring.
> diff --git a/lib/librte_ring/rte_ring_hts_c11_mem.h
> b/lib/librte_ring/rte_ring_hts_c11_mem.h
> new file mode 100644
> index 000000000..0218d0e7d
> --- /dev/null
> +++ b/lib/librte_ring/rte_ring_hts_c11_mem.h
> @@ -0,0 +1,222 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + *
> + * Copyright (c) 2010-2020 Intel Corporation
> + * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
> + * All rights reserved.
> + * Derived from FreeBSD's bufring.h
> + * Used as BSD-3 Licensed with permission from Kip Macy.
> + */
> +
> +#ifndef _RTE_RING_HTS_C11_MEM_H_
> +#define _RTE_RING_HTS_C11_MEM_H_
> +
> +/**
> + * @file rte_ring_hts_c11_mem.h
> + * It is not recommended to include this file directly,
> + * include <rte_ring.h> instead.
> + * Contains internal helper functions for head/tail sync (HTS) ring mode.
> + * For more information please refer to <rte_ring_hts.h>.
> + */
> +
> +/**
> + * @internal get current tail value.
> + * Check that user didn't request to move tail above the head.
> + * In that situation:
> + * - return zero, that will cause abort any pending changes and
> + *   return head to its previous position.
> + * - throw an assert in debug mode.
> + */
> +static __rte_always_inline uint32_t
> +__rte_ring_hts_get_tail(struct rte_ring_hts_headtail *ht, uint32_t *tail,
> +	uint32_t num)
> +{
> +	uint32_t n;
> +	union rte_ring_ht_pos p;
> +
> +	p.raw = __atomic_load_n(&ht->ht.raw, __ATOMIC_RELAXED);
> +	n = p.pos.head - p.pos.tail;
> +
> +	RTE_ASSERT(n >= num);
> +	num = (n >= num) ? num : 0;
> +
> +	*tail = p.pos.tail;
> +	return num;
> +}
> +
> +/**
> + * @internal set new values for head and tail as one atomic 64 bit operation.
> + * Should be used only in conjunction with __rte_ring_hts_get_tail.
> + */
> +static __rte_always_inline void
> +__rte_ring_hts_set_head_tail(struct rte_ring_hts_headtail *ht, uint32_t tail,
> +	uint32_t num, uint32_t enqueue)
> +{
> +	union rte_ring_ht_pos p;
> +
> +	RTE_SET_USED(enqueue);
> +
> +	p.pos.head = tail + num;
> +	p.pos.tail = p.pos.head;
> +
> +	__atomic_store_n(&ht->ht.raw, p.raw, __ATOMIC_RELEASE); }
> +
> +static __rte_always_inline void
> +__rte_ring_hts_update_tail(struct rte_ring_hts_headtail *ht, uint32_t num,
> +	uint32_t enqueue)
> +{
> +	uint32_t tail;
> +
> +	num = __rte_ring_hts_get_tail(ht, &tail, num);
> +	__rte_ring_hts_set_head_tail(ht, tail, num, enqueue); }
> +
> +/**
> + * @internal waits till tail will become equal to head.
> + * Means no writer/reader is active for that ring.
> + * Suppose to work as serialization point.
> + */
> +static __rte_always_inline void
> +__rte_ring_hts_head_wait(const struct rte_ring_hts_headtail *ht,
> +		union rte_ring_ht_pos *p)
> +{
> +	p->raw = __atomic_load_n(&ht->ht.raw, __ATOMIC_ACQUIRE);
> +
> +	while (p->pos.head != p->pos.tail) {
> +		rte_pause();
> +		p->raw = __atomic_load_n(&ht->ht.raw,
> __ATOMIC_ACQUIRE);
> +	}
> +}
> +
> +/**
> + * @internal This function updates the producer head for enqueue
> + *
> + * @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_hts_move_prod_head(struct rte_ring *r, unsigned int num,
> +	enum rte_ring_queue_behavior behavior, uint32_t *old_head,
> +	uint32_t *free_entries)
> +{
> +	uint32_t n;
> +	union rte_ring_ht_pos np, op;
> +
> +	const uint32_t capacity = r->capacity;
> +
> +	do {
> +		/* Reset n to the initial burst count */
> +		n = num;
> +
> +		/* wait for tail to be equal to head, , acquire point */
> +		__rte_ring_hts_head_wait(&r->hts_prod, &op);
> +
> +		/*
> +		 *  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 + r->cons.tail - op.pos.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)
> +			break;
> +
> +		np.pos.tail = op.pos.tail;
> +		np.pos.head = op.pos.head + n;
> +
> +	} while (__atomic_compare_exchange_n(&r->hts_prod.ht.raw,
> +			&op.raw, np.raw,
> +			0, __ATOMIC_RELEASE, __ATOMIC_RELAXED) == 0);
__ATOMIC_RELEASE can be __ATOMIC_RELAXED. The RELEASE while updating after the elements are written is enough.

> +
> +	*old_head = op.pos.head;
> +	return n;
> +}
> +
> +/**
> + * @internal This function updates the consumer head for dequeue
> + *
> + * @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_hts_move_cons_head(struct rte_ring *r, unsigned int num,
> +	enum rte_ring_queue_behavior behavior, uint32_t *old_head,
> +	uint32_t *entries)
> +{
> +	uint32_t n;
> +	union rte_ring_ht_pos np, op;
> +
> +	/* move cons.head atomically */
> +	do {
> +		/* Restore n as it may change every loop */
> +		n = num;
> +
> +		/* wait for tail to be equal to head */
> +		__rte_ring_hts_head_wait(&r->hts_cons, &op);
> +
> +		/* 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 = r->prod.tail - op.pos.head;
> +
> +		/* Set the actual entries for dequeue */
> +		if (n > *entries)
> +			n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 :
> *entries;
> +
> +		if (unlikely(n == 0))
> +			break;
> +
> +		np.pos.tail = op.pos.tail;
> +		np.pos.head = op.pos.head + n;
> +
> +	} while (__atomic_compare_exchange_n(&r->hts_cons.ht.raw,
> +			&op.raw, np.raw,
> +			0, __ATOMIC_RELEASE, __ATOMIC_RELAXED) == 0);
Same here, RELEASE can be RELAXED.

> +
> +	*old_head = op.pos.head;
> +	return n;
> +}
> +
> +#endif /* _RTE_RING_HTS_C11_MEM_H_ */

<snip>

>  /**
>   * @internal Enqueue several objects on the RTS ring.
> diff --git a/lib/librte_ring/rte_ring_rts_c11_mem.h
> b/lib/librte_ring/rte_ring_rts_c11_mem.h
> new file mode 100644
> index 000000000..b72901497
> --- /dev/null
> +++ b/lib/librte_ring/rte_ring_rts_c11_mem.h
> @@ -0,0 +1,198 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + *
> + * Copyright (c) 2010-2017 Intel Corporation
> + * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
> + * All rights reserved.
> + * Derived from FreeBSD's bufring.h
> + * Used as BSD-3 Licensed with permission from Kip Macy.
> + */
> +
> +#ifndef _RTE_RING_RTS_C11_MEM_H_
> +#define _RTE_RING_RTS_C11_MEM_H_
> +
> +/**
> + * @file rte_ring_rts_c11_mem.h
> + * It is not recommended to include this file directly,
> + * include <rte_ring.h> instead.
> + * Contains internal helper functions for Relaxed Tail Sync (RTS) ring mode.
> + * For more information please refer to <rte_ring_rts.h>.
> + */
> +
> +/**
> + * @internal This function updates tail values.
> + */
> +static __rte_always_inline void
> +__rte_ring_rts_update_tail(struct rte_ring_rts_headtail *ht) {
> +	union rte_ring_ht_poscnt h, ot, nt;
> +
> +	/*
> +	 * If there are other enqueues/dequeues in progress that
> +	 * might preceded us, then don't update tail with new value.
> +	 */
> +
> +	ot.raw = __atomic_load_n(&ht->tail.raw, __ATOMIC_ACQUIRE);
This can be RELAXED. This thread is reading the value that it updated earlier, so it should be able to see the value it updated.

> +
> +	do {
> +		/* on 32-bit systems we have to do atomic read here */
> +		h.raw = __atomic_load_n(&ht->head.raw,
> __ATOMIC_RELAXED);
> +
> +		nt.raw = ot.raw;
> +		if (++nt.val.cnt == h.val.cnt)
> +			nt.val.pos = h.val.pos;
> +
> +	} while (__atomic_compare_exchange_n(&ht->tail.raw, &ot.raw,
> nt.raw,
> +			0, __ATOMIC_RELEASE, __ATOMIC_ACQUIRE) == 0); }
> +
> +/**
> + * @internal This function waits till head/tail distance wouldn't
> + * exceed pre-defined max value.
> + */
> +static __rte_always_inline void
> +__rte_ring_rts_head_wait(const struct rte_ring_rts_headtail *ht,
> +	union rte_ring_ht_poscnt *h)
> +{
> +	uint32_t max;
> +
> +	max = ht->htd_max;
> +	h->raw = __atomic_load_n(&ht->head.raw, __ATOMIC_ACQUIRE);
> +
> +	while (h->val.pos - ht->tail.val.pos > max) {
> +		rte_pause();
> +		h->raw = __atomic_load_n(&ht->head.raw,
> __ATOMIC_ACQUIRE);
> +	}
> +}
> +
> +/**
> + * @internal This function updates the producer head for enqueue.
> + *
> + * @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 uint32_t
> +__rte_ring_rts_move_prod_head(struct rte_ring *r, uint32_t num,
> +	enum rte_ring_queue_behavior behavior, uint32_t *old_head,
> +	uint32_t *free_entries)
> +{
> +	uint32_t n;
> +	union rte_ring_ht_poscnt nh, oh;
> +
> +	const uint32_t capacity = r->capacity;
> +
> +	do {
> +		/* Reset n to the initial burst count */
> +		n = num;
> +
> +		/* read prod head (may spin on prod tail, acquire point) */
> +		__rte_ring_rts_head_wait(&r->rts_prod, &oh);
> +
> +		/*
> +		 *  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 + r->cons.tail - oh.val.pos;
> +
> +		/* 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)
> +			break;
> +
> +		nh.val.pos = oh.val.pos + n;
> +		nh.val.cnt = oh.val.cnt + 1;
> +
> +	} while (__atomic_compare_exchange_n(&r->rts_prod.head.raw,
> +			&oh.raw, nh.raw,
> +			0, __ATOMIC_RELEASE, __ATOMIC_RELAXED) == 0);
> +
> +	*old_head = oh.val.pos;
> +	return n;
> +}
> +
> +/**
> + * @internal This function updates the consumer head for dequeue
> + *
> + * @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_rts_move_cons_head(struct rte_ring *r, uint32_t num,
> +	enum rte_ring_queue_behavior behavior, uint32_t *old_head,
> +	uint32_t *entries)
> +{
> +	uint32_t n;
> +	union rte_ring_ht_poscnt nh, oh;
> +
> +	/* move cons.head atomically */
> +	do {
> +		/* Restore n as it may change every loop */
> +		n = num;
> +
> +		/* read cons head (may spin on cons tail, acquire point) */
> +		__rte_ring_rts_head_wait(&r->rts_cons, &oh);
> +
> +		/* 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 = r->prod.tail - oh.val.pos;
> +
> +		/* Set the actual entries for dequeue */
> +		if (n > *entries)
> +			n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 :
> *entries;
> +
> +		if (unlikely(n == 0))
> +			break;
> +
> +		nh.val.pos = oh.val.pos + n;
> +		nh.val.cnt = oh.val.cnt + 1;
> +
> +	} while (__atomic_compare_exchange_n(&r->rts_cons.head.raw,
> +			&oh.raw, nh.raw,
> +			1, __ATOMIC_RELEASE, __ATOMIC_RELAXED) == 0);
> +
> +	*old_head = oh.val.pos;
> +	return n;
> +}
> +
> +#endif /* _RTE_RING_RTS_C11_MEM_H_ */
> diff --git a/lib/librte_ring/rte_ring_rts_elem.h
> b/lib/librte_ring/rte_ring_rts_elem.h
> index 71a331b23..23d8aeec7 100644
> --- a/lib/librte_ring/rte_ring_rts_elem.h
> +++ b/lib/librte_ring/rte_ring_rts_elem.h
> @@ -24,7 +24,11 @@
>  extern "C" {
>  #endif
> 
> +#ifdef RTE_USE_C11_MEM_MODEL
> +#include <rte_ring_rts_c11_mem.h>
> +#else
>  #include <rte_ring_rts_generic.h>
> +#endif
> 
>  /**
>   * @internal Enqueue several objects on the RTS ring.
> --
> 2.17.1