Re: [dpdk-dev] [PATCH v3 3/9] ring: introduce RTS ring mode

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

<snip>

> 
> Introduce relaxed tail sync (RTS) mode for MT ring synchronization.
> Aim to reduce stall times in case when ring is used on overcommited cpus
> (multiple active threads on the same cpu).
> The main difference from original MP/MC algorithm is that tail value is
> increased not by every thread that finished enqueue/dequeue, but only by the
> last one.
> That allows threads to avoid spinning on ring tail value, leaving actual tail
> value change to the last thread in the update queue.
> 
> check-abi.sh reports what I believe is a false-positive about ring cons/prod
> changes. As a workaround, devtools/libabigail.abignore is updated to suppress
> *struct ring* related errors.
This can be removed from the commit message.

> 
> Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> ---
>  devtools/libabigail.abignore           |   7 +
>  lib/librte_ring/Makefile               |   5 +-
>  lib/librte_ring/meson.build            |   5 +-
>  lib/librte_ring/rte_ring.c             | 100 +++++++-
>  lib/librte_ring/rte_ring.h             | 110 ++++++++-
>  lib/librte_ring/rte_ring_elem.h        |  86 ++++++-
>  lib/librte_ring/rte_ring_rts.h         | 316 +++++++++++++++++++++++++
>  lib/librte_ring/rte_ring_rts_elem.h    | 205 ++++++++++++++++
>  lib/librte_ring/rte_ring_rts_generic.h | 210 ++++++++++++++++
>  9 files changed, 1015 insertions(+), 29 deletions(-)  create mode 100644
> lib/librte_ring/rte_ring_rts.h  create mode 100644
> lib/librte_ring/rte_ring_rts_elem.h
>  create mode 100644 lib/librte_ring/rte_ring_rts_generic.h
> 
> diff --git a/devtools/libabigail.abignore b/devtools/libabigail.abignore index
> a59df8f13..cd86d89ca 100644
> --- a/devtools/libabigail.abignore
> +++ b/devtools/libabigail.abignore
> @@ -11,3 +11,10 @@
>          type_kind = enum
>          name = rte_crypto_asym_xform_type
>          changed_enumerators = RTE_CRYPTO_ASYM_XFORM_TYPE_LIST_END
> +; Ignore updates of ring prod/cons
> +[suppress_type]
> +        type_kind = struct
> +        name = rte_ring
> +[suppress_type]
> +        type_kind = struct
> +        name = rte_event_ring
Does this block the reporting of these structures forever?

> diff --git a/lib/librte_ring/Makefile b/lib/librte_ring/Makefile index
> 917c560ad..8f5c284cc 100644
> --- a/lib/librte_ring/Makefile
> +++ b/lib/librte_ring/Makefile
> @@ -18,6 +18,9 @@ SRCS-$(CONFIG_RTE_LIBRTE_RING) := rte_ring.c
> SYMLINK-$(CONFIG_RTE_LIBRTE_RING)-include := rte_ring.h \
>  					rte_ring_elem.h \
>  					rte_ring_generic.h \
> -					rte_ring_c11_mem.h
> +					rte_ring_c11_mem.h \
> +					rte_ring_rts.h \
> +					rte_ring_rts_elem.h \
> +					rte_ring_rts_generic.h
> 
>  include $(RTE_SDK)/mk/rte.lib.mk
> diff --git a/lib/librte_ring/meson.build b/lib/librte_ring/meson.build index
> f2f3ccc88..612936afb 100644
> --- a/lib/librte_ring/meson.build
> +++ b/lib/librte_ring/meson.build
> @@ -5,7 +5,10 @@ sources = files('rte_ring.c')  headers = files('rte_ring.h',
>  		'rte_ring_elem.h',
>  		'rte_ring_c11_mem.h',
> -		'rte_ring_generic.h')
> +		'rte_ring_generic.h',
> +		'rte_ring_rts.h',
> +		'rte_ring_rts_elem.h',
> +		'rte_ring_rts_generic.h')
> 
>  # rte_ring_create_elem and rte_ring_get_memsize_elem are experimental
> allow_experimental_apis = true diff --git a/lib/librte_ring/rte_ring.c
> b/lib/librte_ring/rte_ring.c index fa5733907..222eec0fb 100644
> --- a/lib/librte_ring/rte_ring.c
> +++ b/lib/librte_ring/rte_ring.c
> @@ -45,6 +45,9 @@ EAL_REGISTER_TAILQ(rte_ring_tailq)
>  /* true if x is a power of 2 */
>  #define POWEROF2(x) ((((x)-1) & (x)) == 0)
> 
> +/* by default set head/tail distance as 1/8 of ring capacity */
> +#define HTD_MAX_DEF	8
> +
>  /* return the size of memory occupied by a ring */  ssize_t
> rte_ring_get_memsize_elem(unsigned int esize, unsigned int count) @@ -
> 79,11 +82,84 @@ rte_ring_get_memsize(unsigned int count)
>  	return rte_ring_get_memsize_elem(sizeof(void *), count);  }
> 
> +/*
> + * internal helper function to reset prod/cons head-tail values.
> + */
> +static void
> +reset_headtail(void *p)
> +{
> +	struct rte_ring_headtail *ht;
> +	struct rte_ring_rts_headtail *ht_rts;
> +
> +	ht = p;
> +	ht_rts = p;
> +
> +	switch (ht->sync_type) {
> +	case RTE_RING_SYNC_MT:
> +	case RTE_RING_SYNC_ST:
> +		ht->head = 0;
> +		ht->tail = 0;
> +		break;
> +	case RTE_RING_SYNC_MT_RTS:
> +		ht_rts->head.raw = 0;
> +		ht_rts->tail.raw = 0;
> +		break;
> +	default:
> +		/* unknown sync mode */
> +		RTE_ASSERT(0);
> +	}
> +}
> +
>  void
>  rte_ring_reset(struct rte_ring *r)
>  {
> -	r->prod.head = r->cons.head = 0;
> -	r->prod.tail = r->cons.tail = 0;
> +	reset_headtail(&r->prod);
> +	reset_headtail(&r->cons);
> +}
> +
> +/*
> + * helper function, calculates sync_type values for prod and cons
> + * based on input flags. Returns zero at success or negative
> + * errno value otherwise.
> + */
> +static int
> +get_sync_type(uint32_t flags, enum rte_ring_sync_type *prod_st,
> +	enum rte_ring_sync_type *cons_st)
> +{
> +	static const uint32_t prod_st_flags =
> +		(RING_F_SP_ENQ | RING_F_MP_RTS_ENQ);
> +	static const uint32_t cons_st_flags =
> +		(RING_F_SC_DEQ | RING_F_MC_RTS_DEQ);
> +
> +	switch (flags & prod_st_flags) {
> +	case 0:
> +		*prod_st = RTE_RING_SYNC_MT;
> +		break;
> +	case RING_F_SP_ENQ:
> +		*prod_st = RTE_RING_SYNC_ST;
> +		break;
> +	case RING_F_MP_RTS_ENQ:
> +		*prod_st = RTE_RING_SYNC_MT_RTS;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	switch (flags & cons_st_flags) {
> +	case 0:
> +		*cons_st = RTE_RING_SYNC_MT;
> +		break;
> +	case RING_F_SC_DEQ:
> +		*cons_st = RTE_RING_SYNC_ST;
> +		break;
> +	case RING_F_MC_RTS_DEQ:
> +		*cons_st = RTE_RING_SYNC_MT_RTS;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
>  }
> 
>  int
> @@ -100,16 +176,20 @@ rte_ring_init(struct rte_ring *r, const char *name,
> unsigned count,
>  	RTE_BUILD_BUG_ON((offsetof(struct rte_ring, prod) &
>  			  RTE_CACHE_LINE_MASK) != 0);
> 
> +	RTE_BUILD_BUG_ON(offsetof(struct rte_ring_headtail, sync_type) !=
> +		offsetof(struct rte_ring_rts_headtail, sync_type));
> +	RTE_BUILD_BUG_ON(offsetof(struct rte_ring_headtail, tail) !=
> +		offsetof(struct rte_ring_rts_headtail, tail.val.pos));
> +
>  	/* init the ring structure */
>  	memset(r, 0, sizeof(*r));
>  	ret = strlcpy(r->name, name, sizeof(r->name));
>  	if (ret < 0 || ret >= (int)sizeof(r->name))
>  		return -ENAMETOOLONG;
>  	r->flags = flags;
> -	r->prod.sync_type = (flags & RING_F_SP_ENQ) ?
> -		RTE_RING_SYNC_ST : RTE_RING_SYNC_MT;
> -	r->cons.sync_type = (flags & RING_F_SC_DEQ) ?
> -		RTE_RING_SYNC_ST : RTE_RING_SYNC_MT;
> +	ret = get_sync_type(flags, &r->prod.sync_type, &r->cons.sync_type);
> +	if (ret != 0)
> +		return ret;
> 
>  	if (flags & RING_F_EXACT_SZ) {
>  		r->size = rte_align32pow2(count + 1); @@ -126,8 +206,12
> @@ rte_ring_init(struct rte_ring *r, const char *name, unsigned count,
>  		r->mask = count - 1;
>  		r->capacity = r->mask;
>  	}
> -	r->prod.head = r->cons.head = 0;
> -	r->prod.tail = r->cons.tail = 0;
> +
> +	/* set default values for head-tail distance */
> +	if (flags & RING_F_MP_RTS_ENQ)
> +		rte_ring_set_prod_htd_max(r, r->capacity / HTD_MAX_DEF);
> +	if (flags & RING_F_MC_RTS_DEQ)
> +		rte_ring_set_cons_htd_max(r, r->capacity / HTD_MAX_DEF);
> 
>  	return 0;
>  }
> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h index
> d4775a063..f6f084d79 100644
> --- a/lib/librte_ring/rte_ring.h
> +++ b/lib/librte_ring/rte_ring.h
> @@ -48,6 +48,7 @@ extern "C" {
>  #include <rte_branch_prediction.h>
>  #include <rte_memzone.h>
>  #include <rte_pause.h>
> +#include <rte_debug.h>
> 
>  #define RTE_TAILQ_RING_NAME "RTE_RING"
> 
> @@ -65,10 +66,13 @@ enum rte_ring_queue_behavior {  enum
> rte_ring_sync_type {
>  	RTE_RING_SYNC_MT,     /**< multi-thread safe (default mode) */
>  	RTE_RING_SYNC_ST,     /**< single thread only */
> +#ifdef ALLOW_EXPERIMENTAL_API
> +	RTE_RING_SYNC_MT_RTS, /**< multi-thread relaxed tail sync */
> #endif
>  };
> 
>  /**
> - * structure to hold a pair of head/tail values and other metadata.
> + * structures to hold a pair of head/tail values and other metadata.
>   * Depending on sync_type format of that structure might be different,
>   * but offset for *sync_type* and *tail* values should remain the same.
>   */
> @@ -84,6 +88,21 @@ struct rte_ring_headtail {
>  	};
>  };
> 
> +union rte_ring_ht_poscnt {
nit, this is specific to RTS, may be change this to rte_ring_rts_ht_poscnt?

> +	uint64_t raw;
> +	struct {
> +		uint32_t cnt; /**< head/tail reference counter */
> +		uint32_t pos; /**< head/tail position */
> +	} val;
> +};
> +
> +struct rte_ring_rts_headtail {
> +	volatile union rte_ring_ht_poscnt tail;
> +	enum rte_ring_sync_type sync_type;  /**< sync type of prod/cons */
> +	uint32_t htd_max;   /**< max allowed distance between head/tail */
> +	volatile union rte_ring_ht_poscnt head; };
> +
>  /**
>   * An RTE ring structure.
>   *
> @@ -111,11 +130,21 @@ 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;
> +		struct rte_ring_rts_headtail rts_prod;
> +	}  __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;
> +		struct rte_ring_rts_headtail rts_cons;
> +	}  __rte_cache_aligned;
> +
>  	char pad2 __rte_cache_aligned; /**< empty cache line */  };
> 
> @@ -132,6 +161,9 @@ struct rte_ring {
>  #define RING_F_EXACT_SZ 0x0004
>  #define RTE_RING_SZ_MASK  (0x7fffffffU) /**< Ring size mask */
> 
> +#define RING_F_MP_RTS_ENQ 0x0008 /**< The default enqueue is "MP RTS".
> +*/ #define RING_F_MC_RTS_DEQ 0x0010 /**< The default dequeue is "MC
> +RTS". */
> +
>  #define __IS_SP RTE_RING_SYNC_ST
>  #define __IS_MP RTE_RING_SYNC_MT
>  #define __IS_SC RTE_RING_SYNC_ST
> @@ -461,6 +493,10 @@ rte_ring_sp_enqueue_bulk(struct rte_ring *r, void *
> const *obj_table,
>  			RTE_RING_SYNC_ST, free_space);
>  }
> 
> +#ifdef ALLOW_EXPERIMENTAL_API
> +#include <rte_ring_rts.h>
> +#endif
> +
>  /**
>   * Enqueue several objects on a ring.
>   *
> @@ -484,8 +520,21 @@ static __rte_always_inline unsigned int
> rte_ring_enqueue_bulk(struct rte_ring *r, void * const *obj_table,
>  		      unsigned int n, unsigned int *free_space)  {
> -	return __rte_ring_do_enqueue(r, obj_table, n,
> RTE_RING_QUEUE_FIXED,
> -			r->prod.sync_type, free_space);
> +	switch (r->prod.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mp_enqueue_bulk(r, obj_table, n, free_space);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sp_enqueue_bulk(r, obj_table, n, free_space);
Have you validated if these affect the performance for the existing APIs?
I am also wondering why should we support these new modes in the legacy APIs?
I think users should move to use rte_ring_xxx_elem APIs. If users want to use RTS/HTS it will be a good time for them to move to new APIs. They anyway have to test their code for RTS/HTS, might as well make the change to new APIs and test both.
It will be less code to maintain for the community as well.

> #ifdef
> +ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mp_rts_enqueue_bulk(r, obj_table, n,
> +			free_space);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	return 0;
>  }
> 
>  /**
> @@ -619,8 +668,20 @@ static __rte_always_inline unsigned int
> rte_ring_dequeue_bulk(struct rte_ring *r, void **obj_table, unsigned int n,
>  		unsigned int *available)
>  {
> -	return __rte_ring_do_dequeue(r, obj_table, n,
> RTE_RING_QUEUE_FIXED,
> -				r->cons.sync_type, available);
> +	switch (r->cons.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mc_dequeue_bulk(r, obj_table, n, available);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sc_dequeue_bulk(r, obj_table, n, available);
> #ifdef
> +ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mc_rts_dequeue_bulk(r, obj_table, n,
> available);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	return 0;
>  }
> 
>  /**
> @@ -940,8 +1001,21 @@ static __rte_always_inline unsigned
> rte_ring_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>  		      unsigned int n, unsigned int *free_space)  {
> -	return __rte_ring_do_enqueue(r, obj_table, n,
> RTE_RING_QUEUE_VARIABLE,
> -			r->prod.sync_type, free_space);
> +	switch (r->prod.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mp_enqueue_burst(r, obj_table, n,
> free_space);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sp_enqueue_burst(r, obj_table, n, free_space);
> #ifdef
> +ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mp_rts_enqueue_burst(r, obj_table, n,
> +			free_space);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	return 0;
>  }
> 
>  /**
> @@ -1020,9 +1094,21 @@ static __rte_always_inline unsigned
> rte_ring_dequeue_burst(struct rte_ring *r, void **obj_table,
>  		unsigned int n, unsigned int *available)  {
> -	return __rte_ring_do_dequeue(r, obj_table, n,
> -				RTE_RING_QUEUE_VARIABLE,
> -				r->cons.sync_type, available);
> +	switch (r->cons.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mc_dequeue_burst(r, obj_table, n, available);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sc_dequeue_burst(r, obj_table, n, available);
> #ifdef
> +ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mc_rts_dequeue_burst(r, obj_table, n,
> +			available);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	return 0;
>  }
> 
>  #ifdef __cplusplus
> diff --git a/lib/librte_ring/rte_ring_elem.h b/lib/librte_ring/rte_ring_elem.h
> index 28f9836e6..5de0850dc 100644
> --- a/lib/librte_ring/rte_ring_elem.h
> +++ b/lib/librte_ring/rte_ring_elem.h
> @@ -542,6 +542,8 @@ rte_ring_sp_enqueue_bulk_elem(struct rte_ring *r,
> const void *obj_table,
>  			RTE_RING_QUEUE_FIXED, __IS_SP, free_space);  }
> 
> +#include <rte_ring_rts_elem.h>
> +
>  /**
>   * Enqueue several objects on a ring.
>   *
> @@ -571,6 +573,26 @@ rte_ring_enqueue_bulk_elem(struct rte_ring *r,
> const void *obj_table,  {
>  	return __rte_ring_do_enqueue_elem(r, obj_table, esize, n,
>  			RTE_RING_QUEUE_FIXED, r->prod.sync_type,
> free_space);
> +
> +	switch (r->prod.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mp_enqueue_bulk_elem(r, obj_table, esize, n,
> +			free_space);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sp_enqueue_bulk_elem(r, obj_table, esize, n,
> +			free_space);
> +#ifdef ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mp_rts_enqueue_bulk_elem(r, obj_table,
> esize, n,
> +			free_space);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	if (free_space != NULL)
> +		*free_space = 0;
> +	return 0;
>  }
> 
>  /**
> @@ -733,8 +755,25 @@ static __rte_always_inline unsigned int
> rte_ring_dequeue_bulk_elem(struct rte_ring *r, void *obj_table,
>  		unsigned int esize, unsigned int n, unsigned int *available)  {
> -	return __rte_ring_do_dequeue_elem(r, obj_table, esize, n,
> -			RTE_RING_QUEUE_FIXED, r->cons.sync_type,
> available);
> +	switch (r->cons.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mc_dequeue_bulk_elem(r, obj_table, esize, n,
> +			available);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sc_dequeue_bulk_elem(r, obj_table, esize, n,
> +			available);
> +#ifdef ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mc_rts_dequeue_bulk_elem(r, obj_table,
> esize,
> +			n, available);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	if (available != NULL)
> +		*available = 0;
> +	return 0;
>  }
> 
>  /**
> @@ -901,8 +940,25 @@ static __rte_always_inline unsigned
> rte_ring_enqueue_burst_elem(struct rte_ring *r, const void *obj_table,
>  		unsigned int esize, unsigned int n, unsigned int *free_space)  {
> -	return __rte_ring_do_enqueue_elem(r, obj_table, esize, n,
> -			RTE_RING_QUEUE_VARIABLE, r->prod.sync_type,
> free_space);
> +	switch (r->prod.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mp_enqueue_burst_elem(r, obj_table, esize, n,
> +			free_space);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sp_enqueue_burst_elem(r, obj_table, esize, n,
> +			free_space);
> +#ifdef ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mp_rts_enqueue_burst_elem(r, obj_table,
> esize,
> +			n, free_space);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	if (free_space != NULL)
> +		*free_space = 0;
> +	return 0;
>  }
> 
>  /**
> @@ -993,9 +1049,25 @@ static __rte_always_inline unsigned int
> rte_ring_dequeue_burst_elem(struct rte_ring *r, void *obj_table,
>  		unsigned int esize, unsigned int n, unsigned int *available)  {
> -	return __rte_ring_do_dequeue_elem(r, obj_table, esize, n,
> -				RTE_RING_QUEUE_VARIABLE,
> -				r->cons.sync_type, available);
> +	switch (r->cons.sync_type) {
> +	case RTE_RING_SYNC_MT:
> +		return rte_ring_mc_dequeue_burst_elem(r, obj_table, esize, n,
> +			available);
> +	case RTE_RING_SYNC_ST:
> +		return rte_ring_sc_dequeue_burst_elem(r, obj_table, esize, n,
> +			available);
> +#ifdef ALLOW_EXPERIMENTAL_API
> +	case RTE_RING_SYNC_MT_RTS:
> +		return rte_ring_mc_rts_dequeue_burst_elem(r, obj_table,
> esize,
> +			n, available);
> +#endif
> +	}
> +
> +	/* valid ring should never reach this point */
> +	RTE_ASSERT(0);
> +	if (available != NULL)
> +		*available = 0;
> +	return 0;
>  }
> 
>  #ifdef __cplusplus
> diff --git a/lib/librte_ring/rte_ring_rts.h b/lib/librte_ring/rte_ring_rts.h new
> file mode 100644 index 000000000..18404fe48
> --- /dev/null
> +++ b/lib/librte_ring/rte_ring_rts.h
IMO, we should not provide these APIs.

> @@ -0,0 +1,316 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + *
> + * Copyright (c) 2010-2017 Intel Corporation
nit, the year should change to 2020? Look at others too.

> + * 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_H_
> +#define _RTE_RING_RTS_H_
> +
> +/**
> + * @file rte_ring_rts.h
> + * @b EXPERIMENTAL: this API may change without prior notice
> + * It is not recommended to include this file directly.
> + * Please include <rte_ring.h> instead.
> + *
> + * Contains functions for Relaxed Tail Sync (RTS) ring mode.
> + * The main idea remains the same as for our original MP/MC
                                                                                 ^^^ the
> +synchronization
> + * mechanism.
> + * The main difference is that tail value is increased not
> + * by every thread that finished enqueue/dequeue,
> + * but only by the last one doing enqueue/dequeue.
should we say 'current last' or 'last thread at a given instance'?

> + * That allows threads to skip spinning on tail value,
> + * leaving actual tail value change to last thread in the update queue.
nit, I understand what you mean by 'update queue' here. IMO, we should remove it as it might confuse some.

> + * RTS requires 2 64-bit CAS for each enqueue(/dequeue) operation:
> + * one for head update, second for tail update.
> + * As a gain it allows thread to avoid spinning/waiting on tail value.
> + * In comparision original MP/MC algorithm requires one 32-bit CAS
> + * for head update and waiting/spinning on tail value.
> + *
> + * Brief outline:
> + *  - introduce refcnt for both head and tail.
Suggesting using the same names as used in the structures.

> + *  - increment head.refcnt for each head.value update
> + *  - write head:value and head:refcnt atomically (64-bit CAS)
> + *  - move tail.value ahead only when tail.refcnt + 1 == head.refcnt
May be add '(indicating that this is the last thread updating the tail)'

> + *  - increment tail.refcnt when each enqueue/dequeue op finishes
May be add 'otherwise' at the beginning.

> + *    (no matter is tail:value going to change or not)
nit                            ^^ if
> + *  - write tail.value and tail.recnt atomically (64-bit CAS)
> + *
> + * To avoid producer/consumer starvation:
> + *  - limit max allowed distance between head and tail value (HTD_MAX).
> + *    I.E. thread is allowed to proceed with changing head.value,
> + *    only when:  head.value - tail.value <= HTD_MAX
> + * HTD_MAX is an optional parameter.
> + * With HTD_MAX == 0 we'll have fully serialized ring -
> + * i.e. only one thread at a time will be able to enqueue/dequeue
> + * to/from the ring.
> + * With HTD_MAX >= ring.capacity - no limitation.
> + * By default HTD_MAX == ring.capacity / 8.
> + */
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#include <rte_ring_rts_generic.h>
> +
> +/**
> + * @internal Enqueue several objects on the RTS ring.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @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 free_space
> + *   returns the amount of space after the enqueue operation has finished
> + * @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_do_rts_enqueue(struct rte_ring *r, void * const *obj_table,
> +		uint32_t n, enum rte_ring_queue_behavior behavior,
> +		uint32_t *free_space)
> +{
> +	uint32_t free, head;
> +
> +	n =  __rte_ring_rts_move_prod_head(r, n, behavior, &head, &free);
> +
> +	if (n != 0) {
> +		ENQUEUE_PTRS(r, &r[1], head, obj_table, n, void *);
> +		__rte_ring_rts_update_tail(&r->rts_prod);
> +	}
> +
> +	if (free_space != NULL)
> +		*free_space = free - n;
> +	return n;
> +}
> +
> +/**
> + * @internal Dequeue several objects from the RTS ring.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to pull from the ring.
> + * @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 available
> + *   returns the number of remaining ring entries after the dequeue has
> finished
> + * @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_do_rts_dequeue(struct rte_ring *r, void **obj_table,
> +		uint32_t n, enum rte_ring_queue_behavior behavior,
> +		uint32_t *available)
> +{
> +	uint32_t entries, head;
> +
> +	n = __rte_ring_rts_move_cons_head(r, n, behavior, &head, &entries);
> +
> +	if (n != 0) {
> +		DEQUEUE_PTRS(r, &r[1], head, obj_table, n, void *);
> +		__rte_ring_rts_update_tail(&r->rts_cons);
> +	}
> +
> +	if (available != NULL)
> +		*available = entries - n;
> +	return n;
> +}
> +
> +/**
> + * Enqueue several objects on the RTS ring (multi-producers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @param free_space
> + *   if non-NULL, returns the amount of space in the ring after the
> + *   enqueue operation has finished.
> + * @return
> + *   The number of objects enqueued, either 0 or n
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned int
> +rte_ring_mp_rts_enqueue_bulk(struct rte_ring *r, void * const *obj_table,
> +			 unsigned int n, unsigned int *free_space) {
> +	return __rte_ring_do_rts_enqueue(r, obj_table, n,
> RTE_RING_QUEUE_FIXED,
> +			free_space);
> +}
> +
> +/**
> + * Dequeue several objects from an RTS ring (multi-consumers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects) that will be filled.
> + * @param n
> + *   The number of objects to dequeue from the ring to the obj_table.
> + * @param available
> + *   If non-NULL, returns the number of remaining ring entries after the
> + *   dequeue has finished.
> + * @return
> + *   The number of objects dequeued, either 0 or n
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned int
> +rte_ring_mc_rts_dequeue_bulk(struct rte_ring *r, void **obj_table,
> +		unsigned int n, unsigned int *available) {
> +	return __rte_ring_do_rts_dequeue(r, obj_table, n,
> RTE_RING_QUEUE_FIXED,
> +			available);
> +}
> +
> +/**
> + * Return producer max Head-Tail-Distance (HTD).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @return
> + *   Producer HTD value, if producer is set in appropriate sync mode,
> + *   or UINT32_MAX otherwise.
> + */
> +__rte_experimental
> +static inline uint32_t
> +rte_ring_get_prod_htd_max(const struct rte_ring *r) {
> +	if (r->prod.sync_type == RTE_RING_SYNC_MT_RTS)
> +		return r->rts_prod.htd_max;
> +	return UINT32_MAX;
> +}
> +
> +/**
> + * Set producer max Head-Tail-Distance (HTD).
> + * Note that producer has to use appropriate sync mode (RTS).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param v
> + *   new HTD value to setup.
> + * @return
> + *   Zero on success, or negative error code otherwise.
> + */
> +__rte_experimental
> +static inline int
> +rte_ring_set_prod_htd_max(struct rte_ring *r, uint32_t v) {
> +	if (r->prod.sync_type != RTE_RING_SYNC_MT_RTS)
> +		return -ENOTSUP;
> +
> +	r->rts_prod.htd_max = v;
> +	return 0;
> +}
> +
> +/**
> + * Return consumer max Head-Tail-Distance (HTD).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @return
> + *   Consumer HTD value, if consumer is set in appropriate sync mode,
> + *   or UINT32_MAX otherwise.
> + */
> +__rte_experimental
> +static inline uint32_t
> +rte_ring_get_cons_htd_max(const struct rte_ring *r) {
> +	if (r->cons.sync_type == RTE_RING_SYNC_MT_RTS)
> +		return r->rts_cons.htd_max;
> +	return UINT32_MAX;
> +}
> +
> +/**
> + * Set consumer max Head-Tail-Distance (HTD).
> + * Note that consumer has to use appropriate sync mode (RTS).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param v
> + *   new HTD value to setup.
> + * @return
> + *   Zero on success, or negative error code otherwise.
> + */
> +__rte_experimental
> +static inline int
> +rte_ring_set_cons_htd_max(struct rte_ring *r, uint32_t v) {
> +	if (r->cons.sync_type != RTE_RING_SYNC_MT_RTS)
> +		return -ENOTSUP;
> +
> +	r->rts_cons.htd_max = v;
> +	return 0;
> +}
> +
> +/**
> + * Enqueue several objects on the RTS ring (multi-producers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @param free_space
> + *   if non-NULL, returns the amount of space in the ring after the
> + *   enqueue operation has finished.
> + * @return
> + *   - n: Actual number of objects enqueued.
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned
> +rte_ring_mp_rts_enqueue_burst(struct rte_ring *r, void * const *obj_table,
> +			 unsigned int n, unsigned int *free_space) {
> +	return __rte_ring_do_rts_enqueue(r, obj_table, n,
> +			RTE_RING_QUEUE_VARIABLE, free_space); }
> +
> +/**
> + * Dequeue several objects from an RTS  ring (multi-consumers safe).
> + * When the requested objects are more than the available objects,
> + * only dequeue the actual number of objects.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects) that will be filled.
> + * @param n
> + *   The number of objects to dequeue from the ring to the obj_table.
> + * @param available
> + *   If non-NULL, returns the number of remaining ring entries after the
> + *   dequeue has finished.
> + * @return
> + *   - n: Actual number of objects dequeued, 0 if ring is empty
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned
> +rte_ring_mc_rts_dequeue_burst(struct rte_ring *r, void **obj_table,
> +		unsigned int n, unsigned int *available) {
> +	return __rte_ring_do_rts_dequeue(r, obj_table, n,
> +			RTE_RING_QUEUE_VARIABLE, available); }
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_RING_RTS_H_ */
> diff --git a/lib/librte_ring/rte_ring_rts_elem.h
> b/lib/librte_ring/rte_ring_rts_elem.h
> new file mode 100644
> index 000000000..71a331b23
> --- /dev/null
> +++ b/lib/librte_ring/rte_ring_rts_elem.h
> @@ -0,0 +1,205 @@
> +/* 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_ELEM_H_
> +#define _RTE_RING_RTS_ELEM_H_
> +
> +/**
> + * @file rte_ring_rts_elem.h
> + * @b EXPERIMENTAL: this API may change without prior notice
> + *
> + * It is not recommended to include this file directly.
> + * Please include <rte_ring_elem.h> instead.
> + * Contains *ring_elem* functions for Relaxed Tail Sync (RTS) ring mode.
> + * for more details please refer to <rte_ring_rts.h>.
> + */
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#include <rte_ring_rts_generic.h>
> +
> +/**
> + * @internal Enqueue several objects on the RTS ring.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @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 free_space
> + *   returns the amount of space after the enqueue operation has finished
> + * @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_do_rts_enqueue_elem(struct rte_ring *r, void * const *obj_table,
obj_table should be of type 'const void * obj_table' (looks like copy paste error). Please check the other APIs below too.

> +	uint32_t esize, uint32_t n, enum rte_ring_queue_behavior behavior,
'esize' is not documented in the comments above the function. You can copy the header from rte_ring_elem.h file. Please check other APIs as well.

> +	uint32_t *free_space)
> +{
> +	uint32_t free, head;
> +
> +	n =  __rte_ring_rts_move_prod_head(r, n, behavior, &head, &free);
> +
> +	if (n != 0) {
> +		__rte_ring_enqueue_elems(r, head, obj_table, esize, n);
> +		__rte_ring_rts_update_tail(&r->rts_prod);
> +	}
> +
> +	if (free_space != NULL)
> +		*free_space = free - n;
> +	return n;
> +}
> +
> +/**
> + * @internal Dequeue several objects from the RTS ring.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to pull from the ring.
> + * @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 available
> + *   returns the number of remaining ring entries after the dequeue has
> finished
> + * @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_do_rts_dequeue_elem(struct rte_ring *r, void **obj_table,
> +	uint32_t esize, uint32_t n, enum rte_ring_queue_behavior behavior,
> +	uint32_t *available)
> +{
> +	uint32_t entries, head;
> +
> +	n = __rte_ring_rts_move_cons_head(r, n, behavior, &head, &entries);
> +
> +	if (n != 0) {
> +		__rte_ring_dequeue_elems(r, head, obj_table, esize, n);
> +		__rte_ring_rts_update_tail(&r->rts_cons);
> +	}
> +
> +	if (available != NULL)
> +		*available = entries - n;
> +	return n;
> +}
> +
> +/**
> + * Enqueue several objects on the RTS ring (multi-producers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @param free_space
> + *   if non-NULL, returns the amount of space in the ring after the
> + *   enqueue operation has finished.
> + * @return
> + *   The number of objects enqueued, either 0 or n
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned int
> +rte_ring_mp_rts_enqueue_bulk_elem(struct rte_ring *r, void * const
> *obj_table,
> +	unsigned int esize, unsigned int n, unsigned int *free_space) {
> +	return __rte_ring_do_rts_enqueue_elem(r, obj_table, esize, n,
> +			RTE_RING_QUEUE_FIXED, free_space);
> +}
> +
> +/**
> + * Dequeue several objects from an RTS ring (multi-consumers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects) that will be filled.
> + * @param n
> + *   The number of objects to dequeue from the ring to the obj_table.
> + * @param available
> + *   If non-NULL, returns the number of remaining ring entries after the
> + *   dequeue has finished.
> + * @return
> + *   The number of objects dequeued, either 0 or n
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned int
> +rte_ring_mc_rts_dequeue_bulk_elem(struct rte_ring *r, void **obj_table,
> +	unsigned int esize, unsigned int n, unsigned int *available) {
> +	return __rte_ring_do_rts_dequeue_elem(r, obj_table, esize, n,
> +		RTE_RING_QUEUE_FIXED, available);
> +}
> +
> +/**
> + * Enqueue several objects on the RTS ring (multi-producers safe).
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects).
> + * @param n
> + *   The number of objects to add in the ring from the obj_table.
> + * @param free_space
> + *   if non-NULL, returns the amount of space in the ring after the
> + *   enqueue operation has finished.
> + * @return
> + *   - n: Actual number of objects enqueued.
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned
> +rte_ring_mp_rts_enqueue_burst_elem(struct rte_ring *r, void * const
> *obj_table,
> +	unsigned int esize, unsigned int n, unsigned int *free_space) {
> +	return __rte_ring_do_rts_enqueue_elem(r, obj_table, esize, n,
> +			RTE_RING_QUEUE_VARIABLE, free_space); }
> +
> +/**
> + * Dequeue several objects from an RTS  ring (multi-consumers safe).
> + * When the requested objects are more than the available objects,
> + * only dequeue the actual number of objects.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_table
> + *   A pointer to a table of void * pointers (objects) that will be filled.
> + * @param n
> + *   The number of objects to dequeue from the ring to the obj_table.
> + * @param available
> + *   If non-NULL, returns the number of remaining ring entries after the
> + *   dequeue has finished.
> + * @return
> + *   - n: Actual number of objects dequeued, 0 if ring is empty
> + */
> +__rte_experimental
> +static __rte_always_inline unsigned
> +rte_ring_mc_rts_dequeue_burst_elem(struct rte_ring *r, void **obj_table,
> +	unsigned int esize, unsigned int n, unsigned int *available) {
> +	return __rte_ring_do_rts_dequeue_elem(r, obj_table, esize, n,
> +			RTE_RING_QUEUE_VARIABLE, available); }
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_RING_RTS_ELEM_H_ */
> diff --git a/lib/librte_ring/rte_ring_rts_generic.h
> b/lib/librte_ring/rte_ring_rts_generic.h
> new file mode 100644
> index 000000000..f88460d47
> --- /dev/null
> +++ b/lib/librte_ring/rte_ring_rts_generic.h
I do not know the benefit to providing the generic version. Do you know why this was done in the legacy APIs?
If there is no performance difference between generic and C11 versions, should we just skip the generic version?
The oldest compiler in CI are GCC 4.8.5 and Clang 3.4.2 and C11 built-ins are supported earlier than these compiler versions.
I feel the code is growing exponentially in rte_ring library and we should try to cut non-value-ass code/APIs aggressively.

> @@ -0,0 +1,210 @@
> +/* 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_GENERIC_H_
> +#define _RTE_RING_RTS_GENERIC_H_
> +
> +/**
> + * @file rte_ring_rts_generic.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.
> +	 */
> +
> +	do {
> +		ot.raw = ht->tail.raw;
> +		rte_smp_rmb();
> +
> +		/* on 32-bit systems we have to do atomic read here */
> +		h.raw = rte_atomic64_read((rte_atomic64_t *)
> +			(uintptr_t)&ht->head.raw);
> +
> +		nt.raw = ot.raw;
> +		if (++nt.val.cnt == h.val.cnt)
> +			nt.val.pos = h.val.pos;
> +
> +	} while (rte_atomic64_cmpset(&ht->tail.raw, ot.raw, nt.raw) == 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 = ht->head.raw;
> +	rte_smp_rmb();
> +
> +	while (h->val.pos - ht->tail.val.pos > max) {
> +		rte_pause();
> +		h->raw = ht->head.raw;
> +		rte_smp_rmb();
> +	}
> +}
> +
> +/**
> + * @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) */
> +		__rte_ring_rts_head_wait(&r->rts_prod, &oh);
> +
> +		/* 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 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 (rte_atomic64_cmpset(&r->rts_prod.head.raw,
> +			oh.raw, nh.raw) == 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) */
> +		__rte_ring_rts_head_wait(&r->rts_cons, &oh);
> +
> +
> +		/* 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 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 (rte_atomic64_cmpset(&r->rts_cons.head.raw,
> +			oh.raw, nh.raw) == 0);
> +
> +	*old_head = oh.val.pos;
> +	return n;
> +}
> +
> +#endif /* _RTE_RING_RTS_GENERIC_H_ */
> --
> 2.17.1