Re: [dpdk-dev] [PATCH 1/1] mbuf: move pool pointer in first half

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Saturday, November 7, 2020 4:53 PM
> 
> The mempool pointer in the mbuf struct is moved
> from the second to the first half.
> It should increase performance on most systems having 64-byte cache line,
> i.e. mbuf is split in two cache lines.

Perhaps with the #define DEV_TX_OFFLOAD_MBUF_FAST_FREE mentioned by Konstantin, it might be better moving m->next instead, at least for applications with the opportunity to set this flag (e.g. applications with only one mbuf pool).

Unfortunately, the information about the DEV_TX_OFFLOAD_MBUF_FAST_FREE flag came to light after the techboard meeting, and I don't have any benchmarks to support this suggestion, so I guess it's hard to change the techboard's decision to move the pool pointer.

> 
> Due to this change, tx_offload is moved, so some vector data paths
> may need to be adjusted. Note: OCTEON TX2 check is removed temporarily!

Regardless if m->pool or m->next is moved, it will not affect any issues related to m->tx_offload moving to a new location in the second cache line.

> 
> Moving this field gives more space to dynfield1.
> 
> This is how the mbuf layout looks like (pahole-style):
> 
> word  type                              name                byte  size
>  0    void *                            buf_addr;         /*   0 +  8 */
>  1    rte_iova_t                        buf_iova          /*   8 +  8 */
>       /* --- RTE_MARKER64               rearm_data;                   */
>  2    uint16_t                          data_off;         /*  16 +  2 */
>       uint16_t                          refcnt;           /*  18 +  2 */
>       uint16_t                          nb_segs;          /*  20 +  2 */
>       uint16_t                          port;             /*  22 +  2 */
>  3    uint64_t                          ol_flags;         /*  24 +  8 */
>       /* --- RTE_MARKER                 rx_descriptor_fields1;        */
>  4    uint32_t             union        packet_type;      /*  32 +  4 */
>       uint32_t                          pkt_len;          /*  36 +  4 */
>  5    uint16_t                          data_len;         /*  40 +  2 */
>       uint16_t                          vlan_tci;         /*  42 +  2 */
>  5.5  uint64_t             union        hash;             /*  44 +  8 */
>  6.5  uint16_t                          vlan_tci_outer;   /*  52 +  2 */
>       uint16_t                          buf_len;          /*  54 +  2 */
>  7    struct rte_mempool *              pool;             /*  56 +  8 */
>       /* --- RTE_MARKER                 cacheline1;                   */
>  8    struct rte_mbuf *                 next;             /*  64 +  8 */
>  9    uint64_t             union        tx_offload;       /*  72 +  8 */
> 10    struct rte_mbuf_ext_shared_info * shinfo;           /*  80 +  8 */
> 11    uint16_t                          priv_size;        /*  88 +  2 */
>       uint16_t                          timesync;         /*  90 +  2 */
> 11.5  uint32_t                          dynfield1[9];     /*  92 + 36 */
> 16    /* --- END                                             128      */
> 
> Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
> ---
>  doc/guides/rel_notes/deprecation.rst | 7 -------
>  drivers/net/octeontx2/otx2_ethdev.c  | 2 --
>  lib/librte_kni/rte_kni_common.h      | 3 ++-
>  lib/librte_mbuf/rte_mbuf.h           | 1 -
>  lib/librte_mbuf/rte_mbuf_core.h      | 5 ++---
>  lib/librte_mbuf/rte_mbuf_dyn.c       | 1 -
>  6 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/doc/guides/rel_notes/deprecation.rst
> b/doc/guides/rel_notes/deprecation.rst
> index f3258eb3f7..efb09f0c5e 100644
> --- a/doc/guides/rel_notes/deprecation.rst
> +++ b/doc/guides/rel_notes/deprecation.rst
> @@ -81,13 +81,6 @@ Deprecation Notices
>    us extending existing enum/define.
>    One solution can be using a fixed size array instead of ``.*MAX.*``
> value.
> 
> -* mbuf: Some fields will be converted to dynamic API in DPDK 20.11
> -  in order to reserve more space for the dynamic fields, as explained in
> -  `this presentation <https://www.youtube.com/watch?v=Ttl6MlhmzWY>`_.
> -  As a consequence, the layout of the ``struct rte_mbuf`` will be re-
> arranged,
> -  avoiding impact on vectorized implementation of the driver datapaths,
> -  while evaluating performance gains of a better use of the first cache
> line.
> -
>  * ethdev: The flow director API, including ``rte_eth_conf.fdir_conf``
> field,
>    and the related structures (``rte_fdir_*`` and ``rte_eth_fdir_*``),
>    will be removed in DPDK 20.11.
> diff --git a/drivers/net/octeontx2/otx2_ethdev.c
> b/drivers/net/octeontx2/otx2_ethdev.c
> index 6cebbe677d..d6e0f1dd03 100644
> --- a/drivers/net/octeontx2/otx2_ethdev.c
> +++ b/drivers/net/octeontx2/otx2_ethdev.c
> @@ -748,8 +748,6 @@ nix_tx_offload_flags(struct rte_eth_dev *eth_dev)
>  			 offsetof(struct rte_mbuf, buf_iova) + 16);
>  	RTE_BUILD_BUG_ON(offsetof(struct rte_mbuf, pkt_len) !=
>  			 offsetof(struct rte_mbuf, ol_flags) + 12);
> -	RTE_BUILD_BUG_ON(offsetof(struct rte_mbuf, tx_offload) !=
> -			 offsetof(struct rte_mbuf, pool) + 2 * sizeof(void *));
> 
>  	if (conf & DEV_TX_OFFLOAD_VLAN_INSERT ||
>  	    conf & DEV_TX_OFFLOAD_QINQ_INSERT)
> diff --git a/lib/librte_kni/rte_kni_common.h
> b/lib/librte_kni/rte_kni_common.h
> index 36d66e2ffa..ffb3182731 100644
> --- a/lib/librte_kni/rte_kni_common.h
> +++ b/lib/librte_kni/rte_kni_common.h
> @@ -84,10 +84,11 @@ struct rte_kni_mbuf {
>  	char pad2[4];
>  	uint32_t pkt_len;       /**< Total pkt len: sum of all segment
> data_len. */
>  	uint16_t data_len;      /**< Amount of data in segment buffer. */
> +	char pad3[14];
> +	void *pool;
> 
>  	/* fields on second cache line */
>  	__attribute__((__aligned__(RTE_CACHE_LINE_MIN_SIZE)))
> -	void *pool;
>  	void *next;             /**< Physical address of next mbuf in kernel.
> */
>  };
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 3190a29cce..c4c9ebfaa0 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1107,7 +1107,6 @@ rte_pktmbuf_attach_extbuf(struct rte_mbuf *m, void
> *buf_addr,
>  static inline void
>  rte_mbuf_dynfield_copy(struct rte_mbuf *mdst, const struct rte_mbuf *msrc)
>  {
> -	memcpy(&mdst->dynfield0, msrc->dynfield0, sizeof(mdst->dynfield0));
>  	memcpy(&mdst->dynfield1, msrc->dynfield1, sizeof(mdst->dynfield1));
>  }
> 
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h
> b/lib/librte_mbuf/rte_mbuf_core.h
> index debaace95a..567551deab 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -586,12 +586,11 @@ struct rte_mbuf {
> 
>  	uint16_t buf_len;         /**< Length of segment buffer. */
> 
> -	uint64_t dynfield0[1]; /**< Reserved for dynamic fields. */
> +	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
> 
>  	/* second cache line - fields only used in slow path or on TX */
>  	RTE_MARKER cacheline1 __rte_cache_min_aligned;
> 
> -	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
>  	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
> 
>  	/* fields to support TX offloads */
> @@ -645,7 +644,7 @@ struct rte_mbuf {
>  	/** Timesync flags for use with IEEE1588. */
>  	uint16_t timesync;
> 
> -	uint32_t dynfield1[7]; /**< Reserved for dynamic fields. */
> +	uint32_t dynfield1[9]; /**< Reserved for dynamic fields. */
>  } __rte_cache_aligned;
> 
>  /**
> diff --git a/lib/librte_mbuf/rte_mbuf_dyn.c
> b/lib/librte_mbuf/rte_mbuf_dyn.c
> index fd3e019a22..7d5e942bf0 100644
> --- a/lib/librte_mbuf/rte_mbuf_dyn.c
> +++ b/lib/librte_mbuf/rte_mbuf_dyn.c
> @@ -125,7 +125,6 @@ init_shared_mem(void)
>  		 * rte_mbuf_dynfield_copy().
>  		 */
>  		memset(shm, 0, sizeof(*shm));
> -		mark_free(dynfield0);
>  		mark_free(dynfield1);
> 
>  		/* init free_flags */
> --
> 2.28.0
>