On 29/10/2020 09:27, Thomas Monjalon wrote:
> 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.
> On such system, the first half (also called first cache line) is hotter
> than the second one where the pool pointer was.
>
> 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 uint16_t priv_size; /* 80 + 2 */
> uint16_t timesync; /* 82 + 2 */
> uint32_t seqn; /* 84 + 4 */
> 11 struct rte_mbuf_ext_shared_info * shinfo; /* 88 + 8 */
> 12 uint64_t dynfield1[4]; /* 96 + 32 */
> 16 /* --- END 128 */
>
> Signed-off-by: Thomas Monjalon <tho...@monjalon.net>
> ---
> doc/guides/rel_notes/deprecation.rst | 5 -----
> lib/librte_kni/rte_kni_common.h | 3 ++-
> lib/librte_mbuf/rte_mbuf_core.h | 5 ++---
> 3 files changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/doc/guides/rel_notes/deprecation.rst
> b/doc/guides/rel_notes/deprecation.rst
> index 72dbb25b83..07ca1dcbb2 100644
> --- a/doc/guides/rel_notes/deprecation.rst
> +++ b/doc/guides/rel_notes/deprecation.rst
> @@ -88,11 +88,6 @@ Deprecation Notices
>
> - ``seqn``
>
> - 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 legacy filter API, including
> ``rte_eth_dev_filter_supported()``, ``rte_eth_dev_filter_ctrl()`` as well
> as filter types MACVLAN, ETHERTYPE, FLEXIBLE, SYN, NTUPLE, TUNNEL, FDIR,
> 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_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index 52ca1c842f..ee185fa32b 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -584,12 +584,11 @@ struct rte_mbuf {
>
> uint16_t buf_len; /**< Length of segment buffer. */
>
> - uint64_t unused;
> + 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 */
> @@ -646,7 +645,7 @@ struct rte_mbuf {
> */
> struct rte_mbuf_ext_shared_info *shinfo;
>
> - uint64_t dynfield1[3]; /**< Reserved for dynamic fields. */
> + uint64_t dynfield1[4]; /**< Reserved for dynamic fields. */
> } __rte_cache_aligned;
>
> /**
>
I will let other chime in on the merits of positioning cache alignment of the
mempool pointer.
>From the ABI PoV, depreciate notice has been observed and since mbuf effects
everything doing it outside of a ABI Breakage window is impossible, so it now or
never.
Ray K
