Re: [dpdk-dev] [PATCH v4] lib/table: fix cache alignment issue

[David Marchand <david.marchand@redhat.com>]

On Wed, Jul 22, 2020 at 4:13 AM Ting Xu <ting.xu@intel.com> wrote:
>
> When create softnic hash table with 16 keys, it failed on 32-bit
> environment, because the pointer field in structure rte_bucket_4_16
> is only 32 bits. Add a padding field in 32-bit environment to keep
> the structure to a multiple of 64 bytes. Apply this to 8-byte and
> 32-byte key hash function as well.

Please correct me if I am wrong, but it simply means this part of the
table library never worked for 32-bit.
It seems more adding 32-bit support rather than a fix and then I
wonder if it has its place in rc3.



Now, looking at the details:

For 64-bit on my x86, we have:

struct rte_bucket_4_8 {
    uint64_t                   signature;            /*     0     8 */
    uint64_t                   lru_list;             /*     8     8 */
    struct rte_bucket_4_8 *    next;                 /*    16     8 */
    uint64_t                   next_valid;           /*    24     8 */
    uint64_t                   key[4];               /*    32    32 */
    /* --- cacheline 1 boundary (64 bytes) --- */
    uint8_t                    data[];               /*    64     0 */

    /* size: 64, cachelines: 1, members: 6 */
};


For 32-bit, we have:

struct rte_bucket_4_8 {
    uint64_t                   signature;            /*     0     8 */
    uint64_t                   lru_list;             /*     8     8 */
    struct rte_bucket_4_8 *    next;                 /*    16     4 */
    uint64_t                   next_valid;           /*    20     8 */
    uint64_t                   key[4];               /*    28    32 */
    uint8_t                    data[];               /*    60     0 */

    /* size: 60, cachelines: 1, members: 6 */
    /* last cacheline: 60 bytes */
} __attribute__((__packed__));

^^ it is interesting that a packed attribute ends up here.
I saw no such attribute in the library code.
Compiler black magic at work I guess...


>
> Fixes: 8aa327214c ("table: hash")
> Cc: stable@dpdk.org
>
> Signed-off-by: Ting Xu <ting.xu@intel.com>
>
> ---
> v3->v4: Change design based on comment
> v2->v3: Rebase
> v1->v2: Correct patch time
> ---
>  lib/librte_table/rte_table_hash_key16.c | 17 +++++++++++++++++
>  lib/librte_table/rte_table_hash_key32.c | 17 +++++++++++++++++
>  lib/librte_table/rte_table_hash_key8.c  | 16 ++++++++++++++++
>  3 files changed, 50 insertions(+)
>
> diff --git a/lib/librte_table/rte_table_hash_key16.c b/lib/librte_table/rte_table_hash_key16.c
> index 2cca1c924..c4384b114 100644
> --- a/lib/librte_table/rte_table_hash_key16.c
> +++ b/lib/librte_table/rte_table_hash_key16.c
> @@ -33,6 +33,7 @@
>
>  #endif
>
> +#ifdef RTE_ARCH_64
>  struct rte_bucket_4_16 {
>         /* Cache line 0 */
>         uint64_t signature[4 + 1];
> @@ -46,6 +47,22 @@ struct rte_bucket_4_16 {
>         /* Cache line 2 */
>         uint8_t data[0];
>  };
> +#else
> +struct rte_bucket_4_16 {
> +       /* Cache line 0 */
> +       uint64_t signature[4 + 1];
> +       uint64_t lru_list;
> +       struct rte_bucket_4_16 *next;
> +       uint32_t pad;
> +       uint64_t next_valid;
> +
> +       /* Cache line 1 */
> +       uint64_t key[4][2];
> +
> +       /* Cache line 2 */
> +       uint8_t data[0];
> +};
> +#endif

The change could simply be:

@@ -38,6 +38,9 @@ struct rte_bucket_4_16 {
        uint64_t signature[4 + 1];
        uint64_t lru_list;
        struct rte_bucket_4_16 *next;
+#ifndef RTE_ARCH_64
+       uint32_t pad;
+#endif
        uint64_t next_valid;

        /* Cache line 1 */

It avoids duplicating the whole structure definition (we could miss
updating one side of the #ifdef later).
Idem for the other "8" and "32" structures.


-- 
David Marchand