Re: [PATCH v10 1/4] hash: pack the hitmask for hash in bulk lookup

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

Hello Yoan,

On Wed, Jul 3, 2024 at 7:13 PM Yoan Picchi <yoan.picchi@arm.com> wrote:
>
> Current hitmask includes padding due to Intel's SIMD
> implementation detail. This patch allows non Intel SIMD
> implementations to benefit from a dense hitmask.
> In addition, the new dense hitmask interweave the primary
> and secondary matches which allow a better cache usage and
> enable future improvements for the SIMD implementations
> The default non SIMD path now use this dense mask.
>
> Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Reviewed-by: Nathan Brown <nathan.brown@arm.com>

This patch does too many things at the same time.
There is code movement and behavior modifications all mixed in.

As there was still no review from the lib maintainer... I am going a
bit more in depth this time.
Please split this patch to make it less hard to understand.

I can see the need for at least one patch for isolating the change on
sig_cmp_fn from the exposed API, then one patch for moving the code to
per arch headers with *no behavior change*, and one patch for
introducing/switching to "dense hitmask".

More comments below.


> ---
>  .mailmap                                  |   1 +
>  lib/hash/compare_signatures_arm_pvt.h     |  60 +++++++
>  lib/hash/compare_signatures_generic_pvt.h |  37 +++++
>  lib/hash/compare_signatures_x86_pvt.h     |  49 ++++++
>  lib/hash/hash_sig_cmp_func_pvt.h          |  20 +++
>  lib/hash/rte_cuckoo_hash.c                | 190 +++++++++++-----------
>  lib/hash/rte_cuckoo_hash.h                |  10 +-
>  7 files changed, 267 insertions(+), 100 deletions(-)
>  create mode 100644 lib/hash/compare_signatures_arm_pvt.h
>  create mode 100644 lib/hash/compare_signatures_generic_pvt.h
>  create mode 100644 lib/hash/compare_signatures_x86_pvt.h
>  create mode 100644 lib/hash/hash_sig_cmp_func_pvt.h
>
> diff --git a/.mailmap b/.mailmap
> index f76037213d..ec525981fe 100644
> --- a/.mailmap
> +++ b/.mailmap
> @@ -1661,6 +1661,7 @@ Yixue Wang <yixue.wang@intel.com>
>  Yi Yang <yangyi01@inspur.com> <yi.y.yang@intel.com>
>  Yi Zhang <zhang.yi75@zte.com.cn>
>  Yoann Desmouceaux <ydesmouc@cisco.com>
> +Yoan Picchi <yoan.picchi@arm.com>
>  Yogesh Jangra <yogesh.jangra@intel.com>
>  Yogev Chaimovich <yogev@cgstowernetworks.com>
>  Yongjie Gu <yongjiex.gu@intel.com>
> diff --git a/lib/hash/compare_signatures_arm_pvt.h b/lib/hash/compare_signatures_arm_pvt.h
> new file mode 100644
> index 0000000000..e83bae9912
> --- /dev/null
> +++ b/lib/hash/compare_signatures_arm_pvt.h

I guess pvt stands for private.
No need for such suffix, this header won't be exported in any case.


> @@ -0,0 +1,60 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2010-2016 Intel Corporation
> + * Copyright(c) 2018-2024 Arm Limited
> + */
> +
> +/*
> + * Arm's version uses a densely packed hitmask buffer:
> + * Every bit is in use.
> + */

Please put a header guard.

#ifndef <UPPERCASE_HEADER_NAME>_H
#define <UPPERCASE_HEADER_NAME>_H

> +
> +#include <inttypes.h>
> +#include <rte_common.h>
> +#include <rte_vect.h>
> +
> +#include "rte_cuckoo_hash.h"
> +#include "hash_sig_cmp_func_pvt.h"
> +
> +#define DENSE_HASH_BULK_LOOKUP 1
> +
> +static inline void
> +compare_signatures_dense(uint16_t *hitmask_buffer,
> +                       const uint16_t *prim_bucket_sigs,
> +                       const uint16_t *sec_bucket_sigs,
> +                       uint16_t sig,
> +                       enum rte_hash_sig_compare_function sig_cmp_fn)
> +{
> +
> +       static_assert(sizeof(*hitmask_buffer) >= 2 * (RTE_HASH_BUCKET_ENTRIES / 8),
> +               "hitmask_buffer must be wide enough to fit a dense hitmask");
> +
> +       /* For match mask every bits indicates the match */
> +       switch (sig_cmp_fn) {
> +#if RTE_HASH_BUCKET_ENTRIES <= 8
> +       case RTE_HASH_COMPARE_NEON: {
> +               uint16x8_t vmat, vsig, x;
> +               int16x8_t shift = {0, 1, 2, 3, 4, 5, 6, 7};
> +               uint16_t low, high;
> +
> +               vsig = vld1q_dup_u16((uint16_t const *)&sig);
> +               /* Compare all signatures in the primary bucket */
> +               vmat = vceqq_u16(vsig, vld1q_u16((uint16_t const *)prim_bucket_sigs));
> +               x = vshlq_u16(vandq_u16(vmat, vdupq_n_u16(0x0001)), shift);
> +               low = (uint16_t)(vaddvq_u16(x));
> +               /* Compare all signatures in the secondary bucket */
> +               vmat = vceqq_u16(vsig, vld1q_u16((uint16_t const *)sec_bucket_sigs));
> +               x = vshlq_u16(vandq_u16(vmat, vdupq_n_u16(0x0001)), shift);
> +               high = (uint16_t)(vaddvq_u16(x));
> +               *hitmask_buffer = low | high << RTE_HASH_BUCKET_ENTRIES;
> +
> +               }
> +               break;
> +#endif
> +       default:
> +               for (unsigned int i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) {
> +                       *hitmask_buffer |= (sig == prim_bucket_sigs[i]) << i;
> +                       *hitmask_buffer |=
> +                               ((sig == sec_bucket_sigs[i]) << i) << RTE_HASH_BUCKET_ENTRIES;
> +               }
> +       }
> +}

IIRC, this code is copied in all three headers.
It is a common scalar version, so the ARM code could simply call the
"generic" implementation rather than copy/paste.

[snip]

> diff --git a/lib/hash/compare_signatures_x86_pvt.h b/lib/hash/compare_signatures_x86_pvt.h
> new file mode 100644
> index 0000000000..932912ba19
> --- /dev/null
> +++ b/lib/hash/compare_signatures_x86_pvt.h
> @@ -0,0 +1,49 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2010-2016 Intel Corporation
> + * Copyright(c) 2018-2024 Arm Limited
> + */
> +
> +/*
> + * x86's version uses a sparsely packed hitmask buffer:
> + * Every other bit is padding.
> + */
> +
> +#include <inttypes.h>
> +#include <rte_common.h>
> +#include <rte_vect.h>
> +
> +#include "rte_cuckoo_hash.h"
> +#include "hash_sig_cmp_func_pvt.h"
> +
> +#define DENSE_HASH_BULK_LOOKUP 0
> +
> +static inline void
> +compare_signatures_sparse(uint32_t *prim_hash_matches, uint32_t *sec_hash_matches,
> +                       const struct rte_hash_bucket *prim_bkt,
> +                       const struct rte_hash_bucket *sec_bkt,
> +                       uint16_t sig,
> +                       enum rte_hash_sig_compare_function sig_cmp_fn)
> +{
> +       /* For match mask the first bit of every two bits indicates the match */
> +       switch (sig_cmp_fn) {
> +#if defined(__SSE2__) && RTE_HASH_BUCKET_ENTRIES <= 8

The check on RTE_HASH_BUCKET_ENTRIES <= 8 seems new.
It was not present in the previous implementation for SSE2, and this
difference is not explained.


> +       case RTE_HASH_COMPARE_SSE:
> +               /* Compare all signatures in the bucket */
> +               *prim_hash_matches = _mm_movemask_epi8(_mm_cmpeq_epi16(_mm_load_si128(
> +                       (__m128i const *)prim_bkt->sig_current), _mm_set1_epi16(sig)));
> +               /* Extract the even-index bits only */
> +               *prim_hash_matches &= 0x5555;
> +               /* Compare all signatures in the bucket */
> +               *sec_hash_matches = _mm_movemask_epi8(_mm_cmpeq_epi16(_mm_load_si128(
> +                       (__m128i const *)sec_bkt->sig_current), _mm_set1_epi16(sig)));
> +               /* Extract the even-index bits only */
> +               *sec_hash_matches &= 0x5555;
> +               break;
> +#endif /* defined(__SSE2__) */
> +       default:
> +               for (unsigned int i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) {
> +                       *prim_hash_matches |= (sig == prim_bkt->sig_current[i]) << (i << 1);
> +                       *sec_hash_matches |= (sig == sec_bkt->sig_current[i]) << (i << 1);
> +               }
> +       }
> +}
> diff --git a/lib/hash/hash_sig_cmp_func_pvt.h b/lib/hash/hash_sig_cmp_func_pvt.h
> new file mode 100644
> index 0000000000..d8d2fbffaf
> --- /dev/null
> +++ b/lib/hash/hash_sig_cmp_func_pvt.h

Please rename as compare_signatures.h or maybe a simpler option is to
move this enum declaration in rte_cuckoo_hash.c before including the
per arch headers.

> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2024 Arm Limited
> + */
> +
> +#ifndef _SIG_CMP_FUNC_H_
> +#define _SIG_CMP_FUNC_H_

If keeping a header, this guard must reflect the file name.

> +
> +/** Enum used to select the implementation of the signature comparison function to use

/* is enough, doxygen only parses public headers.


> + * eg: A system supporting SVE might want to use a NEON implementation.
> + * Those may change and are for internal use only
> + */
> +enum rte_hash_sig_compare_function {
> +       RTE_HASH_COMPARE_SCALAR = 0,
> +       RTE_HASH_COMPARE_SSE,
> +       RTE_HASH_COMPARE_NEON,
> +       RTE_HASH_COMPARE_SVE,
> +       RTE_HASH_COMPARE_NUM
> +};
> +
> +#endif

[snip]

> diff --git a/lib/hash/rte_cuckoo_hash.h b/lib/hash/rte_cuckoo_hash.h
> index a528f1d1a0..26a992419a 100644
> --- a/lib/hash/rte_cuckoo_hash.h
> +++ b/lib/hash/rte_cuckoo_hash.h
> @@ -134,14 +134,6 @@ struct rte_hash_key {
>         char key[0];
>  };
>
> -/* All different signature compare functions */
> -enum rte_hash_sig_compare_function {
> -       RTE_HASH_COMPARE_SCALAR = 0,
> -       RTE_HASH_COMPARE_SSE,
> -       RTE_HASH_COMPARE_NEON,
> -       RTE_HASH_COMPARE_NUM
> -};
> -
>  /** Bucket structure */
>  struct __rte_cache_aligned rte_hash_bucket {
>         uint16_t sig_current[RTE_HASH_BUCKET_ENTRIES];
> @@ -199,7 +191,7 @@ struct __rte_cache_aligned rte_hash {
>         /**< Custom function used to compare keys. */
>         enum cmp_jump_table_case cmp_jump_table_idx;
>         /**< Indicates which compare function to use. */
> -       enum rte_hash_sig_compare_function sig_cmp_fn;
> +       unsigned int sig_cmp_fn;

From an ABI perspective, it looks ok.
We may be breaking users that would inspect this public object, but I
think it is ok.

In any case, put this change in a separate patch so it is more visible.


>         /**< Indicates which signature compare function to use. */
>         uint32_t bucket_bitmask;
>         /**< Bitmask for getting bucket index from hash signature. */
> --
> 2.25.1
>


-- 
David Marchand