RE: [PATCH v5] mbuf: optimize segment prefree

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> 
> Refactored rte_pktmbuf_prefree_seg() for both performance and readability.
> 
> With the optimized RTE_MBUF_DIRECT() macro, the common likely code path
> now fits within one instruction cache line on x86-64 when built with GCC.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> Acked-by: Chengwen Feng <fengchengwen@huawei.com>
> Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
> v5:
> * Removed the plain RTE_MBUF_DIRECT() macro, and only use the optimized
>   variant. (Bruce Richardson)
>   Further testing on Godbolt confirmed that other compilers benefit from
>   the optimized macro too.
> * Shortened the description of the RTE_MBUF_DIRECT() macro, and only
>   provide one example of code emitted by a compiler. (Bruce Richardson)
> * Consolidated the static_assert() into one, covering both little and big
>   endian.
>   This also reduces the amount of endian-conditional source code and
>   improves readability.
>   (Bruce Richardson)
> * Added comment about MSB meaning "most significant byte".
> v4:
> * Enabled the optimized RTE_MBUF_DIRECT() macro for GCC on all
>   architectures.
> v3:
> * Rewrote the optimized RTE_MBUF_DIRECT() macro for readability; use
>   numerical value instead of long names. (Bruce Richardson)
> * Enabled the optimized RTE_MBUF_DIRECT() macro for Loongarch too.
> v2:
> * Fixed typo in commit description.
> * Fixed indentation.
> * Added detailed description to the optimized RTE_MBUF_DIRECT() macro.
>   (Stephen Hemminger)
> * Added static_assert() to verify that the optimized RTE_MBUF_DIRECT()
>   macro is valid, specifically that the tested bits are in the MSB of the
>   64-bit field.
> ---
>  lib/mbuf/rte_mbuf.h      | 51 +++++++++++++++-------------------------
>  lib/mbuf/rte_mbuf_core.h | 33 +++++++++++++++++++++++---
>  2 files changed, 49 insertions(+), 35 deletions(-)
> 
> diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> index 3df22125de..2004391f57 100644
> --- a/lib/mbuf/rte_mbuf.h
> +++ b/lib/mbuf/rte_mbuf.h
> @@ -31,6 +31,7 @@
>   * http://www.kohala.com/start/tcpipiv2.html
>   */
> 
> +#include <stdbool.h>
>  #include <stdint.h>
> 
>  #include <rte_common.h>
> @@ -1458,44 +1459,30 @@ static inline int
> __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
>  static __rte_always_inline struct rte_mbuf *
>  rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>  {
> -	__rte_mbuf_sanity_check(m, 0);
> -
> -	if (likely(rte_mbuf_refcnt_read(m) == 1)) {
> -
> -		if (!RTE_MBUF_DIRECT(m)) {
> -			rte_pktmbuf_detach(m);
> -			if (RTE_MBUF_HAS_EXTBUF(m) &&
> -			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> -			    __rte_pktmbuf_pinned_extbuf_decref(m))
> -				return NULL;
> -		}
> -
> -		if (m->next != NULL)
> -			m->next = NULL;
> -		if (m->nb_segs != 1)
> -			m->nb_segs = 1;
> +	bool refcnt_not_one;
> 
> -		return m;
> +	__rte_mbuf_sanity_check(m, 0);
> 
> -	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
> +	refcnt_not_one = unlikely(rte_mbuf_refcnt_read(m) != 1);
> +	if (refcnt_not_one && __rte_mbuf_refcnt_update(m, -1) != 0)
> +		return NULL;
> 
> -		if (!RTE_MBUF_DIRECT(m)) {
> -			rte_pktmbuf_detach(m);
> -			if (RTE_MBUF_HAS_EXTBUF(m) &&
> -			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> -			    __rte_pktmbuf_pinned_extbuf_decref(m))
> -				return NULL;
> -		}
> +	if (unlikely(!RTE_MBUF_DIRECT(m))) {
> +		rte_pktmbuf_detach(m);
> +		if (RTE_MBUF_HAS_EXTBUF(m) &&
> +				RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> +				__rte_pktmbuf_pinned_extbuf_decref(m))
> +			return NULL;
> +	}
> 
> -		if (m->next != NULL)
> -			m->next = NULL;
> -		if (m->nb_segs != 1)
> -			m->nb_segs = 1;
> +	if (refcnt_not_one)
>  		rte_mbuf_refcnt_set(m, 1);
> +	if (m->nb_segs != 1)
> +		m->nb_segs = 1;
> +	if (m->next != NULL)
> +		m->next = NULL;
> 
> -		return m;
> -	}
> -	return NULL;
> +	return m;
>  }
> 
>  /**
> diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h
> index a0df265b5d..1dfeab0511 100644
> --- a/lib/mbuf/rte_mbuf_core.h
> +++ b/lib/mbuf/rte_mbuf_core.h
> @@ -711,9 +711,36 @@ struct rte_mbuf_ext_shared_info {
>   *
>   * If a mbuf embeds its own data after the rte_mbuf structure, this mbuf
>   * can be defined as a direct mbuf.
> - */
> -#define RTE_MBUF_DIRECT(mb) \
> -	(!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL)))
> + *
> + * Note: Macro optimized for code size.
> + *
> + * The plain macro would be:
> + *      #define RTE_MBUF_DIRECT(mb) \
> + *          (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL)))
> + *
> + * The flags RTE_MBUF_F_INDIRECT and RTE_MBUF_F_EXTERNAL are both in the
> MSB (most significant
> + * byte) of the 64-bit ol_flags field, so we only compare this one byte instead of all
> 64 bits.
> + *
> + * E.g., GCC version 16.0.0 20251019 (experimental) generates the following code
> for x86-64.
> + *
> + * With the plain macro, 17 bytes of instructions:
> + *      movabs rax,0x6000000000000000       // 10 bytes
> + *      and    rax,QWORD PTR [rdi+0x18]     // 4 bytes
> + *      sete   al                           // 3 bytes
> + * With this optimized macro, only 7 bytes of instructions:
> + *      test   BYTE PTR [rdi+0x1f],0x60     // 4 bytes
> + *      sete   al                           // 3 bytes
> + */
> +#if RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN
> +/* On little endian architecture, the MSB of a 64-bit integer is at byte offset 7. */
> +#define RTE_MBUF_DIRECT(mb)     !(((const char *)(&(mb)->ol_flags))[7] & 0x60)
> +#elif RTE_BYTE_ORDER == RTE_BIG_ENDIAN
> +/* On big endian architecture, the MSB of a 64-bit integer is at byte offset 0. */
> +#define RTE_MBUF_DIRECT(mb)     !(((const char *)(&(mb)->ol_flags))[0] & 0x60)

A stupid q: why then not simply do:
(mb->ol_flags >> 56) & 0x60
then?
Should help to all these LE/BE casts, etc.

> +#endif
> +/* Verify the optimization above. */
> +static_assert((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) ==
> UINT64_C(0x60) << (7 * CHAR_BIT),
> +	"(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) is not 0x60 at MSB");
> 
>  /** Uninitialized or unspecified port. */
>  #define RTE_MBUF_PORT_INVALID UINT16_MAX
> --
> 2.43.0