RE: [PATCH v7] mbuf: optimize segment prefree

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

> From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> Sent: Saturday, 25 October 2025 12.25
> 
> > >
> > > > > > 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>
> > > > > > ---
> > > > > > v7:
> > > > > > * Go back to long names instead of numerical value in
> > > > > RTE_MBUF_DIRECT()
> > > > > >   macro.
> > > > > >   (Konstantin Ananyev)
> > > > > > * Updated static_assert() accordingly.
> > > >
> > > > [...]
> > > >
> > > > > >   *
> > > > > >   * If a mbuf embeds its own data after the rte_mbuf
> structure, this
> > > > > mbuf
> > > > > >   * can be defined as a direct mbuf.
> > > > > > - */
> > > > > > + *
> > > > > > + * Note: Macro optimized for code size.
> > > > > > + *
> > > > > > + * The plain macro would be:
> > > > > > + * \code{.c}
> > > > > > + *      #define RTE_MBUF_DIRECT(mb) \
> > > > > > + *          (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT |
> > > > > > RTE_MBUF_F_EXTERNAL)))
> > > > > > + * \endcode
> > > > > > + *
> > > > > > + * 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:
> > > > > > + * \code
> > > > > > + *      movabs rax,0x6000000000000000       // 10 bytes
> > > > > > + *      and    rax,QWORD PTR [rdi+0x18]     // 4 bytes
> > > > > > + *      sete   al                           // 3 bytes
> > > > > > + * \endcode
> > > > > > + * With this optimized macro, only 7 bytes of instructions:
> > > > > > + * \code
> > > > > > + *      test   BYTE PTR [rdi+0x1f],0x60     // 4 bytes
> > > > > > + *      sete   al                           // 3 bytes
> > > > > > + * \endcode
> > > > > > + */
> > > > > > +#ifdef __DOXYGEN__
> > > > > > +#define RTE_MBUF_DIRECT(mb) \
> > > > > > +	!(((const char *)(&(mb)->ol_flags))[MSB_OFFSET /* 7 or 0,
> > > > > depending on
> > > > > > endianness */] & \
> > > > > > +	(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 *
> > > > > > CHAR_BIT)))
> > > > > > +#else /* !__DOXYGEN__ */
> > > > > > +#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] & \
> > > > > > +	(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 *
> > > > > > CHAR_BIT)))
> > > > > > +#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) \
> > > > > > -	(!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT |
> > > > > > RTE_MBUF_F_EXTERNAL)))
> > > > > > +	!(((const char *)(&(mb)->ol_flags))[0] & \
> > > > > > +	(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 *
> > > > > > CHAR_BIT)))
> > > > > > +#endif /* RTE_BYTE_ORDER */
> > > > > > +#endif /* !__DOXYGEN__ */
> > > > > > +/* Verify the optimization above. */
> > > > > > +static_assert(((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) &
> > > > > > (UINT64_C(0xFF) << (7 * CHAR_BIT))) ==
> > > > > > +	(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL),
> > > > > > +	"(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) is not at
> > MSB");
> > > > > >
> > > > > >  /** Uninitialized or unspecified port. */
> > > > > >  #define RTE_MBUF_PORT_INVALID UINT16_MAX
> > > > > > --
> > > > >
> > > > > LGTM, thanks for refactoring.
> > > >
> > > > Thank you for reviewing, Konstantin.
> > > >
> > > > I had no preference for v7 or v6, but Bruce and Thomas preferred
> v6, so v6 was
> > > > applied.
> > >
> > > Yes, I saw Thomas email, after I sent my reply already.
> > > Looks like I was late with my vote.
> > > My preference still would be to avoid hard-coded constants in the
> code,
> > > but seems that it is just me.
> >
> > Me too I want to avoid hardcoded constants.
> > But in this case, it is very well documented,
> > and there is a trade-off with length and reading.
> 
> If we allow hard-coded constants in one place,
> then it would be harder for us to disallow them in other places.
> If we allow them everywhere - code will become a mess pretty soon.

Vector code has plenty of hardcoded constants. E.g.:
https://elixir.bootlin.com/dpdk/v25.07/source/drivers/net/intel/i40e/i40e_rxtx_vec_avx512.c#L154
So we already do allow it.

However, I think uses of numerical constants (like 0x60 here) should always be accompanied by a static_assert(), so a change of the underlying value will be caught at build time.

> Anyway, the changes are already merged, so probably not point
> to keep arguing on that subject.

I normally have a strong preference for descriptive names over numbers, but in this case, I was in doubt. A majority of reviewers voted that using 0x60 made the code easier to read, so let's stick with that.

> 
> >
> > The comment starts with
> > * The plain macro would be:
> > * \code{.c}
> > *      #define RTE_MBUF_DIRECT(mb) \
> > *          (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT |
> RTE_MBUF_F_EXTERNAL)))
> > * \endcode
> >
> > so I believe it is very clear already.
> >