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From: Bruce Richardson <bruce.richardson@intel.com>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: Konstantin Ananyev <konstantin.ananyev@huawei.com>,
	<dev@dpdk.org>, Stephen Hemminger <stephen@networkplumber.org>,
	Wathsala Vithanage <wathsala.vithanage@arm.com>,
	Fengchengwen <fengchengwen@huawei.com>
Subject: Re: [PATCH v5] mbuf: optimize segment prefree
Date: Thu, 23 Oct 2025 17:03:46 +0100	[thread overview]
Message-ID: <aPpR4jZtBvDM42pL@bricha3-mobl1.ger.corp.intel.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35F654FE@smartserver.smartshare.dk>

On Thu, Oct 23, 2025 at 05:46:49PM +0200, Morten Brørup wrote:
> > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > Sent: Thursday, 23 October 2025 17.27
> > 
> > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > > > Sent: Thursday, 23 October 2025 16.05
> > > >
> > > > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > > > > > Sent: Thursday, 23 October 2025 10.51
> > > > > >
> > > > > > > -#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.
> > > > >
> > > > > GCC is too stupid for that too.
> > > > >
> > > > > Playing around with Godbolt shows that
> > > > > 	return !((char)(p[3] >> 56) & 0x60);
> > > > > becomes
> > > > > 	movzx  eax,BYTE PTR [rdi+0x1f]	// 4 bytes
> > > > > 	test   al,0x60				// 2 bytes
> > > > > Instead of simply
> > > > > 	test   BYTE PTR [rdi+0x1f],0x60	// 4 bytes
> > > >
> > > > And these 2 extra bytes in instructions, are that really that
> > critical?
> > > > My guess, we wouldn't notice any real diff here.
> > >
> > > The optimized macro made the common code path of the refactored
> > > rte_pktmbuf_prefree_seg() fit into one cache line.
> > > IIRC, all 10 bytes saving were required for this.
> > 
> > I understand that. but is that change will provide a measurable impact,
> > in terms of cycles/op or pps or so?
> 
> L1 instruction cache is important; reducing code size of a per-packet function might have an effect in some cases.
> I don't have other metrics than code size for this optimization.
> 
> I just tested to see if I recalled correctly, and here's the generated code with the two different macros:
> 
> #define RTE_MBUF_DIRECT(mb)     !(((const char *)(&(mb)->ol_flags))[7] & 0x60)
> 
> 0000000000000670 <review_rte_pktmbuf_prefree_seg>:
>      670:       f3 0f 1e fa             endbr64
>      674:       41 57                   push   %r15
>      676:       41 56                   push   %r14
>      678:       41 55                   push   %r13
>      67a:       41 54                   push   %r12
>      67c:       55                      push   %rbp
>      67d:       53                      push   %rbx
>      67e:       48 89 fb                mov    %rdi,%rbx
>      681:       48 83 ec 18             sub    $0x18,%rsp
>      685:       64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
>      68c:       00 00
>      68e:       48 89 44 24 08          mov    %rax,0x8(%rsp)
>      693:       31 c0                   xor    %eax,%eax
>      695:       0f b7 6f 12             movzwl 0x12(%rdi),%ebp
>      699:       66 83 fd 01             cmp    $0x1,%bp
>      69d:       75 51                   jne    6f0 <review_rte_pktmbuf_prefree_seg+0x80>
> ** Look here {
>      69f:       f6 47 1f 60             testb  $0x60,0x1f(%rdi)
>      6a3:       75 6b                   jne    710 <review_rte_pktmbuf_prefree_seg+0xa0>
> ** }
>      6a5:       66 83 7b 14 01          cmpw   $0x1,0x14(%rbx)
>      6aa:       74 09                   je     6b5 <review_rte_pktmbuf_prefree_seg+0x45>
>      6ac:       b8 01 00 00 00          mov    $0x1,%eax
>      6b1:       66 89 43 14             mov    %ax,0x14(%rbx)
>      6b5:       48 83 7b 40 00          cmpq   $0x0,0x40(%rbx)
>      6ba:       74 08                   je     6c4 <review_rte_pktmbuf_prefree_seg+0x54>
>      6bc:       48 c7 43 40 00 00 00    movq   $0x0,0x40(%rbx)
>      6c3:       00
>      6c4:       48 89 d8                mov    %rbx,%rax
>      6c7:       48 8b 54 24 08          mov    0x8(%rsp),%rdx
>      6cc:       64 48 2b 14 25 28 00    sub    %fs:0x28,%rdx
>      6d3:       00 00
>      6d5:       0f 85 3a 02 00 00       jne    915 <review_rte_pktmbuf_prefree_seg+0x2a5>
>      6db:       48 83 c4 18             add    $0x18,%rsp
>      6df:       5b                      pop    %rbx
>      6e0:       5d                      pop    %rbp
>      6e1:       41 5c                   pop    %r12
>      6e3:       41 5d                   pop    %r13
>      6e5:       41 5e                   pop    %r14
>      6e7:       41 5f                   pop    %r15
>      6e9:       c3                      ret
> 
> #define RTE_MBUF_DIRECT(mb) \
> 	!((char)((mb)->ol_flags >> (7 * CHAR_BIT)) & \
> 	(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * CHAR_BIT)))
> 
> 0000000000000690 <review_rte_pktmbuf_prefree_seg>:
>      690:       f3 0f 1e fa             endbr64
>      694:       41 57                   push   %r15
>      696:       41 56                   push   %r14
>      698:       41 55                   push   %r13
>      69a:       41 54                   push   %r12
>      69c:       55                      push   %rbp
>      69d:       53                      push   %rbx
>      69e:       48 89 fb                mov    %rdi,%rbx
>      6a1:       48 83 ec 18             sub    $0x18,%rsp
>      6a5:       64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
>      6ac:       00 00
>      6ae:       48 89 44 24 08          mov    %rax,0x8(%rsp)
>      6b3:       31 c0                   xor    %eax,%eax
>      6b5:       0f b7 6f 12             movzwl 0x12(%rdi),%ebp
>      6b9:       66 83 fd 01             cmp    $0x1,%bp
>      6bd:       75 59                   jne    718 <review_rte_pktmbuf_prefree_seg+0x88>
> ** Look here {
>      6bf:       48 8b 47 18             mov    0x18(%rdi),%rax
>      6c3:       48 89 c2                mov    %rax,%rdx
>      6c6:       48 c1 ea 38             shr    $0x38,%rdx
>      6ca:       83 e2 60                and    $0x60,%edx
>      6cd:       75 71                   jne    740 <review_rte_pktmbuf_prefree_seg+0xb0>
> * }
>      6cf:       66 83 7b 14 01          cmpw   $0x1,0x14(%rbx)
>      6d4:       74 09                   je     6df <review_rte_pktmbuf_prefree_seg+0x4f>
>      6d6:       b8 01 00 00 00          mov    $0x1,%eax
>      6db:       66 89 43 14             mov    %ax,0x14(%rbx)
>      6df:       48 83 7b 40 00          cmpq   $0x0,0x40(%rbx)
>      6e4:       74 08                   je     6ee <review_rte_pktmbuf_prefree_seg+0x5e>
>      6e6:       48 c7 43 40 00 00 00    movq   $0x0,0x40(%rbx)
>      6ed:       00
>      6ee:       48 89 d8                mov    %rbx,%rax
>      6f1:       48 8b 54 24 08          mov    0x8(%rsp),%rdx
>      6f6:       64 48 2b 14 25 28 00    sub    %fs:0x28,%rdx
>      6fd:       00 00
>      6ff:       0f 85 50 02 00 00       jne    955 <review_rte_pktmbuf_prefree_seg+0x2c5>
>      705:       48 83 c4 18             add    $0x18,%rsp
>      709:       5b                      pop    %rbx
>      70a:       5d                      pop    %rbp
>      70b:       41 5c                   pop    %r12
>      70d:       41 5d                   pop    %r13
>      70f:       41 5e                   pop    %r14
>      711:       41 5f                   pop    %r15
>      713:       c3                      ret
> 
> > 
> > > > But if it really is, can I ask you to create a new define for 0x60,
> > > > to avoid hardcoded constants in the code?
> > > > Might be something  like
> > > > #define RTE_MBUF_F_INDIRECT_EXTERNAL_1B ...
> > > > or so.
> > >
> > > I started out using the field names, but Bruce suggested using 0x60
> > for
> > > readability, making the macros shorter, which IMO looks good.
> > >
> > > I don't like adding special names just for this, so either we stick
> > with 0x60 or go for
> > > "(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 *
> > > CHAR_BIT))", something like this:
> > 
> > My vote would be to use the construction above.
> > Might be put it in a new macro for readability.
> > Konstantin
> 
> The optimization requires casting ol_flags as a byte array and then reading the MSB; otherwise GCC and some other compilers are too stupid to perform the optimization.
> So, should I post a v7 with the code proposed below (to get rid of the 0x60 numerical value)?
> 
While I'm not going to massively complain about removing it, I think using
the numeric value is absolutely fine because we check its validity using a
static_assert, which also serves to document where the constant comes from.

/Bruce

  reply	other threads:[~2025-10-23 16:04 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-08-27 21:35 [PATCH] " Morten Brørup
2025-08-27 23:17 ` Stephen Hemminger
2025-10-06 17:46   ` Wathsala Vithanage
2025-10-06 18:26     ` Morten Brørup
2025-10-06 14:49 ` Morten Brørup
2025-10-20 12:02 ` [PATCH v2] " Morten Brørup
2025-10-20 14:24   ` Konstantin Ananyev
2025-10-21  8:38   ` fengchengwen
2025-10-22  9:08   ` Bruce Richardson
2025-10-22 13:53     ` Morten Brørup
2025-10-22 14:12       ` Bruce Richardson
2025-10-22 14:14         ` Morten Brørup
2025-10-22 13:23 ` [PATCH v3] " Morten Brørup
2025-10-22 14:47 ` [PATCH v4] " Morten Brørup
2025-10-22 15:02   ` Bruce Richardson
2025-10-22 18:28     ` Morten Brørup
2025-10-23  7:04     ` Morten Brørup
2025-10-23  8:01 ` [PATCH v5] " Morten Brørup
2025-10-23  8:08   ` Bruce Richardson
2025-10-23  8:51   ` Konstantin Ananyev
2025-10-23 11:17     ` Morten Brørup
2025-10-23 14:04       ` Konstantin Ananyev
2025-10-23 14:48         ` Morten Brørup
2025-10-23 15:27           ` Konstantin Ananyev
2025-10-23 15:46             ` Morten Brørup
2025-10-23 16:03               ` Bruce Richardson [this message]
2025-10-23 16:24                 ` Morten Brørup
2025-10-23 12:48 ` [PATCH v6] " Morten Brørup
2025-10-23 17:13   ` Thomas Monjalon
2025-10-23 16:18 ` [PATCH v7] " Morten Brørup

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