gro: optimise redundant parsing of packets

Currently the IPv6 extension headers are parsed twice: first in ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the network header, we can avoid parsing extension headers a second time in ipv6_gro_complete (which saves multiple memory dereferences and conditional checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6 packets).

The implementation had to handle both inner and outer layers in case of encapsulation (as they can't use the same field). I've applied a similar optimisation to Ethernet.

Performance tests for TCP stream over IPv6 with a varying amount of extension headers demonstrate throughput improvement of ~0.7%.

Quoted 9 lines · Richard Gobert

Currently the IPv6 extension headers are parsed twice: first in ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the network header, we can avoid parsing extension headers a second time in ipv6_gro_complete (which saves multiple memory dereferences and conditional checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6 packets).

Quoted 6 lines

The implementation had to handle both inner and outer layers in case of encapsulation (as they can't use the same field). I've applied a similar optimisation to Ethernet.

Performance tests for TCP stream over IPv6 with a varying amount of extension headers demonstrate throughput improvement of ~0.7%.

I'm surprised that the improvement is measurable: for large aggregate packets a single ipv6_exthdrs_len() call is avoided out of tens calls for the individual pkts. Additionally such figure is comparable to noise level in my tests.

This adds a couple of additional branches for the common (no extensions header) case.

while patch 1/2 could be useful, patch 2/2 overall looks not worthy to me.

I suggest to re-post for inclusion only patch 1, unless others have strong different opinions.

Cheers,

Paolo

Quoted 15 lines · Richard Gobert

Quoted 13 lines

Currently the IPv6 extension headers are parsed twice: first in ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the network header, we can avoid parsing extension headers a second time in ipv6_gro_complete (which saves multiple memory dereferences and conditional checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6 packets).

The implementation had to handle both inner and outer layers in case of encapsulation (as they can't use the same field). I've applied a similar optimisation to Ethernet.

Quoted 7 lines

Quoted 2 lines

Performance tests for TCP stream over IPv6 with a varying amount of extension headers demonstrate throughput improvement of ~0.7%.

I'm surprised that the improvement is measurable: for large aggregate packets a single ipv6_exthdrs_len() call is avoided out of tens calls for the individual pkts. Additionally such figure is comparable to noise level in my tests.

Quoted 2 lines

This adds a couple of additional branches for the common (no extensions header) case.

Quoted 6 lines

while patch 1/2 could be useful, patch 2/2 overall looks not worthy to me.

I suggest to re-post for inclusion only patch 1, unless others have strong different opinions.

+2

I have the same feeling/opinion.

Quoted 4 lines

Cheers,

Paolo

Quoted 27 lines

Quoted 25 lines · Paolo Abeni

Quoted 22 lines · Richard Gobert

Currently the IPv6 extension headers are parsed twice: first in ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the
network header, we can avoid parsing extension headers a second time in
ipv6_gro_complete (which saves multiple memory dereferences and conditional
checks inside ipv6_exthdrs_len for a varying amount of extension headers in
IPv6 packets).

The implementation had to handle both inner and outer layers in case of
encapsulation (as they can't use the same field). I've applied a similar
optimisation to Ethernet.

Performance tests for TCP stream over IPv6 with a varying amount of
extension headers demonstrate throughput improvement of ~0.7%.

Quoted 7 lines

Quoted 7 lines

I'm surprised that the improvement is measurable: for large aggregate
packets a single ipv6_exthdrs_len() call is avoided out of tens calls
for the individual pkts. Additionally such figure is comparable to
noise level in my tests.

It's not simple but I made an effort to make a quiet environment. Correct configuration allows for this kind of measurements to be made as the test is CPU bound and noise is a variance that can be reduced with
enough samples.

Environment example: (100Gbit NIC (mlx5), physical machine, i9 12th gen)

    # power-management and hyperthreading disabled in BIOS
    # sysctl preallocate net mem
    echo 0 > /sys/devices/system/cpu/cpufreq/boost # disable
turboboost
    ethtool -A enp1s0f0np0 rx off tx off autoneg off # no PAUSE
frames

    # Single core performance
    for x in /sys/devices/system/cpu/cpu[1-9]*/online; do echo 0

Quoted 1 line

"$x"; done

    ./network-testing-master/bin/netfilter_unload_modules.sh 2>/dev/null # unload netfilter
    tuned-adm profile latency-performance
    cpupower frequency-set -f 2200MHz # Set core to specific frequency
    systemctl isolate rescue-ssh.target

    # and kill all processes besides init

Quoted 3 lines

Quoted 3 lines

This adds a couple of additional branches for the common (no extensions
header) case.

The additional branch in ipv6_gro_receive would be negligible or even non-existent for a branch predictor in the common case (non-encapsulated packets).
I could wrap it with a likely macro if you wish.
Inside ipv6_gro_complete a couple of branches are saved for the common
case as demonstrated below.

original code ipv6_gro_complete (ipv6_exthdrs_len is inlined):

    // if (skb->encapsulation)

    ffffffff81c4962b: f6 87 81 00 00 00 20 testb
$0x20,0x81(%rdi)
    ffffffff81c49632: 74 2a je
ffffffff81c4965e <ipv6_gro_complete+0x3e>

    ...

    // nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);

    ffffffff81c4969c: eb 1b jmp
ffffffff81c496b9 <ipv6_gro_complete+0x99> <-- jump to beginning of
for loop

    ffffffff81c4968e:   b8 28 00 00 00          mov    $0x28,%eax
    ffffffff81c49693:   31 f6                   xor    %esi,%esi
    ffffffff81c49695:   48 c7 c7 c0 28 aa 82    mov   

$0xffffffff82aa28c0,%rdi
    ffffffff81c4969c: eb 1b jmp
ffffffff81c496b9 <ipv6_gro_complete+0x99>
    ffffffff81c4969e: f6 41 18 01 testb
$0x1,0x18(%rcx)
    ffffffff81c496a2: 74 34 je
ffffffff81c496d8 <ipv6_gro_complete+0xb8> <--- 3rd conditional
check: !((*opps)->flags & INET6_PROTO_GSO_EXTHDR)

    ffffffff81c496a4:   48 98                   cltq  
    ffffffff81c496a6:   48 01 c2                add    %rax,%rdx
    ffffffff81c496a9:   0f b6 42 01             movzbl 0x1(%rdx),%eax
    ffffffff81c496ad:   0f b6 0a                movzbl (%rdx),%ecx
    ffffffff81c496b0:   8d 04 c5 08 00 00 00    lea   
0x8(,%rax,8),%eax
    ffffffff81c496b7:   01 c6                   add    %eax,%esi
    ffffffff81c496b9:   85 c9                   test   %ecx,%ecx    

<--- for loop starts here
    ffffffff81c496bb: 74 e7 je
ffffffff81c496a4 <ipv6_gro_complete+0x84> <--- 1st conditional
check: proto != NEXTHDR_HOP
    ffffffff81c496bd: 48 8b 0c cf mov
(%rdi,%rcx,8),%rcx

    ffffffff81c496c1:   48 85 c9                test   %rcx,%rcx
    ffffffff81c496c4:   75 d8                   jne   

ffffffff81c4969e <ipv6_gro_complete+0x7e> <--- 2nd conditional
check: unlikely(!(*opps))

    ... (indirect call ops->callbacks.gro_complete)

ipv6_exthdrs_len contains a loop which has 3 conditional checks. For the common (no extensions header) case, in the new code, *all 3 branches are completely avoided*

patched code ipv6_gro_complete:

    // if (skb->encapsulation)
    ffffffff81befe58: f6 83 81 00 00 00 20 testb
$0x20,0x81(%rbx)
    ffffffff81befe5f: 74 78 je
ffffffff81befed9 <ipv6_gro_complete+0xb9>

    ...

    // else
    ffffffff81befed9: 0f b6 43 50 movzbl
0x50(%rbx),%eax
    ffffffff81befedd: 0f b7 73 4c movzwl
0x4c(%rbx),%esi
    ffffffff81befee1: 48 8b 0c c5 c0 3f a9 mov -
0x7d56c040(,%rax,8),%rcx

    ... (indirect call ops->callbacks.gro_complete)

Thus, the patch is beneficial for both the common case and the ext hdr
case. I would appreciate a second consideration :)

Quoted 143 lines

Quoted 27 lines

Quoted 25 lines · Paolo Abeni

Quoted 22 lines · Richard Gobert

Currently the IPv6 extension headers are parsed twice: first in ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the
network header, we can avoid parsing extension headers a second time in
ipv6_gro_complete (which saves multiple memory dereferences and conditional
checks inside ipv6_exthdrs_len for a varying amount of extension headers in
IPv6 packets).

The implementation had to handle both inner and outer layers in case of
encapsulation (as they can't use the same field). I've applied a similar
optimisation to Ethernet.

Performance tests for TCP stream over IPv6 with a varying amount of
extension headers demonstrate throughput improvement of ~0.7%.

Quoted 7 lines

Quoted 7 lines

I'm surprised that the improvement is measurable: for large aggregate
packets a single ipv6_exthdrs_len() call is avoided out of tens calls
for the individual pkts. Additionally such figure is comparable to
noise level in my tests.

It's not simple but I made an effort to make a quiet environment. Correct configuration allows for this kind of measurements to be made as the test is CPU bound and noise is a variance that can be reduced with
enough samples.

Environment example: (100Gbit NIC (mlx5), physical machine, i9 12th gen)

    # power-management and hyperthreading disabled in BIOS
    # sysctl preallocate net mem
    echo 0 > /sys/devices/system/cpu/cpufreq/boost # disable
turboboost
    ethtool -A enp1s0f0np0 rx off tx off autoneg off # no PAUSE
frames

# Single core performance
for x in /sys/devices/system/cpu/cpu[1-9]*/online; do echo 0

Quoted 1 line

"$x"; done

./network-testing-master/bin/netfilter_unload_modules.sh
2>/dev/null # unload netfilter
tuned-adm profile latency-performance
cpupower frequency-set -f 2200MHz # Set core to specific
frequency
systemctl isolate rescue-ssh.target
# and kill all processes besides init

Quoted 3 lines

Quoted 3 lines

This adds a couple of additional branches for the common (no extensions
header) case.

The additional branch in ipv6_gro_receive would be negligible or even non-existent for a branch predictor in the common case (non-encapsulated packets).
I could wrap it with a likely macro if you wish.
Inside ipv6_gro_complete a couple of branches are saved for the common
case as demonstrated below.

original code ipv6_gro_complete (ipv6_exthdrs_len is inlined):

    // if (skb->encapsulation)

    ffffffff81c4962b:	f6 87 81 00 00 00 20 	testb 
$0x20,0x81(%rdi)
    ffffffff81c49632:	74 2a                	je    

ffffffff81c4965e <ipv6_gro_complete+0x3e>

    ...

    // nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);

    ffffffff81c4969c:	eb 1b                	jmp   
ffffffff81c496b9 <ipv6_gro_complete+0x99>    <-- jump to beginning of
for loop
    ffffffff81c4968e:   b8 28 00 00 00          mov    $0x28,%eax
    ffffffff81c49693:   31 f6                   xor    %esi,%esi
    ffffffff81c49695:   48 c7 c7 c0 28 aa 82    mov   
$0xffffffff82aa28c0,%rdi
    ffffffff81c4969c:   eb 1b                   jmp   
ffffffff81c496b9 <ipv6_gro_complete+0x99>
    ffffffff81c4969e:   f6 41 18 01             testb 

$0x1,0x18(%rcx)
ffffffff81c496a2: 74 34 je
ffffffff81c496d8 <ipv6_gro_complete+0xb8> <--- 3rd conditional
check: !((*opps)->flags & INET6_PROTO_GSO_EXTHDR)

    ffffffff81c496a4:   48 98                   cltq  
    ffffffff81c496a6:   48 01 c2                add    %rax,%rdx
    ffffffff81c496a9:   0f b6 42 01             movzbl 0x1(%rdx),%eax
    ffffffff81c496ad:   0f b6 0a                movzbl (%rdx),%ecx
    ffffffff81c496b0:   8d 04 c5 08 00 00 00    lea   
0x8(,%rax,8),%eax
    ffffffff81c496b7:   01 c6                   add    %eax,%esi
    ffffffff81c496b9:   85 c9                   test   %ecx,%ecx    

<--- for loop starts here
ffffffff81c496bb: 74 e7 je
ffffffff81c496a4 <ipv6_gro_complete+0x84> <--- 1st conditional
check: proto != NEXTHDR_HOP
ffffffff81c496bd: 48 8b 0c cf mov
(%rdi,%rcx,8),%rcx

    ffffffff81c496c1:   48 85 c9                test   %rcx,%rcx
    ffffffff81c496c4:   75 d8                   jne   

ffffffff81c4969e <ipv6_gro_complete+0x7e> <--- 2nd conditional
check: unlikely(!(*opps))

    ... (indirect call ops->callbacks.gro_complete)

ipv6_exthdrs_len contains a loop which has 3 conditional checks. For the common (no extensions header) case, in the new code, *all 3 branches are completely avoided*

patched code ipv6_gro_complete:

    // if (skb->encapsulation)
    ffffffff81befe58:   f6 83 81 00 00 00 20    testb 

$0x20,0x81(%rbx)
ffffffff81befe5f: 74 78 je
ffffffff81befed9 <ipv6_gro_complete+0xb9>

    ...
    
    // else
    ffffffff81befed9:	0f b6 43 50          	movzbl
0x50(%rbx),%eax
    ffffffff81befedd:	0f b7 73 4c          	movzwl
0x4c(%rbx),%esi
    ffffffff81befee1:	48 8b 0c c5 c0 3f a9 	mov    -

Quoted 7 lines

0x7d56c040(,%rax,8),%rcx

    ... (indirect call ops->callbacks.gro_complete)

Thus, the patch is beneficial for both the common case and the ext hdr
case. I would appreciate a second consideration :)

A problem with the above analysis is that it does not take in consideration the places where the new branch are added:
eth_gro_receive() and ipv6_gro_receive().

Note that such functions are called for each packet on the wire:
multiple times for each aggregate packets.

The above is likely not measurable in terms on pps delta, but the added CPU cycles spent for the common case are definitely there. In my opinion that outlast the benefit for the extensions header case.

Cheers,

Paolo

p.s. please refrain from off-list ping. That is ignored by most and considered rude by some.