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Date: Thu, 2 Dec 2021 15:10:40 +0200
From: Vladimir Oltean <olteanv@...il.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()
Hello,
On Wed, Nov 24, 2021 at 12:24:46PM -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Remove one pair of add/adc instructions and their dependency
> against carry flag.
>
> We can leverage third argument to csum_partial():
>
> X = csum_block_sub(X, csum_partial(start, len, 0), 0);
>
> -->
>
> X = csum_block_add(X, ~csum_partial(start, len, 0), 0);
>
> -->
>
> X = ~csum_partial(start, len, ~X);
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
> include/linux/skbuff.h | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index eba256af64a577b458998845f2dc01a5ec80745a..eae4bd3237a41cc1b60b44c91fbfe21dfdd8f117 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3485,7 +3485,11 @@ __skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int len,
> static inline void skb_postpull_rcsum(struct sk_buff *skb,
> const void *start, unsigned int len)
> {
> - __skb_postpull_rcsum(skb, start, len, 0);
> + if (skb->ip_summed == CHECKSUM_COMPLETE)
> + skb->csum = ~csum_partial(start, len, ~skb->csum);
> + else if (skb->ip_summed == CHECKSUM_PARTIAL &&
> + skb_checksum_start_offset(skb) < 0)
> + skb->ip_summed = CHECKSUM_NONE;
> }
>
> static __always_inline void
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>
I am seeing some errors after this patch, and I am not able to
understand why. Specifically, __skb_gro_checksum_complete() hits this
condition:
wsum = skb_checksum(skb, skb_gro_offset(skb), skb_gro_len(skb), 0);
/* NAPI_GRO_CB(skb)->csum holds pseudo checksum */
sum = csum_fold(csum_add(NAPI_GRO_CB(skb)->csum, wsum));
/* See comments in __skb_checksum_complete(). */
if (likely(!sum)) {
if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
!skb->csum_complete_sw)
netdev_rx_csum_fault(skb->dev, skb);
}
To test, I am using a DSA switch network interface with an IPv4 address
and pinging through it.
The idea is as follows: an enetc port is attached to a switch, and that
switch places a frame header before the Ethernet header.
The enetc calculates the L2 payload (actually what it perceives as L2
payload, since it has no insight into the switch header format) checksum
and puts it in skb->csum:
https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/freescale/enetc/enetc.c#L726
Then, the switch driver packet type handler is invoked, and this strips
that header and recalculates the checksum (then it changes skb->dev and
this is how pinging is done through the DSA interface, but that is
irrelevant).
https://elixir.bootlin.com/linux/latest/source/net/dsa/tag_ocelot.c#L56
There seems to be a disparity when the skb->csum is calculated by
skb_postpull_rcsum as zero. Before, it was calculated as 0xffff.
Below is a dump added by me in skb_postpull_rcsum when the checksums
calculated through both methods differ. I've kept the old implementation
inside skb->csum and this is what skb_dump() sees:
[ 99.891524] skb csum of 20 bytes (20 to left of skb->data) using method 1: 0x0 method 2 0xffffffff, orig 0xf470
[ 99.901701] skb len=84 headroom=98 headlen=84 tailroom=1546
[ 99.901701] mac=(84,-6) net=(78,0) trans=78
[ 99.901701] shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
[ 99.901701] csum(0xffffffff ip_summed=2 complete_sw=0 valid=0 level=0)
[ 99.901701] hash(0x0 sw=0 l4=0) proto=0x00f8 pkttype=3 iif=7
[ 99.929916] dev name=eno2 feat=0x0x00020100001149a9
[ 99.934831] skb headroom: 00000000: 00 c0 b1 a4 ff ff 00 00 00 e0 b1 a4 ff ff 00 00
[ 99.942533] skb headroom: 00000010: 00 6f 5b 02 f8 14 ff ff 40 62 5b 02 f8 14 ff ff
[ 99.950232] skb headroom: 00000020: 21 6f 5b 02 f8 14 ff ff 00 00 00 00 00 00 00 00
[ 99.957931] skb headroom: 00000030: 00 00 00 00 00 00 00 00 00 40 00 00 00 00 00 00
[ 99.965631] skb headroom: 00000040: 88 80 00 0a 00 33 9d 40 f0 41 01 80 00 00 08 0f
[ 99.973330] skb headroom: 00000050: 00 10 00 00 00 04 9f 05 f6 28 ba ae e4 b6 2c 3d
[ 99.981028] skb headroom: 00000060: 08 00
[ 99.985062] skb linear: 00000000: 45 00 00 54 27 ac 00 00 40 01 09 a8 c0 a8 64 03
[ 99.992762] skb linear: 00000010: c0 a8 64 01 00 00 10 e6 01 5c 00 04 49 30 a7 61
[ 100.000462] skb linear: 00000020: 00 00 00 00 3d 55 01 00 00 00 00 00 10 11 12 13
[ 100.008162] skb linear: 00000030: 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23
[ 100.015862] skb linear: 00000040: 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33
[ 100.023561] skb linear: 00000050: 34 35 36 37
And below is the same output as above, but annotated by me with some comments:
[ 99.891524] skb csum of 20 bytes (20 to left of skb->data) using method 1: 0x0 method 2 0xffffffff, orig 0xf470
[ 99.901701] skb len=84 headroom=98 headlen=84 tailroom=1546
[ 99.901701] mac=(84,-6) net=(78,0) trans=78
^^^^^^^^^^^^^^^^^^^^^^
since the print is done from ocelot_rcv, the network and
transport headers haven't yet been updated
[ 99.901701] shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
[ 99.901701] csum(0xffffffff ip_summed=2 complete_sw=0 valid=0 level=0)
[ 99.901701] hash(0x0 sw=0 l4=0) proto=0x00f8 pkttype=3 iif=7
[ 99.929916] dev name=eno2 feat=0x0x00020100001149a9
[ 99.934831] skb headroom: 00000000: 00 c0 b1 a4 ff ff 00 00 00 e0 b1 a4 ff ff 00 00
[ 99.942533] skb headroom: 00000010: 00 6f 5b 02 f8 14 ff ff 40 62 5b 02 f8 14 ff ff
[ 99.950232] skb headroom: 00000020: 21 6f 5b 02 f8 14 ff ff 00 00 00 00 00 00 00 00
[ 99.957931] skb headroom: 00000030: 00 00 00 00 00 00 00 00 00 40 00 00 00 00 00 00
here is where the enetc saw the the "start" variable (old skb->data)
beginning of the frame points here
v v
[ 99.965631] skb headroom: 00000040: 88 80 00 0a 00 33 9d 40 f0 41 01 80 00 00 08 0f
OCELOT_TAG_LEN bytes
later is the real MAC header
v
[ 99.973330] skb headroom: 00000050: 00 10 00 00 00 04 9f 05 f6 28 ba ae e4 b6 2c 3d
[ 99.981028] skb headroom: 00000060: 08 00
^
the skb_postpull_rcsum is called from "start"
until the first byte prior to this one
[ 99.985062] skb linear: 00000000: 45 00 00 54 27 ac 00 00 40 01 09 a8 c0 a8 64 03
[ 99.992762] skb linear: 00000010: c0 a8 64 01 00 00 10 e6 01 5c 00 04 49 30 a7 61
[ 100.000462] skb linear: 00000020: 00 00 00 00 3d 55 01 00 00 00 00 00 10 11 12 13
[ 100.008162] skb linear: 00000030: 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23
[ 100.015862] skb linear: 00000040: 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33
[ 100.023561] skb linear: 00000050: 34 35 36 37
Do you have some suggestions as to what may be wrong? Thanks.
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