From: Eric Dumazet <eric.dumazet@gmail.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
Eric Dumazet <eric.dumazet@gmail.com>
Cc: Alexander Lobakin <alobakin@pm.me>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
Paolo Abeni <pabeni@redhat.com>,
Network Development <netdev@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO
Date: Mon, 9 Nov 2020 19:54:02 +0100 [thread overview]
Message-ID: <3729478a-77b5-aeb5-0192-49f0e0d170ac@gmail.com> (raw)
In-Reply-To: <CA+FuTSc0QLM4QpinZ1XiLreRECBDVbanwoFtMhnF6caEWjXTBw@mail.gmail.com>
On 11/9/20 7:28 PM, Willem de Bruijn wrote:
> On Mon, Nov 9, 2020 at 12:37 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>
>>
>>
>> On 11/9/20 5:56 PM, Alexander Lobakin wrote:
>>> While testing UDP GSO fraglists forwarding through driver that uses
>>> Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order
>>> iperf packets:
>>>
>>> [ ID] Interval Transfer Bitrate Jitter
>>> [SUM] 0.0-40.0 sec 12106 datagrams received out-of-order
>>>
>>> Simple switch to napi_gro_receive() any other method without frag0
>>> shortcut completely resolved them.
>>>
>>> I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive()
>>> callback. While it's probably OK for non-frag0 paths (when all
>>> headers or even the entire frame are already in skb->data), this
>>> inline points to junk when using Fast GRO (napi_gro_frags() or
>>> napi_gro_receive() with only Ethernet header in skb->data and all
>>> the rest in shinfo->frags) and breaks GRO packet compilation and
>>> the packet flow itself.
>>> To support both modes, skb_gro_header_fast() + skb_gro_header_slow()
>>> are typically used. UDP even has an inline helper that makes use of
>>> them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr()
>>> to get rid of the out-of-order delivers.
>>>
>>> Present since the introduction of plain UDP GRO in 5.0-rc1.
>>>
>>> Since v1 [1]:
>>> - added a NULL pointer check for "uh" as suggested by Willem.
>>>
>>> [1] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@cp4-web-034.plabs.ch
>>>
>>> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
>>> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
>>> ---
>>> net/ipv4/udp_offload.c | 7 ++++++-
>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
>>> index e67a66fbf27b..7f6bd221880a 100644
>>> --- a/net/ipv4/udp_offload.c
>>> +++ b/net/ipv4/udp_offload.c
>>> @@ -366,13 +366,18 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
>>> static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
>>> struct sk_buff *skb)
>>> {
>>> - struct udphdr *uh = udp_hdr(skb);
>>> + struct udphdr *uh = udp_gro_udphdr(skb);
>>> struct sk_buff *pp = NULL;
>>> struct udphdr *uh2;
>>> struct sk_buff *p;
>>> unsigned int ulen;
>>> int ret = 0;
>>>
>>> + if (unlikely(!uh)) {
>>
>> How uh could be NULL here ?
>>
>> My understanding is that udp_gro_receive() is called
>> only after udp4_gro_receive() or udp6_gro_receive()
>> validated that udp_gro_udphdr(skb) was not NULL.
>
> Oh indeed. This has already been checked before.
>
>>> + NAPI_GRO_CB(skb)->flush = 1;
>>> + return NULL;
>>> + }
>>> +
>>> /* requires non zero csum, for symmetry with GSO */
>>> if (!uh->check) {
>>> NAPI_GRO_CB(skb)->flush = 1;
>>>
>>
>> Why uh2 is left unchanged ?
>>
>> uh2 = udp_hdr(p);
>
> Isn't that the same as th2 = tcp_hdr(p) in tcp_gro_receive? no frag0
> optimization to worry about for packets on the list.
My feeling was that tcp_gro_receive() is terminal in the GRO stack.
While UDP could be encapsulated in UDP :)
I guess we do not support this yet.
Years ago we made sure to propagate the current header offset into GRO stack
(when we added SIT/IPIP/GRE support to GRO)
299603e8370a93dd5d8e8d800f0dff1ce2c53d36 ("net-gro: Prepare GRO stack for the upcoming tunneling support")
udp_hdr() is using transport header, which is unique for skb "on the list"
next prev parent reply other threads:[~2020-11-09 18:54 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-09 16:56 [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO Alexander Lobakin
2020-11-09 17:08 ` Willem de Bruijn
2020-11-09 17:37 ` Eric Dumazet
2020-11-09 18:26 ` Alexander Lobakin
2020-11-09 19:29 ` Eric Dumazet
2020-11-09 19:56 ` Alexander Lobakin
2020-11-09 18:28 ` Willem de Bruijn
2020-11-09 18:54 ` Eric Dumazet [this message]
2020-11-09 19:06 ` Willem de Bruijn
2020-11-09 19:48 ` Alexander Lobakin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=3729478a-77b5-aeb5-0192-49f0e0d170ac@gmail.com \
--to=eric.dumazet@gmail.com \
--cc=alobakin@pm.me \
--cc=davem@davemloft.net \
--cc=kuba@kernel.org \
--cc=kuznet@ms2.inr.ac.ru \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=pabeni@redhat.com \
--cc=willemdebruijn.kernel@gmail.com \
--cc=yoshfuji@linux-ipv6.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).