From: Jon Maloy <jmaloy@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: passt-dev@passt.top, sbrivio@redhat.com, lvivier@redhat.com,
dgibson@redhat.com
Subject: Re: [PATCH 1/2] tcp: leverage support of SO_PEEK_OFF socket option when available
Date: Thu, 25 Apr 2024 19:23:28 -0400 [thread overview]
Message-ID: <72322857-eb15-26c8-1713-d999acd05364@redhat.com> (raw)
In-Reply-To: <ZihV0L7nzZYkbmNO@zatzit>
On 2024-04-23 20:44, David Gibson wrote:
> On Sat, Apr 20, 2024 at 03:19:19PM -0400, Jon Maloy wrote:
>> The kernel may support recvmsg(MSG_PEEK), starting reading data from a
> Not worth a respin on its own, but I think the comma above is
> misplaced, and for me makes the sentence much harder to read.
>
>> given offset set by the SO_PEEK_OFF socket option. This makes it
>> possible to avoid repeated reading of already read initial bytes of a
>> received message, hence saving read cycles when forwarding TCP messages
>> in the host->name space direction.
>>
>> In this commit, we add functionality to leverage this feature when available,
[...]
>> @@ -2174,6 +2183,15 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>> if (iov_rem)
>> iov_sock[fill_bufs].iov_len = iov_rem;
>>
>> + if (peek_offset_cap) {
>> + /* Don't use discard buffer */
>> + mh_sock.msg_iov = &iov_sock[1];
>> + mh_sock.msg_iovlen -= 1;
>> +
>> + /* Keep kernel sk_peek_off in synch */
>> + set_peek_offset(s, already_sent);
> I thought we didn't need to set SO_PEEK_OFF here - that it would track
> on its own, and we only needed to change it for retransmits. I don't
> think we even need to calculate 'already_sent' when we have
> SO_PEEK_OFF. In fact - if we set already_sent to 0 here, it might
> make things a bit cleaner than having to have special cases for
> adjusting the iov and sendlen.
In theory yes.
I tried it for a while, using SEQ_GE(max_ack_seq, ack_seq) as criteria for
retransmission.
I observed some strange behavior, like retransmits that seemingly did not
come from fast retransmit or timer retransmit, and that the kernel
'sk_peek_off'
didn´t always have the expected value when comparing with 'already_sent´.
Since my focus was on the zero-window issue I decided to skip this for now
and take the safe option.
I may revisit this later.
>
>> + }
>> +
>> /* Receive into buffers, don't dequeue until acknowledged by guest. */
>> do
>> len = recvmsg(s, &mh_sock, MSG_PEEK);
>> @@ -2195,7 +2213,9 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>> return 0;
>> }
>>
[...]
>> + peek_offset_cap = true;
>> + }
>> + close(s);
>> + }
>> + printf("SO_PEEK_OFF%ssupported\n", peek_offset_cap ? " " : " not ")
> Should be an info().
Made it a debug() as suggested by Stefano.
>
>> return 0;
>> }
>>
next prev parent reply other threads:[~2024-04-25 23:24 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-20 19:19 [PATCH 0/2] Support for SO_PEEK_OFF when a available Jon Maloy
2024-04-20 19:19 ` [PATCH 1/2] tcp: leverage support of SO_PEEK_OFF socket option when available Jon Maloy
2024-04-23 17:50 ` Stefano Brivio
2024-04-24 0:48 ` David Gibson
2024-04-24 18:30 ` Stefano Brivio
2024-04-26 3:27 ` David Gibson
2024-04-26 5:58 ` Stefano Brivio
2024-04-29 1:46 ` David Gibson
2024-04-25 23:06 ` Jon Maloy
2024-04-24 0:44 ` David Gibson
2024-04-25 23:23 ` Jon Maloy [this message]
2024-04-26 3:29 ` David Gibson
2024-04-20 19:19 ` [PATCH 2/2] tcp: allow retransmit when peer receive window is zero Jon Maloy
2024-04-24 1:04 ` David Gibson
2024-04-24 18:31 ` Stefano Brivio
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