tcp: refactor cwnd during SACK transmissions and enable TSO

[rscheff]

Summary

Since the dawn of the SACK feature, TSO was never enabled
during SACK loss recovery episodes. With the use of
LRO / GRO leading to ACK compression (partial ACKs and
SACK blocks stepping by more than a singular segment),
and also features like PRR which allow increasing the
cwnd in the final phase of loss recovery (SSRB), this
frequently results in multiple passes of the
tcp_default_output() codepath.

It turns out that previously, the SACK transmission path would
only really work as the restriction to 1 MSS per call to tcp_output()
work, but allowing more than 1 MSS could result in invalid transmission
selection.

Refactoring of cwnd and moving the adjustment for SACKed data into
tcp_output() - cwnd tracking the maximum extend from snd_una - allows
both SACK loss recovery as well as SACK transmissions after RTO during
slow start.

Test Plan

On a TSO enabled NIC, with the receiver also using TSO and
LRO, verify that TSO chunks of more than 1 SMSS show up in a
packet capture. Also, when taking an RTO during a SACK loss
recovery episode, the flight size increases as per SlowStart and
Congestion Avoidance while excluding the re-transmission of
previously SACKed data.
Note that additional losses in that phase result in very small
congestion windows (since cwnd is reduced multiple times),
and entering a loss recovery then can result in lengthy
recovery until snd_max is finally reached.

[rscheff]

• change cwnd during SACK LR to flightsize
• put cwnd inflation during classic retransmissions into tcp_output()
• properly calculate available data during SACK transmissions
• allow cwnd adjustments by more than MSS (LRO/TSO side effect)
• reset scoreboard when entering LR after RTO
• various minor style changes

[rscheff]

With this change, cwnd will reflect the expected flightsize also during SACK transmissions.
This allows the proper operation of skipping previously SACKed data after an RTO, and also the use of TSO while retransmitting.
The SACK related cwin calculation previously seems to only have worked as only a single MSS per tcp_output() call could be sent.
Keep sack_rxmit set when an attempt was made to send from the scoreboard - this prevents a longer codepath before returning. If the snd_cwnd would allow for more than the current hole to be sent, we can cycle back by specifically setting sendalot only in those cases.

While there, also do some various style changes and readability enhancements.

[tuexen]

Is it possible to split this up? At least separate the style changes?

[rscheff]

In D43470#992578, @tuexen wrote:

Is it possible to split this up? At least separate the style changes?

This includes changes in
D43520
D43447
and D43355.

I'll extract the pure style changes with no functional impact as a separate one, but want to keep this homogeneous for testing.

[rscheff]

• (int32_t)min(a,b) is still not the same as imin(a,b)

[rscheff]

• rebase main

[rscheff]

• delta to D43355_sack_after_rto

[rscheff]

• rebase main

[cc]

Because of commit 440f4ba18e3a, please re-base.

sys/netinet/tcp_input.c

2656–2658

For readability, please remove the parentheses () for Relational operators, which have higher precedence than Logical operator. Also re-align.

2679–2681

Re-align.

2862–2866

For readability, please remove the parentheses () for Relational operators, which have higher precedence than Logical operator.

sys/netinet/tcp_output.c

268–273

For readability, please remove the parentheses () for Relational operators, which have higher precedence than Logical operator.

556–563

For readability, please remove the parentheses () for Relational operators, which have higher precedence than Logical operator. Also re-align.

[cc]

Need code update.

[rscheff]

In D43470#1072656, @cc wrote:

Because of commit 440f4ba18e3a, please re-base.

This is exactly the reason for the new diff ;)

git rebase main

Current branch D43470_sacktso is up to date.

commit 02fbfd835eb70d1081a8918acb5cb554afde1fb7 (HEAD -> D43470_sacktso)

sys/netinet/tcp_input.c

2656–2658

See style(9). Not everyone knows the exact precedence for all operators. There are many examples of extra (not strictly needed by the compiler) parenthesis, for ease of readability.
Also, a half-tab is the 2nd level indentation, with all subsequent clauses aligning to this.
For readability, I tend to have logical operators at the end of a line, so that the individual clauses are easier to see and not get drowned.

2679–2681

There are plenty of examples where the used indentation (full tab plus half-tab) is used for "} else if (foo)\n(bar)" constructs.

between

<------><------><------><------><------>} else if (tcp_is_sack_recovery(tp, &to) &&
<------><------><------><------><------><------>    IN_FASTRECOVERY(tp->t_flags) &&
<------><------><------><------><------><------>    SEQ_LT(tp->snd_nxt, tp->snd_max)) {

and

<------><------><------><------><------>} else if (tcp_is_sack_recovery(tp, &to) &&
<------><------><------><------><------>    IN_FASTRECOVERY(tp->t_flags) &&
<------><------><------><------><------>    SEQ_LT(tp->snd_nxt, tp->snd_max)) {

I think the former one is more readable.

[cc]

sys/netinet/tcp_input.c

2770

This line should not be changed. A missing { caused compile error.

[rscheff]

• fix typo after rebase

[rscheff]

thanks!

[cc]

sys/netinet/tcp_output.c

556

Let's keep a minimum change. If the priority of (len > tp->t_maxseg) is higher than others, just need to change this line, and leave the rest of the lines as they were. The rest of the changes are insignificant and can be left for future non-functional refresh.

566

Same here. Don't change anything on this line.

612–614

Unnecessary change, please leave the pair of {} as it was.

1648–1657

Duplicated code.

[rscheff]

• remove unnecessary changes

[rscheff]

sys/netinet/tcp_output.c

556–563

Realign? remember, the linter in upstream is not the one you are used to; and I like "bool" checks (TF_xx and TF2_xx) without fluff (!(foo) instead of (foo) != 0 - which I believe is covered by style(9). And let's agree to disagree what readability with extra parenthesis means - both ways seem to be ok by style; I like to make the precedence explict, and not rely on the implicit precedence ordering of operators which may be different between different languages.

1648–1657

Thanks. Seems when i split the original patch in two, I missed the comment - restoring that here.

[cc]

sys/netinet/tcp_output.c

297

Type of cwin should be int32_t, as it will be compared with other signed variables.

322–327

For readability, there is no need to combine the two lines into one.

330–335

Same here. For readability, there is no need to combine the two lines into one.

346–350

Can be simpler.

414

This line is not a necessary change.

415

Is imin() a better choice for consistence?

556–563

I think the result of a bitwise (e.g.&, |, ^, ~) operation is not boolean, although you may think C treats non-zero values as true and zero as false in conditional expressions like if, while, etc.

So style(9) still applies. :)

Do not use ! for tests unless it is a boolean // from style(9)

int result = 5 & 5;  // 5 is 0101
// Result is 0101 (5 in decimal)

Talking about this block of code, we can just leave as it was (don't change), as it is insignificant/non-functional change, and not related with the patch subject.

sys/netinet/tcp_sack.c

967–974

Looks to be no-op. Can we just add the comment without adding the no-op code? And please proof-reading the comment.

[rscheff]

• implement suggested improvements

[cc]

Better now. But it can be cleaner.

sys/netinet/tcp_output.c

414

This line is still not reverted.

415

Either revert this line or use imin for consistence of the imin in the else clause as below.

len = imin(sbavail(&so->so_snd), sendwin) - off;

556–561

This block change is still not reverted.

[rscheff]

• remove unneccesary style and line wrap changes

[cc]

OK. I am approving it now as my test in https://wiki.freebsd.org/chengcui/testD43470 shows some improvement. Any bug related observations can be fixed later.