Fw: [Bug 217134] New: TCP performance regression on loopback interface

[Stephen Hemminger <stephen@networkplumber.org>]

Begin forwarded message:

Date: Sat, 04 Mar 2023 15:54:34 +0000
From: bugzilla-daemon@kernel.org
To: stephen@networkplumber.org
Subject: [Bug 217134] New: TCP performance regression on loopback interface


https://bugzilla.kernel.org/show_bug.cgi?id=217134

            Bug ID: 217134
           Summary: TCP performance regression on loopback interface
           Product: Networking
           Version: 2.5
    Kernel Version: 5.10-6.2
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
          Assignee: stephen@networkplumber.org
          Reporter: uint8ptr@proton.me
        Regression: No

Created attachment 303849
  --> https://bugzilla.kernel.org/attachment.cgi?id=303849&action=edit  
Packet captures of the GDB communication on Linux 5.9 and 5.10

Somewhere between Linux 5.9-rc1 and Linux 5.10 LTS the TCP performance on the
loopback interface slowed down to a crawl. This issue makes doing some
operations with gdb a lot slower than they should be, in particular when using
Python GDB extensions that require sending and receiving data to/from the GDB
server.

The bug affects all versions of the Linux kernel from 5.10 LTS up to 6.2.
Versions 5.4 LTS and 5.9-rc1 are not affected by this bug.

I attached two PCAP files that shows the problem.

The packet captures were obtained by connecting the mGBA (version 0.10.1) GDB
server and reading 8 bytes from the memory with the following commands:

```
(gdb) target remote localhost:2345
Remote debugging using localhost:2345
0x080008c6 in WaitForVBlank ()
(gdb) x/8bx 0x08000000
0x8000000:      0x7f    0x00    0x00    0xea    0x24    0xff    0xae    0x51
```

I made a table illustrating the performance difference for reading the first
byte. You can trace back the exact packets by referring to the first column of
the tables.

---

Linux 5.9 packet capture summary

Seq |   | Timestamp | Payload
168 | > | 4.457569  | $m8000000,1#22
172 | < | 4.457689  | $7f#9d

Time delta: 0.00011s/110μs

---

Linux 5.10 packet capture summary

Seq |   | Timestamp | Payload
164 | > | 3.583188  | $m8000000,1#22
168 | < | 3.623667  | $7f#9d

Time delta: 0.040479s/40ms

---

I already discussed extensively the issue with the emulator developer, and we
came to the conclusion that the packet is getting delayed by the OS at some
point. The issue is present on any version of the emulator that supports the
GDB server.

Additional information:

During the TCP communication, no new messages appeared by running the dmesg
command.

```
root # uname -a
Linux manjaro 6.1.12-1-MANJARO #1 SMP PREEMPT_DYNAMIC Tue Feb 14 21:59:10 UTC
2023 x86_64 GNU/Linux
root # lspci | grep "Network controller"
02:00.0 Network controller: Intel Corporation Wireless 8265 / 8275 (rev 78)
```

The non-affected Kernel version I used for testing can be downloaded here:
https://kacabenggala.uny.ac.id/manjaro/stable-staging/core/x86_64/linux59-5.9rc1.d0816.g9123e3a-1-x86_64.pkg.tar.zst

Steps to reproduce:

1. Start a game on any version of mGBA that supports the GDB server on Linux
5.10 LTS or newer
2. Go to mGBA > Tools > Start GDB server... > Start
3. Start arm-none-eabi-gdb and run the following commands
   - target remote localhost:2345
   - x/8bx 0x08000000

If you display more bytes on the second command, the issue is more noticeable.

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