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7267e8dcad
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Salvatore Dipietro 7267e8dcad tcp: Add memory barrier to tcp_push() 
On CPUs with weak memory models, reads and updates performed by tcp_push
to the sk variables can get reordered leaving the socket throttled when
it should not. The tasklet running tcp_wfree() may also not observe the
memory updates in time and will skip flushing any packets throttled by
tcp_push(), delaying the sending. This can pathologically cause 40ms
extra latency due to bad interactions with delayed acks.

Adding a memory barrier in tcp_push removes the bug, similarly to the
previous commit bf06200e73 ("tcp: tsq: fix nonagle handling").
smp_mb__after_atomic() is used to not incur in unnecessary overhead
on x86 since not affected.

Patch has been tested using an AWS c7g.2xlarge instance with Ubuntu
22.04 and Apache Tomcat 9.0.83 running the basic servlet below:

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorldServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream(),"UTF-8");
        String s = "a".repeat(3096);
        osw.write(s,0,s.length());
        osw.flush();
    }
}

Load was applied using wrk2 (https://github.com/kinvolk/wrk2) from an AWS
c6i.8xlarge instance. Before the patch an additional 40ms latency from P99.99+
values is observed while, with the patch, the extra latency disappears.

No patch and tcp_autocorking=1
./wrk -t32 -c128 -d40s --latency -R10000  http://172.31.60.173:8080/hello/hello
  ...
 50.000%    0.91ms
 75.000%    1.13ms
 90.000%    1.46ms
 99.000%    1.74ms
 99.900%    1.89ms
 99.990%   41.95ms  <<< 40+ ms extra latency
 99.999%   48.32ms
100.000%   48.96ms

With patch and tcp_autocorking=1
./wrk -t32 -c128 -d40s --latency -R10000  http://172.31.60.173:8080/hello/hello
  ...
 50.000%    0.90ms
 75.000%    1.13ms
 90.000%    1.45ms
 99.000%    1.72ms
 99.900%    1.83ms
 99.990%    2.11ms  <<< no 40+ ms extra latency
 99.999%    2.53ms
100.000%    2.62ms

Patch has been also tested on x86 (m7i.2xlarge instance) which it is not
affected by this issue and the patch doesn't introduce any additional
delay.

Fixes: 7aa5470c2c ("tcp: tsq: move tsq_flags close to sk_wmem_alloc")
Signed-off-by: Salvatore Dipietro <dipiets@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240119190133.43698-1-dipiets@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
2024-01-23 10:22:31 +01:00
arch RTC for 6.8 2024-01-18 17:25:39 -08:00
block for-6.8/block-2024-01-08 2024-01-11 13:58:04 -08:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto crypto: scomp - fix req->dst buffer overflow 2023-12-29 11:25:56 +08:00
Documentation This cycle, I2C removes the currently unused CLASS_DDC support 2024-01-18 17:29:01 -08:00
drivers net: micrel: Fix PTP frame parsing for lan8814 2024-01-22 11:02:23 +00:00
fs More eventfs fixes and a seq_buf fix for 6.8: 2024-01-18 14:45:33 -08:00
include udp: fix busy polling 2024-01-21 18:09:30 +00:00
init Driver core changes for 6.8-rc1 2024-01-18 09:48:40 -08:00
io_uring Generic: 2024-01-17 13:03:37 -08:00
ipc shm: Slim down dependencies 2023-12-20 19:26:31 -05:00
kernel Including fixes from bpf and netfilter. 2024-01-18 17:33:50 -08:00
lib cxl for v6.8 2024-01-18 16:22:43 -08:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm memblock: code readability improvement 2024-01-18 16:46:18 -08:00
net tcp: Add memory barrier to tcp_push() 2024-01-23 10:22:31 +01:00
rust Rust changes for v6.8 2024-01-11 13:05:41 -08:00
samples tracing updates for 6.8: 2024-01-18 14:35:29 -08:00
scripts Char/Misc and other Driver changes for 6.8-rc1 2024-01-17 16:47:17 -08:00
security misc cleanups (the part that hadn't been picked by individual fs trees) 2024-01-11 20:23:50 -08:00
sound This cycle, I2C removes the currently unused CLASS_DDC support 2024-01-18 17:29:01 -08:00
tools selftests: bonding: Increase timeout to 1200s 2024-01-19 21:12:31 -08:00
usr usr/Kconfig: fix typos of "its" 2023-12-20 15:02:58 -08:00
virt Generic: 2024-01-17 13:03:37 -08:00
.clang-format clang-format: Update with v6.7-rc4's for_each macro list 2023-12-08 23:54:38 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: rpm-pkg: generate kernel.spec in rpmbuild/SPECS/ 2023-10-03 20:49:09 +09:00
.mailmap Char/Misc and other Driver changes for 6.8-rc1 2024-01-17 16:47:17 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS Including fixes from bpf and netfilter. 2024-01-18 17:33:50 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Including fixes from bpf and netfilter. 2024-01-18 17:33:50 -08:00
Makefile Quite a lot of kexec work this time around. Many singleton patches in 2024-01-09 11:46:20 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.