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Filename Latest commit message Latest commit date
Salvatore Dipietro da70948068 tcp: Add memory barrier to tcp_push() 
[ Upstream commit 7267e8dcad6b2f9fce05a6a06335d7040acbc2b6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2024-02-23 08:24:50 +01:00
arch x86/CPU/AMD: Fix disabling XSAVES on AMD family 0x17 due to erratum 2024-02-23 08:24:50 +01:00
block block: Remove special-casing of compound pages 2024-02-23 08:24:49 +01:00
certs
crypto crypto: api - Disallow identical driver names 2024-02-23 08:24:48 +01:00
Documentation rename(): fix the locking of subdirectories 2024-02-23 08:24:49 +01:00
drivers mtd: spinand: macronix: Fix MX35LFxGE4AD page size 2024-02-23 08:24:49 +01:00
fs afs: Hide silly-rename files from userspace 2024-02-23 08:24:50 +01:00
include llc: Drop support for ETH_P_TR_802_2. 2024-02-23 08:24:50 +01:00
init rootfs: Fix support for rootfstype= when root= is given 2024-01-25 14:34:30 -08:00
ipc
kernel tracing: Ensure visibility when inserting an element into tracing_map 2024-02-23 08:24:50 +01:00
lib ida: Fix crash in ida_free when the bitmap is empty 2024-01-25 14:34:21 -08:00
LICENSES
mm
net tcp: Add memory barrier to tcp_push() 2024-02-23 08:24:50 +01:00
samples
scripts
security apparmor: avoid crash when parsed profile name is empty 2024-01-25 14:34:31 -08:00
sound ALSA: oxygen: Fix right channel of capture volume mixer 2024-01-25 14:34:30 -08:00
tools perf top: Skip side-band event setup if HAVE_LIBBPF_SUPPORT is not set 2024-01-25 14:34:33 -08:00
usr
virt
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore
.mailmap
COPYING
CREDITS
Kbuild
Kconfig
MAINTAINERS
Makefile Linux 5.4.268 2024-01-25 14:34:33 -08:00
README

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.