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c84897c0ff
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Steven Rostedt (Google) c84897c0ff ring-buffer: Remove 32bit timestamp logic 
Each event has a 27 bit timestamp delta that is used to hold the delta
from the last event. If the time between events is greater than 2^27, then
a timestamp is added that holds a 59 bit absolute timestamp.

Until a389d86f7f ("ring-buffer: Have nested events still record running
time stamp"), if an interrupt interrupted an event in progress, all the
events delta would be zero to not deal with the races that need to be
handled. The commit a389d86f7f changed that to handle the races giving
all events, even those that preempt other events, still have an accurate
timestamp.

To handle those races requires performing 64-bit cmpxchg on the
timestamps. But doing 64-bit cmpxchg on 32-bit architectures is considered
very slow. To try to deal with this the timestamp logic was broken into
two and then three 32-bit cmpxchgs, with the thought that two (or three)
32-bit cmpxchgs are still faster than a single 64-bit cmpxchg on 32-bit
architectures.

Part of the problem with this is that I didn't have any 32-bit
architectures to test on. After hitting several subtle bugs in this code,
an effort was made to try and see if three 32-bit cmpxchgs are indeed
faster than a single 64-bit. After a few people brushed off the dust of
their old 32-bit machines, tests were done, and even though 32-bit cmpxchg
was faster than a single 64-bit, it was in the order of 50% at best, not
300%.

After some more refactoring of the code, all 4 64-bit cmpxchg were removed:

 https://lore.kernel.org/linux-trace-kernel/20231211114420.36dde01b@gandalf.local.home
 https://lore.kernel.org/linux-trace-kernel/20231214222921.193037a7@gandalf.local.home
 https://lore.kernel.org/linux-trace-kernel/20231215081810.1f4f38fe@rorschach.local.home
 https://lore.kernel.org/linux-trace-kernel/20231218230712.3a76b081@gandalf.local.home/

With all the 64-bit cmpxchg removed, the complex 32-bit workaround can also be
removed.

The 32-bit and 64-bit logic is now exactly the same.

Link: https://lore.kernel.org/all/20231213214632.15047c40@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20231219074303.28f9abda@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
2023-12-19 07:47:04 -05:00
arch - Ignore invalid x2APIC entries in order to not waste per-CPU data 2023-11-19 13:46:17 -08:00
block blk-mq: make sure active queue usage is held for bio_integrity_prep() 2023-11-13 08:52:52 -07:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto This push fixes a regression in ahash and hides the Kconfig sub-options for the jitter RNG. 2023-11-09 17:04:58 -08:00
Documentation - Ignore invalid x2APIC entries in order to not waste per-CPU data 2023-11-19 13:46:17 -08:00
drivers tracing: Allow creating instances with specified system events 2023-12-18 23:14:16 -05:00
fs eventfs: Fix events beyond NAME_MAX blocking tasks 2023-12-12 19:00:37 -05:00
include trace_seq: Increase the buffer size to almost two pages 2023-12-18 23:14:16 -05:00
init As usual, lots of singleton and doubleton patches all over the tree and 2023-11-02 20:53:31 -10:00
io_uring io_uring/fdinfo: remove need for sqpoll lock for thread/pid retrieval 2023-11-15 06:35:46 -07:00
ipc Many singleton patches against the MM code. The patch series which are 2023-11-02 19:38:47 -10:00
kernel ring-buffer: Remove 32bit timestamp logic 2023-12-19 07:47:04 -05:00
lib Zstd fixes for v6.7 2023-11-14 23:35:31 -05:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm mm: more ptep_get() conversion 2023-11-15 15:30:09 -08:00
net netfilter pull request 23-11-15 2023-11-16 11:02:52 +01:00
rust Kbuild updates for v6.7 2023-11-04 08:07:19 -10:00
samples tracing: Allow creating instances with specified system events 2023-12-18 23:14:16 -05:00
scripts Kbuild fixes for v6.7 2023-11-19 13:54:28 -08:00
security + Features 2023-11-03 09:48:17 -10:00
sound sound fixes for 6.7-rc2 2023-11-17 09:05:31 -05:00
tools tracing/selftests: Add test to test the trace_marker 2023-12-19 07:35:10 -05:00
usr arch: Remove Itanium (IA-64) architecture 2023-09-11 08:13:17 +00:00
virt ARM: 2023-09-07 13:52:20 -07:00
.clang-format iommu: Add for_each_group_device() 2023-05-23 08:15:51 +02:00
.cocciconfig
.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 As usual, lots of singleton and doubleton patches all over the tree and 2023-11-02 20:53:31 -10:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING
CREDITS USB: Remove Wireless USB and UWB documentation 2023-08-09 14:17:32 +02:00
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 MAINTAINERS: TRACING: Add Mathieu Desnoyers as Reviewer 2023-11-22 18:37:37 -05:00
Makefile Linux 6.7-rc2 2023-11-19 15:02:14 -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.