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Thomas Gleixner 3db6b38dfe rseq: Switch to fast path processing on exit to user 
Now that all bits and pieces are in place, hook the RSEQ handling fast path
function into exit_to_user_mode_prepare() after the TIF work bits have been
handled. If case of fast path failure, TIF_NOTIFY_RESUME has been raised
and the caller needs to take another turn through the TIF handling slow
path.

This only works for architectures which use the generic entry code.
Architectures who still have their own incomplete hacks are not supported
and won't be.

This results in the following improvements:

  Kernel build	       Before		  After		      Reduction

  exit to user         80692981		  80514451
  signal checks:          32581		       121	       99%
  slowpath runs:        1201408   1.49%	       198 0.00%      100%
  fastpath runs:			    675941 0.84%       N/A
  id updates:           1233989   1.53%	     50541 0.06%       96%
  cs checks:            1125366   1.39%	         0 0.00%      100%
    cs cleared:         1125366      100%	 0            100%
    cs fixup:                 0        0%	 0

  RSEQ selftests      Before		  After		      Reduction

  exit to user:       386281778		  387373750
  signal checks:       35661203		          0           100%
  slowpath runs:      140542396 36.38%	        100  0.00%    100%
  fastpath runs:			    9509789  2.51%     N/A
  id updates:         176203599 45.62%	    9087994  2.35%     95%
  cs checks:          175587856 45.46%	    4728394  1.22%     98%
    cs cleared:       172359544   98.16%    1319307   27.90%   99%
    cs fixup:           3228312    1.84%    3409087   72.10%

The 'cs cleared' and 'cs fixup' percentages are not relative to the exit to
user invocations, they are relative to the actual 'cs check' invocations.

While some of this could have been avoided in the original code, like the
obvious clearing of CS when it's already clear, the main problem of going
through TIF_NOTIFY_RESUME cannot be solved. In some workloads the RSEQ
notify handler is invoked more than once before going out to user
space. Doing this once when everything has stabilized is the only solution
to avoid this.

The initial attempt to completely decouple it from the TIF work turned out
to be suboptimal for workloads, which do a lot of quick and short system
calls. Even if the fast path decision is only 4 instructions (including a
conditional branch), this adds up quickly and becomes measurable when the
rate for actually having to handle rseq is in the low single digit
percentage range of user/kernel transitions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.701201365@linutronix.de
2025-11-04 08:34:39 +01:00
arch entry: Remove syscall_enter_from_user_mode_prepare() 2025-11-04 08:31:37 +01:00
block block-6.18-20251031 2025-10-31 12:57:19 -07:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto This push contains the following changes: 2025-10-10 08:56:16 -07:00
Documentation rseq: Provide static branch for runtime debugging 2025-11-04 08:32:49 +01:00
drivers rseq, virt: Retrigger RSEQ after vcpu_run() 2025-11-04 08:30:23 +01:00
fs rseq: Optimize event setting 2025-11-04 08:34:03 +01:00
include rseq: Switch to fast path processing on exit to user 2025-11-04 08:34:39 +01:00
init rseq: Switch to fast path processing on exit to user 2025-11-04 08:34:39 +01:00
io_uring io_uring: fix buffer auto-commit for multishot uring_cmd 2025-10-23 19:41:31 -06:00
ipc namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
kernel rseq: Switch to fast path processing on exit to user 2025-11-04 08:34:39 +01:00
lib Second round of Kbuild fixes for 6.19 2025-11-01 10:00:53 -07:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm slab fixes for 6.18-rc3 2025-10-24 12:40:51 -07:00
net bpf-fixes 2025-10-31 18:22:26 -07:00
rust Driver core fixes for 6.18-rc3 2025-10-25 11:03:46 -07:00
samples Char/Misc/IIO/Binder changes for 6.18-rc1 2025-10-04 16:26:32 -07:00
scripts kconfig/nconf: Initialize the default locale at startup 2025-11-01 00:23:22 -04:00
security integrity-v6.18 2025-10-05 10:48:33 -07:00
sound ASoC: Fixes for v6.18 2025-10-30 13:08:08 +01:00
tools objtool: Fix skip_alt_group() for non-alternative STAC/CLAC 2025-11-01 07:43:20 +01:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt rseq, virt: Retrigger RSEQ after vcpu_run() 2025-11-04 08:30:23 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap linux_kselftest-kunit-fixes-6.18-rc4 2025-10-30 19:11:27 -07:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.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 MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS Second round of Kbuild fixes for 6.19 2025-11-01 10:00:53 -07:00
Makefile Linux 6.18-rc4 2025-11-02 11:28:02 -08:00
README README: Fix spelling 2024-03-18 03:36:32 -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 reStructuredText 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.