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Joel Fernandes bc3705e209 rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early
The RCU grace period mechanism uses a two-phase FQS (Force Quiescent
State) design where the first FQS saves dyntick-idle snapshots and
the second FQS compares them. This results in long and unnecessary latency
for synchronize_rcu() on idle systems (two FQS waits of ~3ms each with
1000HZ) whenever one FQS wait sufficed.

Some investigations showed that the GP kthread's CPU is the holdout CPU
a lot of times after the first FQS as - it cannot be detected as "idle"
because it's actively running the FQS scan in the GP kthread.

Therefore, at the end of rcu_gp_init(), immediately report a quiescent
state for the GP kthread's CPU using rcu_qs() + rcu_report_qs_rdp(). The
GP kthread cannot be in an RCU read-side critical section while running
GP initialization, so this is safe and results in significant latency
improvements.

The following tests were performed:

(1) synchronize_rcu() benchmarking

    100 synchronize_rcu() calls with 32 CPUs, 10 runs each (default fqs
    jiffies settings):

    Baseline (without fix):
    | Run | Mean      | Min      | Max       |
    |-----|-----------|----------|-----------|
    | 1   | 10.088 ms | 9.989 ms | 18.848 ms |
    | 2   | 10.064 ms | 9.982 ms | 16.470 ms |
    | 3   | 10.051 ms | 9.988 ms | 15.113 ms |
    | 4   | 10.125 ms | 9.929 ms | 22.411 ms |
    | 5   |  8.695 ms | 5.996 ms | 15.471 ms |
    | 6   | 10.157 ms | 9.977 ms | 25.723 ms |
    | 7   | 10.102 ms | 9.990 ms | 20.224 ms |
    | 8   |  8.050 ms | 5.985 ms | 10.007 ms |
    | 9   | 10.059 ms | 9.978 ms | 15.934 ms |
    | 10  | 10.077 ms | 9.984 ms | 17.703 ms |

    With fix:
    | Run | Mean     | Min      | Max       |
    |-----|----------|----------|-----------|
    | 1   | 6.027 ms | 5.915 ms |  8.589 ms |
    | 2   | 6.032 ms | 5.984 ms |  9.241 ms |
    | 3   | 6.010 ms | 5.986 ms |  7.004 ms |
    | 4   | 6.076 ms | 5.993 ms | 10.001 ms |
    | 5   | 6.084 ms | 5.893 ms | 10.250 ms |
    | 6   | 6.034 ms | 5.908 ms |  9.456 ms |
    | 7   | 6.051 ms | 5.993 ms | 10.000 ms |
    | 8   | 6.057 ms | 5.941 ms | 10.001 ms |
    | 9   | 6.016 ms | 5.927 ms |  7.540 ms |
    | 10  | 6.036 ms | 5.993 ms |  9.579 ms |

    Summary:
    - Mean latency: 9.75 ms -> 6.04 ms (38% improvement)
    - Max latency:  25.72 ms -> 10.25 ms (60% improvement)

(2) Bridge setup/teardown latency (Uladzislau Rezki)

    x86_64 with 64 CPUs, 100 iterations of bridge add/configure/delete:

                                   real time
    1 - default:                   24.221s
    2 - this patch:                20.754s  (14% faster)
    3 - this patch + wake_from_gp: 15.895s  (34% faster)
    4 - wake_from_gp only:         18.947s  (22% faster)

    Per-synchronize_rcu() latency (in usec):
                  1         2         3       4
    median: 37249.5   31540.5   15765   22480
    min:    7881      7918      9803    7857
    max:    63651     55639     31861   32040

    This patch combined with rcu_normal_wake_from_gp reduces bridge
    setup/teardown time from 24 seconds to 16 seconds.

(3) CPU overhead verification (Uladzislau Rezki)

    System CPU time across 5 runs showed no measurable increase:
      default:     1.698s - 1.937s
      this patch:  1.667s - 1.930s
    Conclusion: variations are within noise, no CPU overhead regression.

(4) rcutorture

    Tested TREE and SRCU configurations - no regressions.

Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Tested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Samir M <samir@linux.ibm.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
History 2026-01-11 20:11:15 +08:00
arch Misc fixes: 2025-12-14 06:10:35 +12:00
block block-6.19-20251211 2025-12-12 22:04:18 +12:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto Networking changes for 6.19. 2025-12-03 17:24:33 -08:00
Documentation Misc fixes: 2025-12-14 06:07:09 +12:00
drivers SCSI misc on 20251214 2025-12-14 15:35:35 +12:00
fs We have a patch that adds an initial set of tracepoints to the MDS 2025-12-14 15:24:10 +12:00
include We have a patch that adds an initial set of tracepoints to the MDS 2025-12-14 15:24:10 +12:00
init Significant patch series in this pull request: 2025-12-06 14:01:20 -08:00
io_uring io_uring-6.19-20251211 2025-12-12 22:01:32 +12:00
ipc Significant patch series in this pull request: 2025-12-06 14:01:20 -08:00
kernel rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early 2026-01-11 20:11:15 +08:00
lib Miscellaneous fixes: 2025-12-14 06:04:16 +12:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm There are no significant series in this small merge. Please see the 2025-12-13 20:55:12 +12:00
net We have a patch that adds an initial set of tracepoints to the MDS 2025-12-14 15:24:10 +12:00
rust USB/Thunderbolt changes for 6.19-rc1 2025-12-06 18:42:12 -08:00
samples USB/Thunderbolt changes for 6.19-rc1 2025-12-06 18:42:12 -08:00
scripts There are no significant series in this small merge. Please see the 2025-12-13 20:55:12 +12:00
security Trivial optimization. 2025-12-14 15:21:02 +12:00
sound soundwire updates for 6.19 2025-12-13 16:26:55 +12:00
tools Futex changes for v6.19: 2025-12-10 17:21:30 +09:00
usr initramfs: add gen_init_cpio to hostprogs unconditionally 2025-11-26 21:55:40 +01:00
virt soc: driver updates for 6.19 2025-12-05 17:29:04 -08:00
.clang-format Significant patch series in this merge are as follows: 2025-12-05 13:52:43 -08:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.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 rust: kbuild: add proc macro library support 2025-11-24 17:15:36 +01:00
.mailmap Significant patch series in this pull request: 2025-12-06 14:01:20 -08:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.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 phy-for-6.19 2025-12-09 06:31:47 +09:00
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 Patch series in this pull request: 2025-12-13 20:35:41 +12:00
Makefile Linux 6.19-rc1 2025-12-14 16:05:07 +12:00
README README: restructure with role-based documentation and guidelines 2025-11-29 08:40:33 -07:00

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

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst



Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst