mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
synced 2026-11-02 19:05:42 +00:00
No description
- C 96.8%
- Assembly 0.9%
- Rust 0.7%
- Shell 0.6%
- Python 0.5%
- Other 0.3%
60ef541561
Marco Crivellari says:
====================
Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:
"workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
=== Current situation: problems ===
Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.
This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
schedule_delayed_work(, 0);
This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:
schedule_delayed_work(, 1);
Will move the timer on an housekeeping CPU, and schedule the work there.
Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.
This lack of consistentcy cannot be addressed without refactoring the API.
=== Plan and future plans ===
This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.
These are the main steps:
1) API refactoring (that this patch is introducing)
- Make more clear and uniform the system wq names, both per-cpu and
unbound. This to avoid any possible confusion on what should be
used.
- Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
introduced in this patchset and used on all the callers that are not
currently using WQ_UNBOUND.
WQ_UNBOUND will be removed in a future release cycle.
Most users don't need to be per-cpu, because they don't have
locality requirements, because of that, a next future step will be
make "unbound" the default behavior.
2) Check who really needs to be per-cpu
- Remove the WQ_PERCPU flag when is not strictly required.
3) Add a new API (prefer local cpu)
- There are users that don't require a local execution, like mentioned
above; despite that, local execution yeld to performance gain.
This new API will prefer the local execution, without requiring it.
=== Introduced Changes by this series ===
1) [P 1-2] Replace use of system_wq and system_unbound_wq
system_wq is a per-CPU workqueue, but his name is not clear.
system_unbound_wq is to be used when locality is not required.
Because of that, system_wq has been renamed in system_percpu_wq, and
system_unbound_wq has been renamed in system_dfl_wq.
2) [P 3] add WQ_PERCPU to remaining alloc_workqueue() users
Every alloc_workqueue() caller should use one among WQ_PERCPU or
WQ_UNBOUND. This is actually enforced warning if both or none of them
are present at the same time.
WQ_UNBOUND will be removed in a next release cycle.
=== For Maintainers ===
There are prerequisites for this series, already merged in the master branch.
The commits are:
|
History
|
|
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| 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.