[PATCH v4 1/1] Documentation: real-time: Add kernel configuration guide

["Ahmed S. Darwish" <darwi@linutronix.de>]

Add a configuration guide for real-time kernels.

List all Kconfig options that are recommended to be either enabled or
disabled.  Explicitly add a table of contents at the top of the document,
so that all the options can be seen in a glance.

Whenever appropriate, link to other kernel guides; e.g. cpuidle, cpufreq,
power management, and no_hz.

Add a summary at the end of the document warning users that there is a no
"one size fits all solution" for configuring a real-time system.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
---

* Changelog v4

 Handle Sashiko's review remarks at
 https://sashiko.dev/#/patchset/ad5_XCnVDlC9Hvup%40lx-t490

 Documentation/core-api/real-time/index.rst    |   1 +
 .../real-time/kernel-configuration.rst        | 310 ++++++++++++++++++
 2 files changed, 311 insertions(+)
 create mode 100644 Documentation/core-api/real-time/kernel-configuration.rst

diff --git a/Documentation/core-api/real-time/index.rst b/Documentation/core-api/real-time/index.rst
index f08d2395a22c..a17a3dec535c 100644
--- a/Documentation/core-api/real-time/index.rst
+++ b/Documentation/core-api/real-time/index.rst
@@ -15,3 +15,4 @@ the required changes compared to a non-PREEMPT_RT configuration.
    differences
    hardware
    architecture-porting
+   kernel-configuration
diff --git a/Documentation/core-api/real-time/kernel-configuration.rst b/Documentation/core-api/real-time/kernel-configuration.rst
new file mode 100644
index 000000000000..73f7730d468e
--- /dev/null
+++ b/Documentation/core-api/real-time/kernel-configuration.rst
@@ -0,0 +1,310 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Real-Time Kernel configuration
+==============================
+
+.. contents:: Table of Contents
+   :depth: 3
+   :local:
+
+Introduction
+============
+
+This document lists the kernel configuration options that might affect a
+real-time kernel's worst-case latency.  It is intended for system integrators.
+
+Configuration options
+=====================
+
+``CONFIG_CPU_FREQ``
+-------------------
+
+:Expectation: enabled
+:Severity: *high*
+
+The CPU frequency scaling subsystem ensures that the processor can operate
+at its maximum supported frequency.  While, in general, bootloaders are
+tasked with setting the CPU clock to the highest speed on boot, some do
+not.  It is thus desirable to keep this option enabled.
+
+.. caution::
+
+  A real-time kernel is not about being "as fast as possible", however
+  real-time requirements may demand that the CPU is clocked at a
+  particular speed.
+
+``CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE``
+-------------------------------------------
+
+:Expectation: enabled
+:Severity: *high*
+
+Real-Time workloads expect a fixed CPU frequency during execution.  Using
+the performance governor is an easy way to achieve that purely from kernel
+configuration.
+
+This is not a blanket rule.  Some setups might prefer to clock the CPU to
+lower speeds due to thermal packaging or other requirements.  The key is
+that the CPU frequency remains constant once set.
+
+``CONFIG_CPU_IDLE``
+-------------------
+
+:Expectation: enabled
+:Severity: *info*
+
+CPU idle states (C-states) allow the processor to enter low-power modes
+during periods of inactivity.  Very-low CPU idle states may require
+flushing the CPU caches and lowering or disabling the clocking.  This can
+lower power consumption, but it also increases the entry and exit latency
+from such states.
+
+While disabling this option eliminates cpuidle-related latencies, doing so
+can significantly impact hardware longevity, warranty, and thermal
+behavior.  Users should cap the maximum C-state to C1 instead.  For ACPI
+platforms, this can be achieved by using the boot parameter [1]_::
+
+  processor.max_cstate=1
+
+Higher C-states can be acceptable depending on the user workload's latency
+requirements.  For ACPI-based platforms, use the ``cpupower idle-info``
+command to inspect the available idle states.
+
+For more information, please see:
+
+- ``linux/tools/power/cpupower``
+- :doc:`/admin-guide/pm/cpuidle`
+- :doc:`/admin-guide/pm/index`
+
+``CONFIG_DRM``
+--------------
+
+:Expectation: disabled
+:Severity: *info*
+
+GPU-accelerated workloads can share system resources with the CPU,
+including last-level cache (LLC) and memory bandwidth.  Modern integrated
+GPUs optimize graphics performance at the expense of CPU determinism.
+
+Examples of affected platforms:
+
+- Intel processors with integrated graphics (Gen9 and later)
+- AMD APUs with Radeon Graphics
+- Xilinx Zynq UltraScale+ MPSoC EG/EV series
+
+If graphics workloads must run alongside real-time tasks, users must
+conduct thorough stress testing using tools like ``glmark2`` while
+measuring the overall system latency.
+
+For more information, please check:
+
+- :doc:`Regarding hardware (System memory and cache) </core-api/real-time/hardware>`
+- :doc:`/filesystems/resctrl`
+- `Real-Time and Graphics: A Contradiction?`_
+
+``CONFIG_EFI_DISABLE_RUNTIME``
+------------------------------
+
+:Expectation: enabled
+:Severity: *medium*
+
+EFI is the standard boot and firmware interface for multiple
+architectures.  EFI runtime services provide callback functions to be
+called from the kernel; e.g., as utilized by (``CONFIG_EFI_VARS*``) or
+(``CONFIG_RTC_DRV_EFI``).  For the former, the kernel calls into EFI to
+update the EFI variables.
+
+Calling into EFI means invoking firmware callbacks.  During such
+invocations, the system might not be able to react to interrupts and will
+thus not be able to perform a context switch.  This can cause significant
+latency spikes for the real-time system.
+
+``CONFIG_PREEMPT_RT`` enables this option by default.  If this option is
+manually disabled at build time, the following boot parameter [1]_ may be
+used to disable EFI runtime at boot up::
+
+  efi=noruntime
+
+There is ongoing `development work`_ to allow access to EFI variables for a
+real-time Linux system.
+
+``CONFIG_NO_HZ`` / ``CONFIG_NO_HZ_FULL``
+----------------------------------------
+
+:Expectation: disabled
+:Severity: *medium*
+
+Tickless operation can increase kernel-to-userspace transition latency due
+to the extra accounting and state book-keeping.
+
+*Guidance by real-time workload type:*
+
+- For periodic workloads; e.g., control loops executing every 100 µs, avoid
+  ``NO_HZ`` modes.  Consistent kernel ticks are preferable.
+
+- For computation-intensive workloads; e.g. extended userspace execution,
+  ``NO_HZ_FULL`` may be beneficial.  In such cases, users should offload
+  the kernel housekeeping to dedicated CPUs and isolate compute cores.
+
+See also :doc:`/timers/no_hz`.
+
+``CONFIG_PREEMPT_RT``
+---------------------
+
+:Expectation: enabled
+:Severity: **fatal**
+
+This option must be enabled, or the resulting kernel will not be fully
+preemptible and real-time capable.
+
+``CONFIG_TRACING`` (and tracing options)
+----------------------------------------
+
+:Expectation: enabled
+:Severity: *info*
+
+Shipping kernels with tracing support enabled (but not actively running) is
+highly recommended.  This will allow the users to extract more information
+if latency problems arise.  Nonetheless, some tracers do incur latency
+overhead by just being enabled; see :ref:`tracers`.
+
+.. caution::
+
+  Users should *not* make use of tracers or trace events during production
+  real-time kernel operation as they can add considerable overhead and
+  degrade the system's latency.
+
+Non-performance CPU frequency governors
+---------------------------------------
+
+:Expectation: disabled
+:Severity: *medium*
+
+To ensure reproducible system latency measurements, disable the
+non-``PERFORMANCE`` CPU frequency governors when possible.  This avoids the
+risk of unknown userspace tasks implicitly or explicitly setting a
+different CPU frequency governor, and thus achieving different latency
+results across the system's runtime.
+
+If disabling other frequency governors is not an option, then
+``CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE`` should be enabled.  In that case,
+users should set a *stable* CPU frequency setting during the system
+runtime, as changing the CPU frequency will increase the system latency and
+affect latency measurements reproducibility.  If a lower CPU frequency is
+desired, then ``CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE`` should be set.
+
+The ``ONDEMAND`` CPU frequency governor should *not* be enabled in a
+real-time system since it dramatically affects determinism depending on the
+workload.
+
+For more information, please check :doc:`/admin-guide/pm/cpufreq`.
+
+Kernel Debug Options
+====================
+
+Most kernel debug options add runtime overhead that increases the
+worst-case latency.
+
+.. caution::
+
+  During development and early testing, users are encouraged to run their
+  real-time workloads and peripherals with lockdep (:ref:`lockdep`) and
+  other kernel debug options enabled, for a considerable amount of time.
+  Such workloads might trigger kernel code paths that were not triggered
+  during the internal Linux real-time kernel development, thus helping to
+  uncover locking and other types of kernel bugs.
+
+Problematic debug options
+-------------------------
+
+.. _tracers:
+
+``CONFIG_IRQSOFF_TRACER`` and ``CONFIG_PREEMPT_TRACER``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Severity: *high*
+
+These tracers do incur measurable latency overhead even when tracing is not
+currently active.
+
+``CONFIG_LOCKUP_DETECTOR``
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Severity: *high*
+
+The lockup detector creates kernel timer callbacks that execute every few
+seconds, in hard-IRQ context, even on real-time kernels.  These periodic
+interrupts can cause latency spikes.
+
+Users should use hardware watchdogs instead, which will provide a similar
+functionality without the software-induced latency.
+
+.. _lockdep:
+
+``CONFIG_PROVE_LOCKING``
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Severity: *high*
+
+Proving the correctness of all kernel locking adds substantial overhead
+and significantly increases worst-case latency.
+
+Allowed kernel debug options
+----------------------------
+
+Kernel debug options which are not included in this list should be enabled
+with caution, after extensive auditing of their impact on system latency.
+
+``CONFIG_DEBUG_ATOMIC_SLEEP``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This sanity check catches common kernel programming errors with
+a tolerable latency cost.
+
+``CONFIG_DEBUG_BUGVERBOSE``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This improves the debugging capabilities without affecting normal
+operation latency.
+
+``CONFIG_DEBUG_FS``
+^^^^^^^^^^^^^^^^^^^
+
+This is safe to include in real-time kernels, *provided that debugfs is
+not accessed during production runtime*.
+
+``CONFIG_DEBUG_INFO``
+^^^^^^^^^^^^^^^^^^^^^
+
+This increases the kernel image size but has no latency impact.  It is
+also essential for meaningful crash dumps and profiling.
+
+``CONFIG_DEBUG_KERNEL``
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Meta-option which allows debug features to be enabled.  This configuration
+option has no runtime impact, but be aware of any debug features that it
+may have allowed to be enabled.
+
+Summary
+=======
+
+There is no "one size fits all" solution for configuring a real-time Linux
+system.  Beginning with the system real-time requirements, integrators
+must consider the features and functions of the system's hardware, kernel,
+and userspace.  All such components must be properly configured in order
+to establish and constrain the system's maximum latency.
+
+With that in mind, any incorrect real-time kernel configuration could cause
+a new maximum latency that shows up at the wrong time and is catastrophic
+for the real-time system's latency.
+
+References
+==========
+
+.. [1] See :doc:`/admin-guide/kernel-parameters`
+
+.. _development work: https://lore.kernel.org/r/20260227170103.4042157-1-bigeasy@linutronix.de
+
+.. _Real-Time and Graphics\: A Contradiction?: https://web.archive.org/web/20221025085614/https://linutronix.de/PDF/Realtime_and_graphics-acontradiction2021.pdf
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