[PATCH] docs: timers: hrtimers: clarify expiry modes and ktimersd on PREEMPT_RT

[Liang Hao <haohlliang@gmail.com>]

Documentation/timers/hrtimers.rst described the high-resolution timer
subsystem but did not cover the PREEMPT_RT expiry-mode semantics.  On a
PREEMPT_RT kernel a timer that is not explicitly marked
HRTIMER_MODE_HARD is forced into softirq expiry and its callback runs on
the per-CPU ktimers/%u thread at SCHED_FIFO priority 1, regardless of
the priority of the task that armed it -- a SCHED_FIFO task's priority
does not extend into its timer callback.  This is a recurring source of
hard-to-diagnose latency for RT/DL authors who assume the opposite.

Add a dedicated "Expiry modes and PREEMPT_RT" section that documents:

 - the distinction between HRTIMER_MODE_HARD and HRTIMER_MODE_SOFT;
 - the fact that unmarked timers are forced into softirq expiry on
   PREEMPT_RT (__hrtimer_setup());
 - the role of the per-CPU ktimers/%u thread and its fixed SCHED_FIFO
   priority 1 (sched_set_fifo_low());
 - the absence of priority inheritance between the arming task and the
   timer callback on RT;
 - the sleeper exception, where hrtimer_setup_sleeper() automatically
   marks RT/DL-armed timers HRTIMER_MODE_HARD
   (__hrtimer_setup_sleeper());
 - the critical distinction between the *requested* expiry mode (passed
   by the caller) and the *effective* execution context (chosen by the
   kernel and stored in timer->is_soft).

The hrtimer_start tracepoint logs the *requested* mode, not the
effective one.  On PREEMPT_RT a timer armed with the default mode
(e.g. ABS or REL without HARD/SOFT flags) is implicitly forced into
softirq expiry, so the effective soft nature must be inferred from the
*absence* of explicit mode flags in the trace rather than read directly.

Documentation only; no code or behaviour change.

Signed-off-by: Liang Hao <haohlliang@gmail.com>
---
 Documentation/timers/hrtimers.rst | 66 +++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/Documentation/timers/hrtimers.rst b/Documentation/timers/hrtimers.rst
index f88ff8bae89c..ff42377668c1 100644
--- a/Documentation/timers/hrtimers.rst
+++ b/Documentation/timers/hrtimers.rst
@@ -171,3 +171,69 @@ hrtimers-based high-resolution clock implementation, so the hrtimers
 code got a healthy amount of testing and use in practice.
 
 	Thomas Gleixner, Ingo Molnar
+
+
+Expiry modes and PREEMPT_RT
+---------------------------
+
+Each hrtimer carries an expiry mode that determines the execution context
+of its callback:
+
+  * ``HRTIMER_MODE_HARD`` -- the callback runs in hard interrupt context.
+    It must be hardirq-safe (no sleeping locks, no allocations, no
+    scheduling).
+  * ``HRTIMER_MODE_SOFT`` -- the callback runs in softirq context and may
+    use operations that are not hardirq-safe.
+  * Default (neither flag) -- the mode is selected by the subsystem or
+    the kernel configuration.
+
+On ``CONFIG_PREEMPT_RT`` the choice is not optional for most timers:
+any timer not explicitly marked ``HRTIMER_MODE_HARD`` is forced into
+softirq expiry (see ``__hrtimer_setup()``). Instead of executing in
+hardirq context or within the context of the task that armed it, the
+callback runs on the per-CPU ``ktimers/%u`` thread.
+
+The ``ktimersd`` thread operates at ``SCHED_FIFO`` priority 1 (established
+via ``sched_set_fifo_low()``). This priority is fixed and **does not
+inherit the priority of the task that armed the timer**. The effective
+execution context is determined internally by the hrtimer subsystem at
+setup time and is reflected in ``timer->is_soft``; it is not directly
+visible as a mode flag at arming time.
+
+Consequently, even if a timer is armed by a ``SCHED_FIFO`` task with
+priority 99, its callback will execute only when the ``ktimersd`` thread
+(priority 1) is selected to run.
+
+This design ensures that timer processing does not interfere with
+higher-priority real-time workloads, while still providing bounded
+latency relative to ``SCHED_OTHER`` tasks. However, it also means that
+latency-sensitive processing must not rely on implicit priority
+inheritance through the timer arming path.
+
+A notable exception is the sleeper path: a timer set up via
+``hrtimer_setup_sleeper()`` (used by ``clock_nanosleep()`` and similar)
+that is armed by an RT or DEADLINE task is automatically marked
+``HRTIMER_MODE_HARD``, so its wakeup runs in hardirq context and does
+not go through ``ktimersd`` (see ``__hrtimer_setup_sleeper()``).
+
+**Guidelines for RT authors:**
+
+- If the timer callback contains logic that must execute at the priority
+  of the owning RT task, the timer must be declared as
+  ``HRTIMER_MODE_HARD``. Ensure the callback adheres to hardirq context
+  constraints.
+- Alternatively, move the latency-sensitive logic out of the timer
+  callback and into a dedicated, properly prioritized kthread which is
+  woken by the timer.
+
+**Debugging context:** The ``hrtimer_start`` tracepoint logs the
+*requested* expiry mode passed by the caller (e.g. ``ABS``, ``REL``,
+``ABS|HARD``), not the effective mode chosen by the kernel. On
+PREEMPT_RT, any timer armed with the default mode (i.e. the trace shows
+``ABS`` or ``REL`` without ``|SOFT`` or ``|HARD``) is implicitly forced
+into softirq expiry. When such a timer is armed by an RT or DEADLINE
+task, the consequence -- the callback running at ``ktimersd`` priority
+rather than the arming task's -- is the case to watch. The effective
+soft nature of such timers is inferred from the *absence* of an explicit
+mode flag in the trace, combined with the ``CONFIG_PREEMPT_RT``
+configuration.
-- 
2.50.1 (Apple Git-155)