[PATCH v4] docs: real-time: mention the hrtimer sleeper HARD path

[Liang Hao]

The Timers section of Documentation/core-api/real-time/differences.rst
describes the PREEMPT_RT default (softirq / ktimers) and HRTIMER_MODE_HARD,
but not the sleeper helper: hrtimer_setup_sleeper_on_stack() marks the
timer HRTIMER_MODE_HARD when the current task is RT or DEADLINE and
HRTIMER_MODE_SOFT was not requested, so the wakeup runs in hard interrupt
context.

Document that behaviour. Also rename "ktimersd" to "ktimers/%u" to match
the per-CPU thread name.

No code or behaviour change.

Signed-off-by: Liang Hao <haohlliang@xxxxxxxxx>
---
v3 -> v4:
- drop the Documentation/timers/hrtimers.rst section; extend differences.rst
  instead, per review
- add only the sleeper HARD wording and the ktimers/%u rename
- do not restate lowest-priority / cancel-PI points already covered elsewhere
- avoid vague "and similar" caller lists

 Documentation/core-api/real-time/differences.rst | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/Documentation/core-api/real-time/differences.rst b/Documentation/core-api/real-time/differences.rst
index a129570dab5a..c8acfba051eb 100644
--- a/Documentation/core-api/real-time/differences.rst
+++ b/Documentation/core-api/real-time/differences.rst
@@ -119,12 +119,17 @@ timers initialized with the HRTIMER_MODE_SOFT flag, which are executed in
 softirq context.
 
 On a PREEMPT_RT kernel, this behavior is reversed: hrtimers are executed in
-softirq context by default, typically within the ktimersd thread. This thread
-runs at the lowest real-time priority, ensuring it executes before any
-SCHED_OTHER tasks but does not interfere with higher-priority real-time
+softirq context by default, typically within the per-CPU ktimers/%u thread.
+This thread runs at the lowest real-time priority, ensuring it executes before
+any SCHED_OTHER tasks but does not interfere with higher-priority real-time
 threads. To explicitly request execution in hard interrupt context on
 PREEMPT_RT, the timer must be marked with the HRTIMER_MODE_HARD flag.
 
+hrtimer_setup_sleeper_on_stack() marks the sleeper HRTIMER_MODE_HARD when the
+current task is in a real-time or deadline scheduling class and
+HRTIMER_MODE_SOFT was not requested, so the wakeup runs in hard interrupt
+context rather than on ktimers/%u.
+
 Memory allocation
 -----------------
 
-- 
2.50.1 (Apple Git-155)