[PATCH v5] docs: real-time: mention the hrtimer sleeper HARD path

[Liang Hao <haohlliang@gmail.com>]

The Timers section documents the PREEMPT_RT softirq default and
HRTIMER_MODE_HARD, but not the sleeper helper used by userspace sleeps.
Describe that path, and rename "ktimersd" to "ktimers" to match
ksoftirqd naming in this document.

No code or behaviour change.

Signed-off-by: Liang Hao <haohlliang@gmail.com>
---
Thanks!

v4 -> v5:
- use "ktimers" instead of "ktimers/%u"
- replace the sleeper paragraph with wording suggested by Sebastian

 Documentation/core-api/real-time/differences.rst | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/core-api/real-time/differences.rst b/Documentation/core-api/real-time/differences.rst
index a129570dab5a..04be5789b5fc 100644
--- a/Documentation/core-api/real-time/differences.rst
+++ b/Documentation/core-api/real-time/differences.rst
@@ -119,12 +119,20 @@ 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
+softirq context by default, typically within the ktimers 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.
 
+Userland sleepers usually deploy a hrtimer to guarantee a precise wakeup
+time. The timer is initialized with hrtimer_setup_sleeper_on_stack(), which
+distinguishes between real-time and regular tasks. The hrtimer of a task
+without a real-time priority is initialized with HRTIMER_MODE_SOFT, but for
+real-time priorities HRTIMER_MODE_HARD is used. This ensures that real-time
+tasks are woken up as soon as possible while ordinary tasks cannot block the
+CPU with a thundering herd of wakeups.
+
 Memory allocation
 -----------------
 
-- 
2.50.1 (Apple Git-155)