[PATCH rcu 1/2] doc: Update Tasks RCU and Tasks Rude RCU description in Requirements.rst

[Paul E. McKenney]

This commit adds more detail to the Tasks RCU and Tasks Rude RCU
descriptions in Requirements.rst.  While in the area, add Tasks Trace
RCU to the Tasks-RCU table of contents.

Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 .../RCU/Design/Requirements/Requirements.rst     | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index cccafdaa1f849..f511476b45506 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2357,6 +2357,7 @@ section.
 #. `Sched Flavor (Historical)`_
 #. `Sleepable RCU`_
 #. `Tasks RCU`_
+#. `Tasks Trace RCU`_
 
 Bottom-Half Flavor (Historical)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -2610,6 +2611,16 @@ critical sections that are delimited by voluntary context switches, that
 is, calls to schedule(), cond_resched(), and
 synchronize_rcu_tasks(). In addition, transitions to and from
 userspace execution also delimit tasks-RCU read-side critical sections.
+Idle tasks are ignored by Tasks RCU, and Tasks Rude RCU may be used to
+interact with them.
+
+Note well that involuntary context switches are *not* Tasks-RCU quiescent
+states.  After all, in preemptible kernels, a task executing code in a
+trampoline might be preempted.  In this case, the Tasks-RCU grace period
+clearly cannot end until that task resumes and its execution leaves that
+trampoline.  This means, among other things, that cond_resched() does
+not provide a Tasks RCU quiescent state.  (Instead, use rcu_softirq_qs()
+from softirq or rcu_tasks_classic_qs() otherwise.)
 
 The tasks-RCU API is quite compact, consisting only of
 call_rcu_tasks(), synchronize_rcu_tasks(), and
@@ -2632,6 +2643,11 @@ moniker.  And this operation is considered to be quite rude by real-time
 workloads that don't want their ``nohz_full`` CPUs receiving IPIs and
 by battery-powered systems that don't want their idle CPUs to be awakened.
 
+Once kernel entry/exit and deep-idle functions have been properly tagged
+``noinstr``, Tasks RCU can start paying attention to idle tasks (except
+those that are idle from RCU's perspective) and then Tasks Rude RCU can
+be removed from the kernel.
+
 The tasks-rude-RCU API is also reader-marking-free and thus quite compact,
 consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(),
 and rcu_barrier_tasks_rude().
-- 
2.40.1