doc: Update Tasks RCU and Tasks Rude RCU description in Requirements.rst

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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Frederic Weisbecker <[email protected]>

Author: paulmckrcu

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().