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
