On Mon, 18 Apr 2022 15:50:32 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > This commit belatedly adds documentation of Tasks Rude RCU and Tasks > Trace RCU to RCU's requirements document. > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > --- > .../RCU/Design/Requirements/Requirements.rst | 32 +++++++++++++++++++ > 1 file changed, 32 insertions(+) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst > index 45278e2974c0..ff2be1ac54c4 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > @@ -2654,6 +2654,38 @@ synchronize_rcu(), and rcu_barrier(), respectively. In > three APIs are therefore implemented by separate functions that check > for voluntary context switches. > > +Tasks Rude RCU > +~~~~~~~~~~~~~~ > + > +Some forms of tracing need to wait for all preemption-disabled regions > +of code running on any online CPU, including those executed when RCU is > +not watching. This means that synchronize_rcu() is insufficient, and > +Tasks Rude RCU must be used instead. This flavor of RCU does its work by > +forcing a workqueue to be scheduled on each online CPU, hence the "Rude" > +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. > + > +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(). Are we going to be able to get rid of the "rude" version once we have all tracing in a RCU visible section? > + > +Tasks Trace RCU > +~~~~~~~~~~~~~~~ > + > +Some forms of tracing need to sleep in readers, but cannot tolerate > +SRCU's read-side overhead, which includes a full memory barrier in both > +srcu_read_lock() and srcu_read_unlock(). This need is handled by a > +Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with > +readers. Real-time systems that cannot tolerate IPIs may build their > +kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at > +the expense of adding full memory barriers to the read-side primitives. If NOHZ_FULL is enabled, is there a way to also be able to have this full mb on RT removed as well? > + > +The tasks-trace-RCU API is also reasonably compact, > +consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(), > +rcu_read_lock_trace_held(), call_rcu_tasks_trace(), > +synchronize_rcu_tasks_trace(), and rcu_barrier_tasks_trace(). > + > Possible Future Changes > ----------------------- > For the documentation: Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx> -- Steve
