On Wed, Apr 20, 2022 at 11:32:31AM -0400, Steven Rostedt wrote: > 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? You tell me! ;-) If there are no longer any users, I would be happy to get rid of it. As of v5.18-rc1, the only user is ftrace. > > + > > +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> I will apply this on the next rebase, thank you! Thanx, Paul
