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docs: Add documentation for rude and trace RCU flavors 

This commit belatedly adds documentation of Tasks Rude RCU and Tasks
Trace RCU to RCU's requirements document.

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-02-17 12:26:00 -08:00
parent 3123109284
commit 6172de3c7f
1 changed files with 32 additions and 0 deletions
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Documentation/RCU/Design/Requirements/Requirements.rst
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@ -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().
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.
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
-----------------------