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authorPaul E. McKenney <paulmck@kernel.org>2022-02-17 23:26:00 +0300
committerPaul E. McKenney <paulmck@kernel.org>2022-04-21 01:57:02 +0300
commit6172de3c7f1171e55314bfc5ee4ae6edd225b048 (patch)
tree5d789928c265a96f5e88bba489d821a09d7f0771 /Documentation/RCU
parent3123109284176b1532874591f7c81f3837bbdc17 (diff)
downloadlinux-6172de3c7f1171e55314bfc5ee4ae6edd225b048.tar.xz
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>
Diffstat (limited to 'Documentation/RCU')
-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.rst32
1 files changed, 32 insertions, 0 deletions
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().
+
+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
-----------------------