|author||Paul E. McKenney <firstname.lastname@example.org>||2021-02-25 10:26:00 -0800|
|committer||Paul E. McKenney <email@example.com>||2021-03-08 14:22:02 -0800|
|parent||rcu-tasks: Rectify kernel-doc for struct rcu_tasks (diff)|
rcu-tasks: Add block comment laying out RCU Tasks Trace design
This commit adds a block comment that gives a high-level overview of how RCU tasks trace grace periods progress. It also adds a note about how exiting tasks are handled, plus it gives an overview of the memory ordering. Reported-by: Peter Zijlstra <firstname.lastname@example.org> Reported-by: Mathieu Desnoyers <email@example.com> [ paulmck: Fix commit log per Mathieu Desnoyers feedback. ] Signed-off-by: Paul E. McKenney <firstname.lastname@example.org>
1 files changed, 36 insertions, 0 deletions
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 17c8ebe131af..350ebf5051f9 100644
@@ -726,6 +726,42 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
// readers can operate from idle, offline, and exception entry/exit in no
// way allows rcu_preempt and rcu_sched readers to also do so.
+// The implementation uses rcu_tasks_wait_gp(), which relies on function
+// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread()
+// function sets these function pointers up so that rcu_tasks_wait_gp()
+// invokes these functions in this order:
+// Initialize the count of readers and block CPU-hotplug operations.
+// rcu_tasks_trace_pertask(), invoked on every non-idle task:
+// Initialize per-task state and attempt to identify an immediate
+// quiescent state for that task, or, failing that, attempt to
+// set that task's .need_qs flag so that task's next outermost
+// rcu_read_unlock_trace() will report the quiescent state (in which
+// case the count of readers is incremented). If both attempts fail,
+// the task is added to a "holdout" list.
+// Initialize state and attempt to identify an immediate quiescent
+// state as above (but only for idle tasks), unblock CPU-hotplug
+// operations, and wait for an RCU grace period to avoid races with
+// tasks that are in the process of exiting.
+// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
+// Scans the holdout list, attempting to identify a quiescent state
+// for each task on the list. If there is a quiescent state, the
+// corresponding task is removed from the holdout list.
+// Wait for the count of readers do drop to zero, reporting any stalls.
+// Also execute full memory barriers to maintain ordering with code
+// executing after the grace period.
+// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
+// Pre-grace-period update-side code is ordered before the grace
+// period via the ->cbs_lock and barriers in rcu_tasks_kthread().
+// Pre-grace-period read-side code is ordered before the grace period by
+// atomic_dec_and_test() of the count of readers (for IPIed readers) and by
+// scheduler context-switch ordering (for locked-down non-running readers).
// The lockdep state must be outside of #ifdef to be useful.