linux-kernel - [PATCH tip/core/rcu 2/2] rcu-tasks: Add block comment laying out RCU Tasks Trace design

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Message-Id: <20210304003245.24327-2-paulmck@kernel.org>
Date:   Wed,  3 Mar 2021 16:32:45 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 2/2] rcu-tasks: Add block comment laying out RCU Tasks Trace design

From: "Paul E. McKenney" <paulmck@...nel.org>

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 <peterz@...radead.org>
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
[ paulmck: Fix commit log per Mathieu Desnoyers feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tasks.h | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 17c8ebe..f818357 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -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:
+//
+// rcu_tasks_trace_pregp_step():
+//	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 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.
+// rcu_tasks_trace_postscan():
+//	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.
+// rcu_tasks_trace_postgp():
+//	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.
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
-- 
2.9.5