[PATCH rcu 30/32] rcu-tasks: Update comments

Paul E. McKenney
2022-06-20T22:56:36Z

This commit updates comments to reflect the changes in the series
of commits that eliminated the full task-list scan.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
---
kernel/rcu/tasks.h | 71 +++++++++++++++++++++-------------------------
1 file changed, 33 insertions(+), 38 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index bf9cc5bc4ae52..df6b2cb2f205d 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1138,11 +1138,10 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
// 3. Avoids expensive read-side instructions, having overhead similar
// to that of Preemptible RCU.
//
-// There are of course downsides. The grace-period code can send IPIs to
-// CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.
-// It is necessary to scan the full tasklist, much as for Tasks RCU. There
-// is a single callback queue guarded by a single lock, again, much as for
-// Tasks RCU. If needed, these downsides can be at least partially remedied.
+// There are of course downsides. For example, the grace-period code
+// can send IPIs to CPUs, even when those CPUs are in the idle loop or
+// in nohz_full userspace. If needed, these downsides can be at least
+// partially remedied.
//
// Perhaps most important, this variant of RCU does not affect the vanilla
// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
@@ -1155,38 +1154,30 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
// 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 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. Note that IPIs are used
-// to invoke trc_read_check_handler() in the context of running tasks
-// in order to avoid ordering overhead on common-case shared-variable
-// accessses.
+// Disables CPU hotplug, adds all currently executing tasks to the
+// holdout list, then checks the state of all tasks that blocked
+// or were preempted within their current RCU Tasks Trace read-side
+// critical section, adding them to the holdout list if appropriate.
+// Finally, this function re-enables CPU hotplug.
+// The ->pertask_func() pointer is NULL, so there is no per-task processing.
// 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.
+// Invokes synchronize_rcu() to wait for late-stage exiting tasks
+// to finish 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.
+// corresponding task is removed from the holdout list. Once this
+// list is empty, the grace period has completed.
// 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.
+// Provides the needed full memory barrier and does debug checks.
//
// 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).
+// 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 operations
+// on .b.need_qs flag of each task involved in this process, or 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
@@ -1245,7 +1236,10 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
}
EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);

-/* If we are the last reader, wake up the grace-period kthread. */
+/*
+ * If we are the last reader, signal the grace-period kthread.
+ * Also remove from the per-CPU list of blocked tasks.
+ */
void rcu_read_unlock_trace_special(struct task_struct *t)
{
unsigned long flags;
@@ -1336,9 +1330,9 @@ static void trc_read_check_handler(void *t_in)
if (unlikely(nesting < 0))
goto reset_ipi;

- // Get here if the task is in a read-side critical section. Set
- // its state so that it will awaken the grace-period kthread upon
- // exit from that critical section.
+ // Get here if the task is in a read-side critical section.
+ // Set its state so that it will update state for the grace-period
+ // kthread upon exit from that critical section.
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);

reset_ipi:
@@ -1387,7 +1381,7 @@ static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
return 0; // In QS, so done.
}
if (nesting < 0)
- return -EINVAL; // QS transitioning, try again later.
+ return -EINVAL; // Reader transitioning, try again later.

// The task is in a read-side critical section, so set up its
// state so that it will update state upon exit from that critical
@@ -1492,11 +1486,12 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));

- // Disable CPU hotplug across the CPU scan.
- // This also waits for all readers in CPU-hotplug code paths.
+ // Disable CPU hotplug across the CPU scan for the benefit of
+ // any IPIs that might be needed. This also waits for all readers
+ // in CPU-hotplug code paths.
cpus_read_lock();

- // These smp_call_function_single() calls are serialized to
+ // These rcu_tasks_trace_pertask_prep() calls are serialized to
// allow safe access to the hop list.
for_each_online_cpu(cpu) {
rcu_read_lock();
@@ -1608,7 +1603,7 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
{
struct task_struct *g, *t;

- // Disable CPU hotplug across the holdout list scan.
+ // Disable CPU hotplug across the holdout list scan for IPIs.
cpus_read_lock();

list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
--
2.31.1.189.g2e36527f23