rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built

Systems running CPU-bound real-time task do not want IPIs sent to CPUs
executing nohz_full userspace tasks.  Battery-powered systems don't
want IPIs sent to idle CPUs in low-power mode.  Unfortunately, RCU tasks
trace can and will send such IPIs in some cases.

Both of these situations occur only when the target CPU is in RCU
dyntick-idle mode, in other words, when RCU is not watching the
target CPU.  This suggests that CPUs in dyntick-idle mode should use
memory barriers in outermost invocations of rcu_read_lock_trace()
and rcu_read_unlock_trace(), which would allow the RCU tasks trace
grace period to directly read out the target CPU's read-side state.
One challenge is that RCU tasks trace is not targeting a specific
CPU, but rather a task.  And that task could switch from one CPU to
another at any time.

This commit therefore uses try_invoke_on_locked_down_task()
and checks for task_curr() in trc_inspect_reader_notrunning().
When this condition holds, the target task is running and cannot move.
If CONFIG_TASKS_TRACE_RCU_READ_MB=y, the new rcu_dynticks_zero_in_eqs()
function can be used to check if the specified integer (in this case,
t->trc_reader_nesting) is zero while the target CPU remains in that same
dyntick-idle sojourn.  If so, the target task is in a quiescent state.
If not, trc_read_check_handler() must indicate failure so that the
grace-period kthread can take appropriate action or retry after an
appropriate delay, as the case may be.

With this change, given CONFIG_TASKS_TRACE_RCU_READ_MB=y, if a given
CPU remains idle or a given task continues executing in nohz_full mode,
the RCU tasks trace grace-period kthread will detect this without the
need to send an IPI.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0bbcbf398169..573fd78a7bca 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -252,6 +252,7 @@ static void rcu_dynticks_eqs_enter(void)
 	 * critical sections, and we also must force ordering with the
 	 * next idle sojourn.
 	 */
+	rcu_dynticks_task_trace_enter();  // Before ->dynticks update!
 	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
 	// RCU is no longer watching.  Better be in extended quiescent state!
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
@@ -278,6 +279,7 @@ static void rcu_dynticks_eqs_exit(void)
 	 */
 	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
 	// RCU is now watching.  Better not be in an extended quiescent state!
+	rcu_dynticks_task_trace_exit();  // After ->dynticks update!
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 		     !(seq & RCU_DYNTICK_CTRL_CTR));
 	if (seq & RCU_DYNTICK_CTRL_MASK) {
@@ -350,6 +352,28 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
 }
 
 /*
+ * Return true if the referenced integer is zero while the specified
+ * CPU remains within a single extended quiescent state.
+ */
+bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	int snap;
+
+	// If not quiescent, force back to earlier extended quiescent state.
+	snap = atomic_read(&rdp->dynticks) & ~(RCU_DYNTICK_CTRL_MASK |
+					       RCU_DYNTICK_CTRL_CTR);
+
+	smp_rmb(); // Order ->dynticks and *vp reads.
+	if (READ_ONCE(*vp))
+		return false;  // Non-zero, so report failure;
+	smp_rmb(); // Order *vp read and ->dynticks re-read.
+
+	// If still in the same extended quiescent state, we are good!
+	return snap == (atomic_read(&rdp->dynticks) & ~RCU_DYNTICK_CTRL_MASK);
+}
+
+/*
  * Set the special (bottom) bit of the specified CPU so that it
  * will take special action (such as flushing its TLB) on the
  * next exit from an extended quiescent state.  Returns true if