rcu-tasks: Add rcu_dynticks_zero_in_eqs() effectiveness statistics

This commit adds counts of the number of calls and number of successful
calls to rcu_dynticks_zero_in_eqs(), which are printed at the end
of rcutorture runs and at stall time.  This allows evaluation of the
effectiveness of rcu_dynticks_zero_in_eqs().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index f272e8f..ce65883 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -725,6 +725,11 @@ DECLARE_WAIT_QUEUE_HEAD(trc_wait); // List of holdout tasks.
 // Record outstanding IPIs to each CPU.  No point in sending two...
 static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
 
+// The number of detections of task quiescent state relying on
+// heavyweight readers executing explicit memory barriers.
+unsigned long n_heavy_reader_attempts;
+unsigned long n_heavy_reader_updates;
+
 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
 DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
                 "RCU Tasks Trace");
@@ -830,9 +835,11 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
                // If heavyweight readers are enabled on the remote task,
                // we can inspect its state despite its currently running.
                // However, we cannot safely change its state.
+               n_heavy_reader_attempts++;
                if (!ofl && // Check for "running" idle tasks on offline CPUs.
                    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
                        return false; // No quiescent state, do it the hard way.
+               n_heavy_reader_updates++;
                in_qs = true;
        } else {
                in_qs = likely(!t->trc_reader_nesting);
@@ -1147,9 +1154,11 @@ core_initcall(rcu_spawn_tasks_trace_kthread);
 
 static void show_rcu_tasks_trace_gp_kthread(void)
 {
-       char buf[32];
+       char buf[64];
 
-       sprintf(buf, "N%d", atomic_read(&trc_n_readers_need_end));
+       sprintf(buf, "N%d h:%lu/%lu", atomic_read(&trc_n_readers_need_end),
+               data_race(n_heavy_reader_updates),
+               data_race(n_heavy_reader_attempts));
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
 }