rcu: Add tracing for RCU_FAST_NO_HZ

This commit adds trace_rcu_prep_idle(), which is invoked from
rcu_prepare_for_idle() and rcu_wake_cpu() to trace attempts on
the part of RCU to force CPUs into dyntick-idle mode.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

2 files changed, 52 insertions, 3 deletions

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 7f6877a35051..debe453c9623 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -275,6 +275,42 @@ TRACE_EVENT(rcu_dyntick,
 );
 
 /*
+ * Tracepoint for RCU preparation for idle, the goal being to get RCU
+ * processing done so that the current CPU can shut off its scheduling
+ * clock and enter dyntick-idle mode.  One way to accomplish this is
+ * to drain all RCU callbacks from this CPU, and the other is to have
+ * done everything RCU requires for the current grace period.  In this
+ * latter case, the CPU will be awakened at the end of the current grace
+ * period in order to process the remainder of its callbacks.
+ *
+ * These tracepoints take a string as argument:
+ *
+ *      "No callbacks": Nothing to do, no callbacks on this CPU.
+ *      "In holdoff": Nothing to do, holding off after unsuccessful attempt.
+ *      "Dyntick with callbacks": Callbacks remain, but RCU doesn't need CPU.
+ *      "Begin holdoff": Attempt failed, don't retry until next jiffy.
+ *      "More callbacks": Still more callbacks, try again to clear them out.
+ *      "Callbacks drained": All callbacks processed, off to dyntick idle!
+ *      "CPU awakened at GP end":
+ */
+TRACE_EVENT(rcu_prep_idle,
+
+        TP_PROTO(char *reason),
+
+        TP_ARGS(reason),
+
+        TP_STRUCT__entry(
+                __field(char *, reason)
+        ),
+
+        TP_fast_assign(
+                __entry->reason = reason;
+        ),
+
+        TP_printk("%s", __entry->reason)
+);
+
+/*
  * Tracepoint for the registration of a single RCU callback function.
  * The first argument is the type of RCU, the second argument is
  * a pointer to the RCU callback itself, and the third element is the
@@ -482,6 +518,7 @@ TRACE_EVENT(rcu_torture_read,
 #define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, grplo, grphi, gp_tasks) do { } while (0)
 #define trace_rcu_fqs(rcuname, gpnum, cpu, qsevent) do { } while (0)
 #define trace_rcu_dyntick(polarity, oldnesting, newnesting) do { } while (0)
+#define trace_rcu_prep_idle(reason) do { } while (0)
 #define trace_rcu_callback(rcuname, rhp, qlen) do { } while (0)
 #define trace_rcu_kfree_callback(rcuname, rhp, offset, qlen) do { } while (0)
 #define trace_rcu_batch_start(rcuname, qlen, blimit) do { } while (0)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index b70ca8cc52e1..6467f5669ab7 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2031,10 +2031,13 @@ static void rcu_prepare_for_idle(int cpu)
         /* If no callbacks or in the holdoff period, enter dyntick-idle. */
         if (!rcu_cpu_has_callbacks(cpu)) {
                 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
+                trace_rcu_prep_idle("No callbacks");
                 return;
         }
-        if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies)
+        if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies) {
+                trace_rcu_prep_idle("In holdoff");
                 return;
+        }
 
         /* Check and update the rcu_dyntick_drain sequencing. */
         if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
@@ -2044,9 +2047,11 @@ static void rcu_prepare_for_idle(int cpu)
                 /* We have hit the limit, so time to give up. */
                 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
                 if (!rcu_pending(cpu)) {
+                        trace_rcu_prep_idle("Dyntick with callbacks");
                         per_cpu(rcu_awake_at_gp_end, cpu) = 1;
                         return;  /* Nothing to do immediately. */
                 }
+                trace_rcu_prep_idle("Begin holdoff");
                 invoke_rcu_core();  /* Force the CPU out of dyntick-idle. */
                 return;
         }
@@ -2073,9 +2078,15 @@ static void rcu_prepare_for_idle(int cpu)
                 c = c || per_cpu(rcu_bh_data, cpu).nxtlist;
         }
 
-        /* If RCU callbacks are still pending, RCU still needs this CPU. */
-        if (c)
+        /*
+         * If RCU callbacks are still pending, RCU still needs this CPU.
+         * So try forcing the callbacks through the grace period.
+         */
+        if (c) {
+                trace_rcu_prep_idle("More callbacks");
                 invoke_rcu_core();
+        } else
+                trace_rcu_prep_idle("Callbacks drained");
 }
 
 /*
@@ -2085,6 +2096,7 @@ static void rcu_prepare_for_idle(int cpu)
  */
 static void rcu_wake_cpu(void *unused)
 {
+        trace_rcu_prep_idle("CPU awakened at GP end");
         invoke_rcu_core();
 }