rcu: Add event-tracing for RCU callback invocation

There was recently some controversy about the overhead of invoking RCU
callbacks.  Add TRACE_EVENT()s to obtain fine-grained timings for the
start and stop of a batch of callbacks and also for each callback invoked.

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

4 files changed, 121 insertions, 4 deletions

diff --git a/kernel/rcu.h b/kernel/rcu.h
new file mode 100644
index 000000000000..7bc16436aba0
--- /dev/null
+++ b/kernel/rcu.h
@@ -0,0 +1,79 @@
+/*
+ * Read-Copy Update definitions shared among RCU implementations.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright IBM Corporation, 2011
+ *
+ * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+ */
+
+#ifndef __LINUX_RCU_H
+#define __LINUX_RCU_H
+
+/*
+ * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
+ * by call_rcu() and rcu callback execution, and are therefore not part of the
+ * RCU API. Leaving in rcupdate.h because they are used by all RCU flavors.
+ */
+
+#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+# define STATE_RCU_HEAD_READY   0
+# define STATE_RCU_HEAD_QUEUED  1
+
+extern struct debug_obj_descr rcuhead_debug_descr;
+
+static inline void debug_rcu_head_queue(struct rcu_head *head)
+{
+        WARN_ON_ONCE((unsigned long)head & 0x3);
+        debug_object_activate(head, &rcuhead_debug_descr);
+        debug_object_active_state(head, &rcuhead_debug_descr,
+                                  STATE_RCU_HEAD_READY,
+                                  STATE_RCU_HEAD_QUEUED);
+}
+
+static inline void debug_rcu_head_unqueue(struct rcu_head *head)
+{
+        debug_object_active_state(head, &rcuhead_debug_descr,
+                                  STATE_RCU_HEAD_QUEUED,
+                                  STATE_RCU_HEAD_READY);
+        debug_object_deactivate(head, &rcuhead_debug_descr);
+}
+#else   /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
+static inline void debug_rcu_head_queue(struct rcu_head *head)
+{
+}
+
+static inline void debug_rcu_head_unqueue(struct rcu_head *head)
+{
+}
+#endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
+
+extern void kfree(const void *);
+
+static inline void __rcu_reclaim(struct rcu_head *head)
+{
+        unsigned long offset = (unsigned long)head->func;
+
+        if (__is_kfree_rcu_offset(offset)) {
+                trace_rcu_invoke_kfree_callback(head, offset);
+                kfree((void *)head - offset);
+        } else {
+                trace_rcu_invoke_callback(head);
+                head->func(head);
+        }
+}
+
+#endif /* __LINUX_RCU_H */
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 09b3b1b54e02..ca0d23b6b3e8 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -46,6 +46,11 @@
 #include <linux/module.h>
 #include <linux/hardirq.h>
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/rcu.h>
+
+#include "rcu.h"
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 static struct lock_class_key rcu_lock_key;
 struct lockdep_map rcu_lock_map =
diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index f544e343256a..19453ba1392e 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -37,6 +37,25 @@
 #include <linux/cpu.h>
 #include <linux/prefetch.h>
 
+#ifdef CONFIG_RCU_TRACE
+
+#include <trace/events/rcu.h>
+
+#else /* #ifdef CONFIG_RCU_TRACE */
+
+/* No by-default tracing in TINY_RCU: Keep TINY_RCU tiny! */
+static void trace_rcu_invoke_kfree_callback(struct rcu_head *rhp,
+                                            unsigned long offset)
+{
+}
+static void trace_rcu_invoke_callback(struct rcu_head *head)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_TRACE */
+
+#include "rcu.h"
+
 /* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
 static struct task_struct *rcu_kthread_task;
 static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
@@ -161,11 +180,15 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
         RCU_TRACE(int cb_count = 0);
 
         /* If no RCU callbacks ready to invoke, just return. */
-        if (&rcp->rcucblist == rcp->donetail)
+        if (&rcp->rcucblist == rcp->donetail) {
+                RCU_TRACE(trace_rcu_batch_start(0, -1));
+                RCU_TRACE(trace_rcu_batch_end(0));
                 return;
+        }
 
         /* Move the ready-to-invoke callbacks to a local list. */
         local_irq_save(flags);
+        RCU_TRACE(trace_rcu_batch_start(0, -1));
         list = rcp->rcucblist;
         rcp->rcucblist = *rcp->donetail;
         *rcp->donetail = NULL;
@@ -187,6 +210,7 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
                 RCU_TRACE(cb_count++);
         }
         RCU_TRACE(rcu_trace_sub_qlen(rcp, cb_count));
+        RCU_TRACE(trace_rcu_batch_end(cb_count));
 }
 
 /*
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a7c6bce1af83..45dcc2036a1e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -52,6 +52,9 @@
 #include <linux/prefetch.h>
 
 #include "rcutree.h"
+#include <trace/events/rcu.h>
+
+#include "rcu.h"
 
 /* Data structures. */
 
@@ -1190,17 +1193,22 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 {
         unsigned long flags;
         struct rcu_head *next, *list, **tail;
-        int count;
+        int bl, count;
 
         /* If no callbacks are ready, just return.*/
-        if (!cpu_has_callbacks_ready_to_invoke(rdp))
+        if (!cpu_has_callbacks_ready_to_invoke(rdp)) {
+                trace_rcu_batch_start(0, 0);
+                trace_rcu_batch_end(0);
                 return;
+        }
 
         /*
          * Extract the list of ready callbacks, disabling to prevent
          * races with call_rcu() from interrupt handlers.
          */
         local_irq_save(flags);
+        bl = rdp->blimit;
+        trace_rcu_batch_start(rdp->qlen, bl);
         list = rdp->nxtlist;
         rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
         *rdp->nxttail[RCU_DONE_TAIL] = NULL;
@@ -1218,11 +1226,12 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
                 debug_rcu_head_unqueue(list);
                 __rcu_reclaim(list);
                 list = next;
-                if (++count >= rdp->blimit)
+                if (++count >= bl)
                         break;
         }
 
         local_irq_save(flags);
+        trace_rcu_batch_end(count);
 
         /* Update count, and requeue any remaining callbacks. */
         rdp->qlen -= count;