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Subject[PATCH tip/core/rcu 14/55] rcu: Add event-tracing for RCU callback invocation
Date
From: Paul E. McKenney <paul.mckenney@linaro.org>

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>
---
include/linux/rcupdate.h | 50 ----------------------
include/trace/events/rcu.h | 98 ++++++++++++++++++++++++++++++++++++++++++++
kernel/rcu.h | 79 +++++++++++++++++++++++++++++++++++
kernel/rcupdate.c | 5 ++
kernel/rcutiny.c | 26 +++++++++++-
kernel/rcutree.c | 15 +++++-
6 files changed, 219 insertions(+), 54 deletions(-)
create mode 100644 include/trace/events/rcu.h
create mode 100644 kernel/rcu.h

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 6433a6f..c61a535 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -794,44 +794,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
#define RCU_INIT_POINTER(p, v) \
p = (typeof(*v) __force __rcu *)(v)

-/*
- * 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 */
-
static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
{
return offset < 4096;
@@ -850,18 +812,6 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset)
call_rcu(head, (rcu_callback)offset);
}

-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))
- kfree((void *)head - offset);
- else
- head->func(head);
-}
-
/**
* kfree_rcu() - kfree an object after a grace period.
* @ptr: pointer to kfree
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
new file mode 100644
index 0000000..db3f6e9
--- /dev/null
+++ b/include/trace/events/rcu.h
@@ -0,0 +1,98 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM rcu
+
+#if !defined(_TRACE_RCU_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_RCU_H
+
+#include <linux/tracepoint.h>
+
+/*
+ * Tracepoint for calling rcu_do_batch, performed to start callback invocation:
+ */
+TRACE_EVENT(rcu_batch_start,
+
+ TP_PROTO(long callbacks_ready, int blimit),
+
+ TP_ARGS(callbacks_ready, blimit),
+
+ TP_STRUCT__entry(
+ __field( long, callbacks_ready )
+ __field( int, blimit )
+ ),
+
+ TP_fast_assign(
+ __entry->callbacks_ready = callbacks_ready;
+ __entry->blimit = blimit;
+ ),
+
+ TP_printk("CBs=%ld bl=%d", __entry->callbacks_ready, __entry->blimit)
+);
+
+/*
+ * Tracepoint for the invocation of a single RCU callback
+ */
+TRACE_EVENT(rcu_invoke_callback,
+
+ TP_PROTO(struct rcu_head *rhp),
+
+ TP_ARGS(rhp),
+
+ TP_STRUCT__entry(
+ __field( void *, rhp )
+ __field( void *, func )
+ ),
+
+ TP_fast_assign(
+ __entry->rhp = rhp;
+ __entry->func = rhp->func;
+ ),
+
+ TP_printk("rhp=%p func=%pf", __entry->rhp, __entry->func)
+);
+
+/*
+ * Tracepoint for the invocation of a single RCU kfree callback
+ */
+TRACE_EVENT(rcu_invoke_kfree_callback,
+
+ TP_PROTO(struct rcu_head *rhp, unsigned long offset),
+
+ TP_ARGS(rhp, offset),
+
+ TP_STRUCT__entry(
+ __field(void *, rhp )
+ __field(unsigned long, offset )
+ ),
+
+ TP_fast_assign(
+ __entry->rhp = rhp;
+ __entry->offset = offset;
+ ),
+
+ TP_printk("rhp=%p func=%ld", __entry->rhp, __entry->offset)
+);
+
+/*
+ * Tracepoint for leaving rcu_do_batch, performed after callback invocation:
+ */
+TRACE_EVENT(rcu_batch_end,
+
+ TP_PROTO(int callbacks_invoked),
+
+ TP_ARGS(callbacks_invoked),
+
+ TP_STRUCT__entry(
+ __field( int, callbacks_invoked )
+ ),
+
+ TP_fast_assign(
+ __entry->callbacks_invoked = callbacks_invoked;
+ ),
+
+ TP_printk("CBs-invoked=%d", __entry->callbacks_invoked)
+);
+
+#endif /* _TRACE_RCU_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
diff --git a/kernel/rcu.h b/kernel/rcu.h
new file mode 100644
index 0000000..7bc1643
--- /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 a088c90..5031caf 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 f544e34..19453ba 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 a7c6bce..45dcc20 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;
--
1.7.3.2


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