[PATCH] rudimentary tracing for Classic RCU

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello!

This is a tracing patch for Classic RCU, which creates an "rcu/rcucb"
file in debugfs.  This patch can be handy when you need to work out
why RCU is refusing to end the current grace period.

Reading from the file results in something like the following:

	rcu: cur=1129  completed=1128  np=0  s=0
		0,3,7
	rcu_bh: cur=-287  completed=-287  np=0  s=0

	online: 0-7

The first two lines are for rcu, the second two for rcu_bh.
The cur= is the current grace-period number, and the completed=
is the number of the last completed grace period.  If these two
numbers are equal, the corresponding flavor of RCU is idle.
The np= is a flag indicating that an additional RCU grace period
beyond the current one will be required.  The s=, if non-zero,
indicates that a round of reschedule IPIs has been send to
attempt to expedite the current grace period.

The second and fourth lines are a comma/dash-separated list of
the CPUs that have not yet reported a quiescent state for the
current grace period (CPUs 0, 3, and 7 for "rcu" above).

The last line lists the online CPUs.

Tested on x86 and Power.

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

 include/linux/rcuclassic.h |    3 +
 kernel/Kconfig.preempt     |    1 
 kernel/Makefile            |    2 
 kernel/rcuclassic.c        |    4 -
 kernel/rcuclassic_trace.c  |  126 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 133 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 4ab8436..061a272 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -54,6 +54,9 @@ struct rcu_ctrlblk {
 				 /* for current batch to proceed.        */
 } ____cacheline_internodealigned_in_smp;
 
+extern struct rcu_ctrlblk rcu_ctrlblk;
+extern struct rcu_ctrlblk rcu_bh_ctrlblk;
+
 /* Is batch a before batch b ? */
 static inline int rcu_batch_before(long a, long b)
 {
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 9fdba03..ba32338 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -68,7 +68,6 @@ config PREEMPT_RCU
 
 config RCU_TRACE
 	bool "Enable tracing for RCU - currently stats in debugfs"
-	depends on PREEMPT_RCU
 	select DEBUG_FS
 	default y
 	help
diff --git a/kernel/Makefile b/kernel/Makefile
index 4e1d7df..e0bfce7 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -77,6 +77,8 @@ obj-$(CONFIG_CLASSIC_RCU) += rcuclassic.o
 obj-$(CONFIG_PREEMPT_RCU) += rcupreempt.o
 ifeq ($(CONFIG_PREEMPT_RCU),y)
 obj-$(CONFIG_RCU_TRACE) += rcupreempt_trace.o
+else
+obj-$(CONFIG_RCU_TRACE) += rcuclassic_trace.o
 endif
 obj-$(CONFIG_RELAY) += relay.o
 obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index aad93cd..517eaa9 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -57,13 +57,13 @@ EXPORT_SYMBOL_GPL(rcu_lock_map);
 
 
 /* Definition for rcupdate control block. */
-static struct rcu_ctrlblk rcu_ctrlblk = {
+struct rcu_ctrlblk rcu_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
 	.cpumask = CPU_MASK_NONE,
 };
-static struct rcu_ctrlblk rcu_bh_ctrlblk = {
+struct rcu_ctrlblk rcu_bh_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
diff --git a/kernel/rcuclassic_trace.c b/kernel/rcuclassic_trace.c
new file mode 100644
index 0000000..f8b9313
--- /dev/null
+++ b/kernel/rcuclassic_trace.c
@@ -0,0 +1,126 @@
+/*
+ * Read-Copy Update tracing for classic implementation
+ *
+ * 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, 2008
+ *
+ * Papers:  http://www.rdrop.com/users/paulmck/RCU
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * 		Documentation/RCU
+ *
+ */
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/rcupdate.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <asm/atomic.h>
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/completion.h>
+#include <linux/moduleparam.h>
+#include <linux/percpu.h>
+#include <linux/notifier.h>
+#include <linux/cpu.h>
+#include <linux/mutex.h>
+#include <linux/debugfs.h>
+
+static DEFINE_MUTEX(rcuclassic_trace_mutex);
+static char *rcuclassic_trace_buf;
+#define RCUCLASSIC_TRACE_BUF_SIZE (20*NR_CPUS + 100)
+
+static int print_one_rcu_ctrlblk(struct rcu_ctrlblk *rcp, char *buf, char *ebuf)
+{
+	int cnt = 0;
+
+	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt], "cur=%ld  completed=%ld  "
+			"np=%d  s=%d\n\t",
+			rcp->cur, rcp->completed,
+			rcp->next_pending, rcp->signaled);
+	cnt += cpulist_scnprintf(&buf[cnt], ebuf - &buf[cnt], rcp->cpumask);
+	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt], "\n");
+	return cnt;
+}
+
+static ssize_t rcucb_read(struct file *filp, char __user *buffer,
+				size_t count, loff_t *ppos)
+{
+	ssize_t bcount;
+	char *buf = rcuclassic_trace_buf;
+	char *ebuf = &rcuclassic_trace_buf[RCUCLASSIC_TRACE_BUF_SIZE];
+
+	mutex_lock(&rcuclassic_trace_mutex);
+	buf += snprintf(buf, ebuf - buf, "rcu: ");
+	buf += print_one_rcu_ctrlblk(&rcu_ctrlblk, buf, ebuf);
+	buf += snprintf(buf, ebuf - buf, "rcu_bh: ");
+	buf += print_one_rcu_ctrlblk(&rcu_bh_ctrlblk, buf, ebuf);
+	buf += snprintf(buf, ebuf - buf, "online: ");
+	buf += cpulist_scnprintf(buf, ebuf - buf, cpu_online_map);
+	buf += snprintf(buf, ebuf - buf, "\n");
+	bcount = simple_read_from_buffer(buffer, count, ppos,
+			rcuclassic_trace_buf, strlen(rcuclassic_trace_buf));
+	mutex_unlock(&rcuclassic_trace_mutex);
+	return bcount;
+}
+
+static struct file_operations rcucb_fops = {
+	.owner = THIS_MODULE,
+	.read = rcucb_read,
+};
+
+static struct dentry *rcudir, *cbdir;
+static int rcuclassic_debugfs_init(void)
+{
+	rcudir = debugfs_create_dir("rcu", NULL);
+	if (!rcudir)
+		goto out;
+	cbdir = debugfs_create_file("rcucb", 0444, rcudir, NULL, &rcucb_fops);
+	if (!cbdir)
+		goto free_out;
+	return 0;
+free_out:
+	debugfs_remove(rcudir);
+out:
+	return 1;
+}
+
+static int __init rcuclassic_trace_init(void)
+{
+	int ret;
+
+	rcuclassic_trace_buf = kmalloc(RCUCLASSIC_TRACE_BUF_SIZE, GFP_KERNEL);
+	if (!rcuclassic_trace_buf)
+		return 1;
+	ret = rcuclassic_debugfs_init();
+	if (ret)
+		kfree(rcuclassic_trace_buf);
+	return ret;
+}
+
+static void __exit rcuclassic_trace_cleanup(void)
+{
+	debugfs_remove(cbdir);
+	debugfs_remove(rcudir);
+	kfree(rcuclassic_trace_buf);
+}
+
+
+module_init(rcuclassic_trace_init);
+module_exit(rcuclassic_trace_cleanup);