[PATCH] v3 rudimentary tracing for Classic RCU

[Paul E. McKenney]

Hello!

This is v3 of a tracing patch for Classic RCU, which creates "rcu/rcucb"
and "rcu/rcudata" files in debugfs.  This patch can be handy when you
need to work out why RCU is refusing to end the current grace period.

Should be ready for inclusion in tip/core/rcu, Ingo, please apply.

Changes since v2:  Add flag to rcu_data to avoid printing rcu_data for
		   CPUs that have never been online.

		   Add documentation (below).

Changes since v1:  Adds (crude) tracing for rcu_data structures.

Reading from the "rcu/rcucb" file results in something like the following:

	rcu: cur=1129  completed=1128  pending=0  s=0
		0,3,7
	rcu_bh: cur=-287  completed=-287  pending=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 pending= is the furthest
future batch number that is required, if equal to cur=, no additional
grace periods are 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.


Reading from the "rcu/rcudata" file results in the following:

	rcu:
	  0 qb=885 b=884 pq=1 qsp=0 ql=0 bl=10
	  1 qb=885 b=882 pq=1 qsp=0 ql=2 bl=10
	  2 qb=885 b=854 pq=1 qsp=0 ql=0 bl=10
	  3 qb=885 b=885 pq=1 qsp=0 ql=0 bl=10
	rcu_bh:
	  0 qb=-291 b=-291 pq=1 qsp=0 ql=0 bl=10
	  1 qb=-291 b=0 pq=1 qsp=0 ql=0 bl=10
	  2 qb=-291 b=0 pq=1 qsp=0 ql=0 bl=10
	  3 qb=-291 b=-298 pq=1 qsp=0 ql=0 bl=10

This output is again split into rcu and rcu_bh portions.  Within each
portion, there is one line per CPU, but only for those CPUs that have
been online at least once since boot.  The number at the beginning of
each line is the CPU number, followed by an "!" if the corresponding CPU
is currently offline.  The qb= is the batch number for the RCU core,
the b= is the batch number corresponding to the callbacks waiting for
the current grace period for this CPU, the pq= is a flag indicating that
this CPU has passed through a quiescent state for the current grace
period, the qsp= is a flag indicating that the RCU core has been
informed that this CPU has passed through a quiescent state for the
current grace period, the ql= is the number of RCU callbacks currently
enqueued on this CPU (regardless of their state), and the bl= is the
current limit of the number of callbacks to be invoked at one shot.

Tested on x86 and Power, rebased to -tip.

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

 include/linux/rcuclassic.h |    4 +
 kernel/Kconfig.preempt     |    1 
 kernel/Makefile            |    2 
 kernel/rcuclassic.c        |    5 -
 kernel/rcuclassic_trace.c  |  179 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 188 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 5f89b62..ce183a8 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -63,6 +63,9 @@ 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)
 {
@@ -81,6 +84,7 @@ struct rcu_data {
 	long		quiescbatch;     /* Batch # for grace period */
 	int		passed_quiesc;	 /* User-mode/idle loop etc. */
 	int		qs_pending;	 /* core waits for quiesc state */
+	bool		beenonline;	 /* CPU online at least once */
 
 	/* 2) batch handling */
 	/*
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 37f72e5..54bd23b 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -58,14 +58,14 @@ 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,
 	.pending = -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,
 	.pending = -300,
@@ -725,6 +725,7 @@ static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
 	rdp->donetail = &rdp->donelist;
 	rdp->quiescbatch = rcp->completed;
 	rdp->qs_pending = 0;
+	rdp->beenonline = 1;
 	rdp->cpu = cpu;
 	rdp->blimit = blimit;
 	spin_unlock_irqrestore(&rcp->lock, flags);
diff --git a/kernel/rcuclassic_trace.c b/kernel/rcuclassic_trace.c
new file mode 100644
index 0000000..d19780b
--- /dev/null
+++ b/kernel/rcuclassic_trace.c
@@ -0,0 +1,179 @@
+/*
+ * 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 (128 * num_possible_cpus() + 100)
+
+static int print_one_rcu_data(struct rcu_data *rdp, char *buf, char *ebuf)
+{
+	int cnt = 0;
+
+	if (!rdp->beenonline)
+		return 0;
+	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
+		"%3d%cqb=%ld b=%ld pq=%d qsp=%d ql=%ld bl=%ld\n",
+		rdp->cpu, cpu_is_offline(rdp->cpu) ? '!' : ' ',
+		rdp->quiescbatch, rdp->batch, rdp->passed_quiesc,
+		rdp->qs_pending, rdp->qlen, rdp->blimit);
+	return cnt;
+}
+
+#define PRINT_RCU_DATA(name, buf, ebuf) \
+	do { \
+		int _p_r_d_i; \
+		\
+		for_each_possible_cpu(_p_r_d_i) \
+			(buf) += print_one_rcu_data(&per_cpu(name, _p_r_d_i), \
+						    buf, ebuf); \
+	} while (0)
+
+static ssize_t rcudata_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:\n");
+	PRINT_RCU_DATA(rcu_data, buf, ebuf);
+	buf += snprintf(buf, ebuf - buf, "rcu_bh:\n");
+	PRINT_RCU_DATA(rcu_bh_data, buf, ebuf);
+	bcount = simple_read_from_buffer(buffer, count, ppos,
+			rcuclassic_trace_buf, strlen(rcuclassic_trace_buf));
+	mutex_unlock(&rcuclassic_trace_mutex);
+	return bcount;
+}
+
+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  "
+			"pending=%ld  s=%d\n\t",
+			rcp->cur, rcp->completed,
+			rcp->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 rcudata_fops = {
+	.owner = THIS_MODULE,
+	.read = rcudata_read,
+};
+
+static struct file_operations rcucb_fops = {
+	.owner = THIS_MODULE,
+	.read = rcucb_read,
+};
+
+static struct dentry *rcudir, *datadir, *cbdir;
+static int rcuclassic_debugfs_init(void)
+{
+	rcudir = debugfs_create_dir("rcu", NULL);
+	if (!rcudir)
+		goto out;
+	datadir = debugfs_create_file("rcudata", 0444, rcudir,
+				      NULL, &rcudata_fops);
+	if (!datadir)
+		goto free_out;
+	cbdir = debugfs_create_file("rcucb", 0444, rcudir, NULL, &rcucb_fops);
+	if (!cbdir)
+		goto free_out;
+	return 0;
+free_out:
+	if (datadir)
+		debugfs_remove(datadir);
+	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(datadir);
+	debugfs_remove(cbdir);
+	debugfs_remove(rcudir);
+	kfree(rcuclassic_trace_buf);
+}
+
+
+module_init(rcuclassic_trace_init);
+module_exit(rcuclassic_trace_cleanup);
--
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