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Subject[PATCH] v2 rudimentary tracing for Classic RCU
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.

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

Additional work needed: Don't print rcu_data for offline CPUs.
Documentation update.

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 | 179 +++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 186 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..a478256
--- /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 (cpu_is_offline(rdp->cpu))
+ 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 "
+ "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 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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