[RFC][PATCH] lib: stacklog_debug: Introduce helper tool for collecting and displaying stacktraces

[John Stultz]

When debugging, its often useful to understand how a function is
called and the different paths taken to get there. Usually
dump_stack() can be used for this purpose. However there are a
number of cases where a function is called very frequently,
making dump_stack far too noisy to be useful.

This is a little debug tool that utilizes stackdepot to capture
unique stack traces and store them in a circular buffer.

In the code, the developer adds: stacklog_debug_save() calls
at points of interest (as they might with stack_dump()).

Then after running the kernel, the developer can dump the unique
stack traces from the buffer via:
   cat /sys/kernel/debug/stacklog_debug

This is pretty trivial, but I've had this hanging around for
awhile and recently hit another case where it was helpful, so I
figured it would be worth sending it out for feedback as to if
others thought it would be useful enough to merge upstream or to
possibly rework into stackdepot itself? 

Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
 include/linux/stacklog_debug.h |  13 ++++
 lib/Kconfig                    |   8 +++
 lib/Makefile                   |   1 +
 lib/stacklog_debug.c           | 110 +++++++++++++++++++++++++++++++++
 4 files changed, 132 insertions(+)
 create mode 100644 include/linux/stacklog_debug.h
 create mode 100644 lib/stacklog_debug.c

diff --git a/include/linux/stacklog_debug.h b/include/linux/stacklog_debug.h
new file mode 100644
index 000000000000..d88f05d7000a
--- /dev/null
+++ b/include/linux/stacklog_debug.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef _LINUX_STACKLOG_DEBUG_H
+#define _LINUX_STACKLOG_DEBUG_H
+
+#ifdef CONFIG_STACKLOG_DEBUG
+void stacklog_debug_save(void);
+#else
+static inline void stacklog_debug_save(void)
+{
+}
+#endif
+#endif
diff --git a/lib/Kconfig b/lib/Kconfig
index b38849af6f13..3a90b9d4f8de 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -725,6 +725,14 @@ config REF_TRACKER
 	depends on STACKTRACE_SUPPORT
 	select STACKDEPOT
 
+config STACKLOG_DEBUG
+	bool "Debug tool for logging and later displaying stacktraces"
+	select STACKDEPOT
+	select STACKDEPOT_ALWAYS_INIT
+	help
+	  Enables debug infrastructure for logging unique stack traces at
+	  a specific point, which can be later displayed from userland.
+
 config SBITMAP
 	bool
 
diff --git a/lib/Makefile b/lib/Makefile
index 773adf88af41..5a07573be73c 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -264,6 +264,7 @@ obj-$(CONFIG_IRQ_POLL) += irq_poll.o
 
 obj-$(CONFIG_POLYNOMIAL) += polynomial.o
 
+obj-$(CONFIG_STACKLOG_DEBUG) += stacklog_debug.o
 # stackdepot.c should not be instrumented or call instrumented functions.
 # Prevent the compiler from calling builtins like memcmp() or bcmp() from this
 # file.
diff --git a/lib/stacklog_debug.c b/lib/stacklog_debug.c
new file mode 100644
index 000000000000..72ffbacee4b7
--- /dev/null
+++ b/lib/stacklog_debug.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2014, The Linux Foundation. All rights reserved.
+ */
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/vmalloc.h>
+#include <linux/stackdepot.h>
+#include <linux/debugfs.h>
+#include <linux/stacklog_debug.h>
+
+#define STACKDEPTH 32
+#define BUFSZ 4096
+
+#define LIST_ENTRIES 512
+DEFINE_SPINLOCK(stack_lock);
+depot_stack_handle_t stack_list[LIST_ENTRIES];
+int head, tail;
+
+void stacklog_debug_save(void)
+{
+	unsigned long entries[STACKDEPTH];
+	depot_stack_handle_t stack_hash;
+	unsigned long flags;
+	unsigned int n;
+	int i;
+
+	n = stack_trace_save(entries, ARRAY_SIZE(entries), 1);
+	stack_hash = stack_depot_save(entries, n, GFP_NOWAIT);
+	if (!stack_hash)
+		return;
+
+	spin_lock_irqsave(&stack_lock, flags);
+	for (i = head; i < tail; i++)
+		if (stack_list[i % LIST_ENTRIES] == stack_hash)
+			goto out;
+
+	stack_list[(tail++ % LIST_ENTRIES)] = stack_hash;
+
+	if (tail % LIST_ENTRIES == head % LIST_ENTRIES)
+		head++;
+
+	if (tail >= 2 * LIST_ENTRIES) {
+		head %= LIST_ENTRIES;
+		tail %= LIST_ENTRIES;
+		if (tail < head)
+			tail += LIST_ENTRIES;
+	}
+out:
+	spin_unlock_irqrestore(&stack_lock, flags);
+}
+
+#ifdef CONFIG_DEBUG_FS
+static int stacklog_stats_show(struct seq_file *s, void *unused)
+{
+	char *buf = kmalloc(BUFSZ, GFP_NOWAIT);
+	unsigned int nr_entries;
+	unsigned long flags;
+	int i, start, stop;
+
+	if (!buf)
+		return -ENOMEM;
+
+	spin_lock_irqsave(&stack_lock, flags);
+	start = head;
+	stop = tail;
+	spin_unlock_irqrestore(&stack_lock, flags);
+
+	if (start == stop)
+		goto out;
+
+	for (i = start; i < stop; i++) {
+		unsigned long *ent;
+		u32 hash;
+
+		/*
+		 * We avoid holdings the lock over the entire loop
+		 * just to be careful as we don't want to trip a
+		 * call path that calls back into stacklog_debug_save
+		 * which would deadlock, so hold the lock minimally
+		 * (and be ok with the data changing between loop
+		 * iterations).
+		 */
+		spin_lock_irqsave(&stack_lock, flags);
+		hash = stack_list[i % LIST_ENTRIES];
+		spin_unlock_irqrestore(&stack_lock, flags);
+
+		nr_entries = stack_depot_fetch(hash, &ent);
+		stack_trace_snprint(buf, BUFSZ, ent, nr_entries, 0);
+		seq_printf(s, "[idx: %i hash: %ld]====================\n%s\n\n",
+			   i - start, (long)hash, buf);
+	}
+out:
+	kfree(buf);
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(stacklog_stats);
+
+static int __init stacklog_debug_init(void)
+{
+	debugfs_create_file("stacklog_debug", 0400, NULL, NULL,
+			    &stacklog_stats_fops);
+	return 0;
+}
+
+late_initcall(stacklog_debug_init);
+#endif
-- 
2.47.0.rc1.288.g06298d1525-goog