stackdepot: add stats counters exported via debugfs

Add a few basic stats counters for stack depot that can be used to derive
if stack depot is working as intended.  This is a snapshot of the new
stats after booting a system with a KASAN-enabled kernel:

 $ cat /sys/kernel/debug/stackdepot/stats
 pools: 838
 allocations: 29861
 frees: 6561
 in_use: 23300
 freelist_size: 1840

Generally, "pools" should be well below the max; once the system is
booted, "in_use" should remain relatively steady.

Link: https://lkml.kernel.org/r/20240118110216.2539519-1-elver@google.com
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: melver

lib/stackdepot.c

@@ -14,6 +14,7 @@
 
 #define pr_fmt(fmt) "stackdepot: " fmt
 
+#include <linux/debugfs.h>
 #include <linux/gfp.h>
 #include <linux/jhash.h>
 #include <linux/kernel.h>
@@ -115,6 +116,23 @@ static bool new_pool_required = true;
 /* Lock that protects the variables above. */
 static DEFINE_RWLOCK(pool_rwlock);
 
+/* Statistics counters for debugfs. */
+enum depot_counter_id {
+	DEPOT_COUNTER_ALLOCS,
+	DEPOT_COUNTER_FREES,
+	DEPOT_COUNTER_INUSE,
+	DEPOT_COUNTER_FREELIST_SIZE,
+	DEPOT_COUNTER_COUNT,
+};
+static long counters[DEPOT_COUNTER_COUNT];
+static const char *const counter_names[] = {
+	[DEPOT_COUNTER_ALLOCS]		= "allocations",
+	[DEPOT_COUNTER_FREES]		= "frees",
+	[DEPOT_COUNTER_INUSE]		= "in_use",
+	[DEPOT_COUNTER_FREELIST_SIZE]	= "freelist_size",
+};
+static_assert(ARRAY_SIZE(counter_names) == DEPOT_COUNTER_COUNT);
+
 static int __init disable_stack_depot(char *str)
 {
 	return kstrtobool(str, &stack_depot_disabled);
@@ -277,6 +295,7 @@ static void depot_init_pool(void *pool)
 		stack->handle.extra = 0;
 
 		list_add(&stack->list, &free_stacks);
+		counters[DEPOT_COUNTER_FREELIST_SIZE]++;
 	}
 
 	/* Save reference to the pool to be used by depot_fetch_stack(). */
@@ -376,6 +395,7 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	/* Get and unlink the first entry from the freelist. */
 	stack = list_first_entry(&free_stacks, struct stack_record, list);
 	list_del(&stack->list);
+	counters[DEPOT_COUNTER_FREELIST_SIZE]--;
 
 	/* Limit number of saved frames to CONFIG_STACKDEPOT_MAX_FRAMES. */
 	if (size > CONFIG_STACKDEPOT_MAX_FRAMES)
@@ -394,6 +414,8 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	 */
 	kmsan_unpoison_memory(stack, DEPOT_STACK_RECORD_SIZE);
 
+	counters[DEPOT_COUNTER_ALLOCS]++;
+	counters[DEPOT_COUNTER_INUSE]++;
 	return stack;
 }
 
@@ -426,6 +448,10 @@ static void depot_free_stack(struct stack_record *stack)
 	lockdep_assert_held_write(&pool_rwlock);
 
 	list_add(&stack->list, &free_stacks);
+
+	counters[DEPOT_COUNTER_FREELIST_SIZE]++;
+	counters[DEPOT_COUNTER_FREES]++;
+	counters[DEPOT_COUNTER_INUSE]--;
 }
 
 /* Calculates the hash for a stack. */
@@ -690,3 +716,30 @@ unsigned int stack_depot_get_extra_bits(depot_stack_handle_t handle)
 	return parts.extra;
 }
 EXPORT_SYMBOL(stack_depot_get_extra_bits);
+
+static int stats_show(struct seq_file *seq, void *v)
+{
+	/*
+	 * data race ok: These are just statistics counters, and approximate
+	 * statistics are ok for debugging.
+	 */
+	seq_printf(seq, "pools: %d\n", data_race(pools_num));
+	for (int i = 0; i < DEPOT_COUNTER_COUNT; i++)
+		seq_printf(seq, "%s: %ld\n", counter_names[i], data_race(counters[i]));
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(stats);
+
+static int depot_debugfs_init(void)
+{
+	struct dentry *dir;
+
+	if (stack_depot_disabled)
+		return 0;
+
+	dir = debugfs_create_dir("stackdepot", NULL);
+	debugfs_create_file("stats", 0444, dir, NULL, &stats_fops);
+	return 0;
+}
+late_initcall(depot_debugfs_init);