[PATCH v6 3/9] fs: add percpu counters for significant multigrain timestamp events

[Jeff Layton <jlayton@kernel.org>]

Four percpu counters for counting various stats around mgtimes, and a
new debugfs file for displaying them:

- number of attempted ctime updates
- number of successful i_ctime_nsec swaps
- number of fine-grained timestamp fetches
- number of floor value swaps

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 869994285e87..fff844345c35 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -21,6 +21,8 @@
 #include <linux/list_lru.h>
 #include <linux/iversion.h>
 #include <linux/rw_hint.h>
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
 #include <trace/events/writeback.h>
 #define CREATE_TRACE_POINTS
 #include <trace/events/timestamp.h>
@@ -80,6 +82,10 @@ EXPORT_SYMBOL(empty_aops);
 
 static DEFINE_PER_CPU(unsigned long, nr_inodes);
 static DEFINE_PER_CPU(unsigned long, nr_unused);
+static DEFINE_PER_CPU(unsigned long, mg_ctime_updates);
+static DEFINE_PER_CPU(unsigned long, mg_fine_stamps);
+static DEFINE_PER_CPU(unsigned long, mg_floor_swaps);
+static DEFINE_PER_CPU(unsigned long, mg_ctime_swaps);
 
 static struct kmem_cache *inode_cachep __ro_after_init;
 
@@ -101,6 +107,42 @@ static inline long get_nr_inodes_unused(void)
 	return sum < 0 ? 0 : sum;
 }
 
+static long get_mg_ctime_updates(void)
+{
+	int i;
+	long sum = 0;
+	for_each_possible_cpu(i)
+		sum += per_cpu(mg_ctime_updates, i);
+	return sum < 0 ? 0 : sum;
+}
+
+static long get_mg_fine_stamps(void)
+{
+	int i;
+	long sum = 0;
+	for_each_possible_cpu(i)
+		sum += per_cpu(mg_fine_stamps, i);
+	return sum < 0 ? 0 : sum;
+}
+
+static long get_mg_floor_swaps(void)
+{
+	int i;
+	long sum = 0;
+	for_each_possible_cpu(i)
+		sum += per_cpu(mg_floor_swaps, i);
+	return sum < 0 ? 0 : sum;
+}
+
+static long get_mg_ctime_swaps(void)
+{
+	int i;
+	long sum = 0;
+	for_each_possible_cpu(i)
+		sum += per_cpu(mg_ctime_swaps, i);
+	return sum < 0 ? 0 : sum;
+}
+
 long get_nr_dirty_inodes(void)
 {
 	/* not actually dirty inodes, but a wild approximation */
@@ -2655,6 +2697,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 
 			/* Get a fine-grained time */
 			fine = ktime_get();
+			this_cpu_inc(mg_fine_stamps);
 
 			/*
 			 * If the cmpxchg works, we take the new floor value. If
@@ -2663,11 +2706,14 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 			 * as good, so keep it.
 			 */
 			old = floor;
-			if (!atomic64_try_cmpxchg(&ctime_floor, &old, fine))
+			if (atomic64_try_cmpxchg(&ctime_floor, &old, fine))
+				this_cpu_inc(mg_floor_swaps);
+			else
 				fine = old;
 			now = ktime_mono_to_real(fine);
 		}
 	}
+	this_cpu_inc(mg_ctime_updates);
 	now_ts = timestamp_truncate(ktime_to_timespec64(now), inode);
 	cur = cns;
 
@@ -2682,6 +2728,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 		/* If swap occurred, then we're (mostly) done */
 		inode->i_ctime_sec = now_ts.tv_sec;
 		trace_ctime_ns_xchg(inode, cns, now_ts.tv_nsec, cur);
+		this_cpu_inc(mg_ctime_swaps);
 	} else {
 		/*
 		 * Was the change due to someone marking the old ctime QUERIED?
@@ -2751,3 +2798,24 @@ umode_t mode_strip_sgid(struct mnt_idmap *idmap,
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);
+
+static int mgts_show(struct seq_file *s, void *p)
+{
+	long ctime_updates = get_mg_ctime_updates();
+	long ctime_swaps = get_mg_ctime_swaps();
+	long fine_stamps = get_mg_fine_stamps();
+	long floor_swaps = get_mg_floor_swaps();
+
+	seq_printf(s, "%lu %lu %lu %lu\n",
+		   ctime_updates, ctime_swaps, fine_stamps, floor_swaps);
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(mgts);
+
+static int __init mg_debugfs_init(void)
+{
+	debugfs_create_file("multigrain_timestamps", S_IFREG | S_IRUGO, NULL, NULL, &mgts_fops);
+	return 0;
+}
+late_initcall(mg_debugfs_init);

-- 
2.45.2