linux-kernel - [PATCH v3 3/9] fs: add percpu counters to count fine vs. coarse timestamps

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Message-Id: <20240705-mgtime-v3-3-85b2daa9b335@kernel.org>
Date: Fri, 05 Jul 2024 13:02:37 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Masami Hiramatsu <mhiramat@...nel.org>, 
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
 Chandan Babu R <chandan.babu@...cle.com>, 
 "Darrick J. Wong" <djwong@...nel.org>, Theodore Ts'o <tytso@....edu>, 
 Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>, 
 Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, 
 Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
 Jonathan Corbet <corbet@....net>
Cc: Dave Chinner <david@...morbit.com>, Andi Kleen <ak@...ux.intel.com>, 
 Christoph Hellwig <hch@...radead.org>, kernel-team@...com, 
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-trace-kernel@...r.kernel.org, linux-xfs@...r.kernel.org, 
 linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-mm@...ck.org, 
 linux-nfs@...r.kernel.org, linux-doc@...r.kernel.org, 
 Jeff Layton <jlayton@...nel.org>
Subject: [PATCH v3 3/9] fs: add percpu counters to count fine vs. coarse
 timestamps

Keep a pair of percpu counters so we can track what proportion of
timestamps is fine-grained.

Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
 fs/inode.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 4ab7aee3558c..2e5610ebb205 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>
@@ -69,6 +71,11 @@ static __cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_hash_lock);
  * converted to the realtime clock on an as-needed basis.
  */
 static __cacheline_aligned_in_smp ktime_t ctime_floor;
+
+static struct percpu_counter mg_ctime_updates;
+static struct percpu_counter mg_floor_swaps;
+static struct percpu_counter mg_ctime_swaps;
+
 /*
  * Empty aops. Can be used for the cases where the user does not
  * define any of the address_space operations.
@@ -2662,11 +2669,14 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 			 * as good, so keep it.
 			 */
 			old = cmpxchg(&ctime_floor, floor, fine);
-			if (old != floor)
+			if (old == floor)
+				percpu_counter_inc(&mg_floor_swaps);
+			else
 				fine = old;
 			now = ktime_mono_to_real(fine);
 		}
 	}
+	percpu_counter_inc(&mg_ctime_updates);
 	now_ts = ktime_to_timespec64(now);
 retry:
 	/* Try to swap the nsec value into place. */
@@ -2676,6 +2686,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 	/* If swap occurred, then we're (mostly) done */
 	if (cur == cns) {
 		inode->i_ctime_sec = now_ts.tv_sec;
+		percpu_counter_inc(&mg_ctime_swaps);
 	} else {
 		/*
 		 * Was the change due to someone marking the old ctime QUERIED?
@@ -2745,3 +2756,39 @@ 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)
+{
+	u64 ctime_updates = percpu_counter_sum(&mg_ctime_updates);
+	u64 floor_swaps = percpu_counter_sum(&mg_floor_swaps);
+	u64 ctime_swaps = percpu_counter_sum(&mg_ctime_swaps);
+
+	seq_printf(s, "%llu %llu %llu\n", ctime_updates, ctime_swaps, floor_swaps);
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(mgts);
+
+static int __init mg_debugfs_init(void)
+{
+	int ret = percpu_counter_init(&mg_ctime_updates, 0, GFP_KERNEL);
+
+	if (ret)
+		return ret;
+
+	ret = percpu_counter_init(&mg_floor_swaps, 0, GFP_KERNEL);
+	if (ret) {
+		percpu_counter_destroy(&mg_ctime_updates);
+		return ret;
+	}
+
+	ret = percpu_counter_init(&mg_ctime_swaps, 0, GFP_KERNEL);
+	if (ret) {
+		percpu_counter_destroy(&mg_floor_swaps);
+		percpu_counter_destroy(&mg_ctime_updates);
+		return ret;
+	}
+	debugfs_create_file("multigrain_timestamps", S_IFREG | S_IRUGO, NULL, NULL, &mgts_fops);
+	return 0;
+}
+late_initcall(mg_debugfs_init);

-- 
2.45.2