[PATCH 04/24] netfs: Record contention stats for writeback lock

[David Howells <dhowells@redhat.com>]

Record statistics for contention upon the writeback serialisation lock that
prevents racing writeback calls from causing each other to interleave their
writebacks.  These can be viewed in /proc/fs/netfs/stats on the WbLock line,
with skip=N indicating the number of non-SYNC writebacks skipped and wait=N
indicating the number of SYNC writebacks that waited.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/internal.h    |  2 ++
 fs/netfs/stats.c       |  5 +++++
 fs/netfs/write_issue.c | 10 +++++++---
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index 7773f3d855a9..9e6e0e59d7e4 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -117,6 +117,8 @@ extern atomic_t netfs_n_wh_upload_failed;
 extern atomic_t netfs_n_wh_write;
 extern atomic_t netfs_n_wh_write_done;
 extern atomic_t netfs_n_wh_write_failed;
+extern atomic_t netfs_n_wb_lock_skip;
+extern atomic_t netfs_n_wb_lock_wait;
 
 int netfs_stats_show(struct seq_file *m, void *v);
 
diff --git a/fs/netfs/stats.c b/fs/netfs/stats.c
index 95ed2d2623a8..5fe1c396e24f 100644
--- a/fs/netfs/stats.c
+++ b/fs/netfs/stats.c
@@ -39,6 +39,8 @@ atomic_t netfs_n_wh_upload_failed;
 atomic_t netfs_n_wh_write;
 atomic_t netfs_n_wh_write_done;
 atomic_t netfs_n_wh_write_failed;
+atomic_t netfs_n_wb_lock_skip;
+atomic_t netfs_n_wb_lock_wait;
 
 int netfs_stats_show(struct seq_file *m, void *v)
 {
@@ -78,6 +80,9 @@ int netfs_stats_show(struct seq_file *m, void *v)
 		   atomic_read(&netfs_n_rh_rreq),
 		   atomic_read(&netfs_n_rh_sreq),
 		   atomic_read(&netfs_n_wh_wstream_conflict));
+	seq_printf(m, "WbLock : skip=%u wait=%u\n",
+		   atomic_read(&netfs_n_wb_lock_skip),
+		   atomic_read(&netfs_n_wb_lock_wait));
 	return fscache_stats_show(m);
 }
 EXPORT_SYMBOL(netfs_stats_show);
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 9258d30cffe3..e7d4f39b8fdc 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -503,10 +503,14 @@ int netfs_writepages(struct address_space *mapping,
 	struct folio *folio;
 	int error = 0;
 
-	if (wbc->sync_mode == WB_SYNC_ALL)
+	if (!mutex_trylock(&ictx->wb_lock)) {
+		if (wbc->sync_mode == WB_SYNC_NONE) {
+			netfs_stat(&netfs_n_wb_lock_skip);
+			return 0;
+		}
+		netfs_stat(&netfs_n_wb_lock_wait);
 		mutex_lock(&ictx->wb_lock);
-	else if (!mutex_trylock(&ictx->wb_lock))
-		return 0;
+	}
 
 	/* Need the first folio to be able to set up the op. */
 	folio = writeback_iter(mapping, wbc, NULL, &error);