[PATCH 05/17] netfs: Record contention stats for writeback lock

[David Howells]

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 <dhowe...@redhat.com>
cc: Jeff Layton <jlay...@kernel.org>
cc: Steve French <sfre...@samba.org>
cc: ne...@lists.linux.dev
cc: linux-fsde...@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 95e281a8af78..42443d99967d 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -126,6 +126,8 @@ 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 ec6cf8707fb0..cd3ddf07ab49 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -502,10 +502,14 @@ 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);