netfs: Record contention stats for writeback lock

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
Link: https://lore.kernel.org/r/20240814203850.2240469-5-dhowells@redhat.com/
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index 7773f3d855a9598f97ecc9b94842590aeded47b7..9e6e0e59d7e4caadde8ccaa1e012cbd7f0e26154 100644 (file)
--- 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 95ed2d2623a881eba3e12bb309681fe740df2938..5fe1c396e24f3293d71c301194b2afe2ee82b2a2 100644 (file)
--- 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 3f7e37e50c7d02456af24ae7cbdc9429d6875580..44f35a0faaca14b765d4504bc7df118a8d90b777 100644 (file)
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -505,10 +505,14 @@ 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);