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

David Howells
2024-06-20T17:32:43Z

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 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);