Btrfs: optimize fsync for the single writer case

This patch optimizes the tree logging stuff so it doesn't always wait 1 jiffie
for new people to join the logging transaction if there is only ever 1 writer.
This helps a little bit with latency where we have something like RPM where it
will fdatasync every file it writes, and so waiting the 1 jiffie for every
fdatasync really starts to add up.

Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

2 files changed, 13 insertions, 1 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index a362dd617e97..16ddb19dda86 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1010,6 +1010,8 @@ struct btrfs_root {
         atomic_t log_commit[2];
         unsigned long log_transid;
         unsigned long log_batch;
+        pid_t log_start_pid;
+        bool log_multiple_pids;
 
         u64 objectid;
         u64 last_trans;
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 4d7d9abef42f..78f6254ac2d9 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -137,11 +137,20 @@ static int start_log_trans(struct btrfs_trans_handle *trans,
 
         mutex_lock(&root->log_mutex);
         if (root->log_root) {
+                if (!root->log_start_pid) {
+                        root->log_start_pid = current->pid;
+                        root->log_multiple_pids = false;
+                } else if (root->log_start_pid != current->pid) {
+                        root->log_multiple_pids = true;
+                }
+
                 root->log_batch++;
                 atomic_inc(&root->log_writers);
                 mutex_unlock(&root->log_mutex);
                 return 0;
         }
+        root->log_multiple_pids = false;
+        root->log_start_pid = current->pid;
         mutex_lock(&root->fs_info->tree_log_mutex);
         if (!root->fs_info->log_root_tree) {
                 ret = btrfs_init_log_root_tree(trans, root->fs_info);
@@ -1985,7 +1994,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         if (atomic_read(&root->log_commit[(index1 + 1) % 2]))
                 wait_log_commit(trans, root, root->log_transid - 1);
 
-        while (1) {
+        while (root->log_multiple_pids) {
                 unsigned long batch = root->log_batch;
                 mutex_unlock(&root->log_mutex);
                 schedule_timeout_uninterruptible(1);
@@ -2011,6 +2020,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         root->log_batch = 0;
         root->log_transid++;
         log->log_transid = root->log_transid;
+        root->log_start_pid = 0;
         smp_mb();
         /*
          * log tree has been flushed to disk, new modifications of