btrfs: avoid taking the device_list_mutex in btrfs_run_dev_stats()

btrfs_run_dev_stats() is called during the critical section of a
transaction commit and it takes the device_list_mutex, which is also
acquired by fitrim, which does discard operations while holding that
mutex. Most of the time, if we are on a healthy filesystem, we don't have
new stat updates to persist in the device tree, so blocking on the
device_list_mutex is just wasting time and making any tasks that need to
start a new transaction wait longer that necessary.

Since the device list is RCU safe/protected, make btrfs_run_dev_stats()
do an initial check for device stat updates using RCU and quit without
taking the device_list_mutex in case there are no new device stats that
need to be persisted in the device tree.

Also note that adding/removing devices also requires starting a
transaction, and since btrfs_run_dev_stats() is called from the critical
section of a transaction commit, no one can be concurrently adding or
removing a device while btrfs_run_dev_stats() is called.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index deaf1997a3a593c7d7c6fef8f9a05b5e413f68b2..33fa7366853474f09e7f506da49d0ffba5bc5feb 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -8250,6 +8250,36 @@ int btrfs_run_dev_stats(struct btrfs_trans_handle *trans)
        struct btrfs_device *device;
        int stats_cnt;
        int ret = 0;
+       bool need_update_dev_stats = false;
+
+       /*
+        * Do an initial pass using RCU to see if we need to update any dev
+        * stats item. This is to avoid taking the device_list_mutex which is
+        * acquired by the fitrim operation and can take a while since it does
+        * discard operations while holding that mutex. Most of the time, if
+        * we are on a healthy filesystem, we don't have new stat updates, so
+        * this avoids blocking on that mutex, which is specially important
+        * because we are called during the critical section of a transaction
+        * commit, therefore blocking new transactions from starting while
+        * discard is running.
+        *
+        * Also note that adding/removing devices also requires starting a
+        * transaction, and since we are called from the critical section of a
+        * transaction commit, no one can be concurrently adding or removing a
+        * device.
+        */
+       rcu_read_lock();
+       list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
+               if (device->dev_stats_valid &&
+                   atomic_read(&device->dev_stats_ccnt) != 0) {
+                       need_update_dev_stats = true;
+                       break;
+               }
+       }
+       rcu_read_unlock();
+
+       if (!need_update_dev_stats)
+               return 0;
 
        mutex_lock(&fs_devices->device_list_mutex);
        list_for_each_entry(device, &fs_devices->devices, dev_list) {