Commit cee4cfd6 authored by Filipe Manana's avatar Filipe Manana Committed by David Sterba
Browse files

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: default avatarFilipe Manana <fdmanana@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent e0a85137

fs/btrfs/volumes.c

+30 −0
Original line number Diff line number Diff line
@@ -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) {