From: Filipe Manana fdmanana@suse.com
commit 28a9579561bcb9082715e720eac93012e708ab94 upstream.
We are incrementing the log_batch atomic counter of the root log tree but we never use that counter, it's used only for the log trees of subvolume roots. We started doing it when we moved the log_batch and log_write counters from the global, per fs, btrfs_fs_info structure, into the btrfs_root structure in commit 7237f1833601dc ("Btrfs: fix tree logs parallel sync").
So just stop doing it for the log root tree and add a comment over the field declaration so inform it's used only for log trees of subvolume roots.
This patch is part of a series that has the following patches:
1/4 btrfs: only commit the delayed inode when doing a full fsync 2/4 btrfs: only commit delayed items at fsync if we are logging a directory 3/4 btrfs: stop incremening log_batch for the log root tree when syncing log 4/4 btrfs: remove no longer needed use of log_writers for the log root tree
After the entire patchset applied I saw about 12% decrease on max latency reported by dbench. The test was done on a qemu vm, with 8 cores, 16Gb of ram, using kvm and using a raw NVMe device directly (no intermediary fs on the host). The test was invoked like the following:
mkfs.btrfs -f /dev/sdk mount -o ssd -o nospace_cache /dev/sdk /mnt/sdk dbench -D /mnt/sdk -t 300 8 umount /mnt/dsk
CC: stable@vger.kernel.org # 5.4+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/ctree.h | 1 + fs/btrfs/tree-log.c | 1 - 2 files changed, 1 insertion(+), 1 deletion(-)
--- a/fs/btrfs/ctree.h +++ b/fs/btrfs/ctree.h @@ -1040,6 +1040,7 @@ struct btrfs_root { struct list_head log_ctxs[2]; atomic_t log_writers; atomic_t log_commit[2]; + /* Used only for log trees of subvolumes, not for the log root tree */ atomic_t log_batch; int log_transid; /* No matter the commit succeeds or not*/ --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -3125,7 +3125,6 @@ int btrfs_sync_log(struct btrfs_trans_ha btrfs_init_log_ctx(&root_log_ctx, NULL);
mutex_lock(&log_root_tree->log_mutex); - atomic_inc(&log_root_tree->log_batch); atomic_inc(&log_root_tree->log_writers);
index2 = log_root_tree->log_transid % 2;