From 938051408905dcd69fb65a84f0648b58e8851f1b Mon Sep 17 00:00:00 2001
From: Filipe Manana <fdmanana@suse.com>
Date: Thu, 2 Jul 2020 12:32:31 +0100
Subject: [PATCH] btrfs: stop incremening log_batch for the log root tree when
syncing log
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(-)
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 36cd210ee2ef..a29c2d0ec96e 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -992,6 +992,7 @@ 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*/
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index f46afbff668e..cca31f7f6c01 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3140,7 +3140,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
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;