Commit 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc") cleaned up how we do delalloc shrinking by utilizing some infrastructure we have in place to flush inodes that we use for device replace and snapshot. However this introduced a pretty serious performance regression. To reproduce the user untarred the source tarball of Firefox, and would see it take anywhere from 5 to 20 times as long to untar in 5.10 compared to 5.9.
The root cause is because before we would generally use the normal writeback path to reclaim delalloc space, and for this we would provide it with the number of pages we wanted to flush. The referenced commit changed this to flush that many inodes, which drastically increased the amount of space we were flushing in certain cases, which severely affected performance.
We cannot revert this patch unfortunately because of
btrfs: fix deadlock when cloning inline extent and low on free metadata space
which requires the ability to skip flushing inodes that are being cloned in certain scenarios, which means we need to keep using our flushing infrastructure or risk re-introducing the deadlock.
Instead to fix this problem we can go back to providing btrfs_start_delalloc_roots with a number of pages to flush, and then set up a writeback_control and utilize sync_inode() to handle the flushing for us. This gives us the same behavior we had prior to the fix, while still allowing us to avoid the deadlock that was fixed by Filipe. I redid the users original test and got the following results on one of our test machines (256gib of ram, 56 cores, 2tib Intel NVME drive)
5.9 0m54.258s 5.10 1m26.212s 5.10+patch 0m38.800s
5.10+patch is significantly faster than plain 5.9 because of my patch series "Change data reservations to use the ticketing infra" which contained the patch that introduced the regression, but generally improved the overall ENOSPC flushing mechanisms.
CC: stable@vger.kernel.org # 5.10 Reported-by: René Rebe rene@exactcode.de Fixes: 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc") Signed-off-by: Josef Bacik josef@toxicpanda.com --- v2->v3: - modified the changelog to add information about the patches referenced, and detail the specs of the machine I used for the performance numbers.
fs/btrfs/inode.c | 60 +++++++++++++++++++++++++++++++------------ fs/btrfs/space-info.c | 4 ++- 2 files changed, 46 insertions(+), 18 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 070716650df8..a8e0a6b038d3 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -9390,7 +9390,8 @@ static struct btrfs_delalloc_work *btrfs_alloc_delalloc_work(struct inode *inode * some fairly slow code that needs optimization. This walks the list * of all the inodes with pending delalloc and forces them to disk. */ -static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot, +static int start_delalloc_inodes(struct btrfs_root *root, + struct writeback_control *wbc, bool snapshot, bool in_reclaim_context) { struct btrfs_inode *binode; @@ -9399,6 +9400,7 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot struct list_head works; struct list_head splice; int ret = 0; + bool full_flush = wbc->nr_to_write == LONG_MAX;
INIT_LIST_HEAD(&works); INIT_LIST_HEAD(&splice); @@ -9427,18 +9429,24 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot if (snapshot) set_bit(BTRFS_INODE_SNAPSHOT_FLUSH, &binode->runtime_flags); - work = btrfs_alloc_delalloc_work(inode); - if (!work) { - iput(inode); - ret = -ENOMEM; - goto out; - } - list_add_tail(&work->list, &works); - btrfs_queue_work(root->fs_info->flush_workers, - &work->work); - if (*nr != U64_MAX) { - (*nr)--; - if (*nr == 0) + if (full_flush) { + work = btrfs_alloc_delalloc_work(inode); + if (!work) { + iput(inode); + ret = -ENOMEM; + goto out; + } + list_add_tail(&work->list, &works); + btrfs_queue_work(root->fs_info->flush_workers, + &work->work); + } else { + ret = sync_inode(inode, wbc); + if (!ret && + test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT, + &BTRFS_I(inode)->runtime_flags)) + ret = sync_inode(inode, wbc); + btrfs_add_delayed_iput(inode); + if (ret || wbc->nr_to_write <= 0) goto out; } cond_resched(); @@ -9464,18 +9472,29 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot
int btrfs_start_delalloc_snapshot(struct btrfs_root *root) { + struct writeback_control wbc = { + .nr_to_write = LONG_MAX, + .sync_mode = WB_SYNC_NONE, + .range_start = 0, + .range_end = LLONG_MAX, + }; struct btrfs_fs_info *fs_info = root->fs_info; - u64 nr = U64_MAX;
if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) return -EROFS;
- return start_delalloc_inodes(root, &nr, true, false); + return start_delalloc_inodes(root, &wbc, true, false); }
int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr, bool in_reclaim_context) { + struct writeback_control wbc = { + .nr_to_write = (nr == U64_MAX) ? LONG_MAX : (unsigned long)nr, + .sync_mode = WB_SYNC_NONE, + .range_start = 0, + .range_end = LLONG_MAX, + }; struct btrfs_root *root; struct list_head splice; int ret; @@ -9489,6 +9508,13 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr, spin_lock(&fs_info->delalloc_root_lock); list_splice_init(&fs_info->delalloc_roots, &splice); while (!list_empty(&splice) && nr) { + /* + * Reset nr_to_write here so we know that we're doing a full + * flush. + */ + if (nr == U64_MAX) + wbc.nr_to_write = LONG_MAX; + root = list_first_entry(&splice, struct btrfs_root, delalloc_root); root = btrfs_grab_root(root); @@ -9497,9 +9523,9 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr, &fs_info->delalloc_roots); spin_unlock(&fs_info->delalloc_root_lock);
- ret = start_delalloc_inodes(root, &nr, false, in_reclaim_context); + ret = start_delalloc_inodes(root, &wbc, false, in_reclaim_context); btrfs_put_root(root); - if (ret < 0) + if (ret < 0 || wbc.nr_to_write <= 0) goto out; spin_lock(&fs_info->delalloc_root_lock); } diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c index 67e55c5479b8..e8347461c8dd 100644 --- a/fs/btrfs/space-info.c +++ b/fs/btrfs/space-info.c @@ -532,7 +532,9 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
loops = 0; while ((delalloc_bytes || dio_bytes) && loops < 3) { - btrfs_start_delalloc_roots(fs_info, items, true); + u64 nr_pages = min(delalloc_bytes, to_reclaim) >> PAGE_SHIFT; + + btrfs_start_delalloc_roots(fs_info, nr_pages, true);
loops++; if (wait_ordered && !trans) {