On Mon, Apr 11, 2016 at 05:37:40PM -0400, Josef Bacik wrote:
> Using the offwakecputime bpf script I noticed most of our time was spent waiting
> on the delayed ref throttling. This is what is supposed to happen, but
> sometimes the transaction can commit and then we're waiting for throttling that
> doesn't matter anymore. So change this stuff to be a little smarter by tracking
> the transid we were in when we initiated the throttling. If the transaction we
> get is different then we can just bail out. This resulted in a 50% speedup in
> my fs_mark test, and reduced the amount of time spent throttling by 60 seconds
> over the entire run (which is about 30 minutes). Thanks,
Does the bpf script show where it's waiting on? delayed_refs spinlock?
Maybe we can make it even smarter.
In __btrfs_end_transaction(), the only case it won't wait for running delayed refs
is when trans is JOIN_NOLOCK or ATTACH and "must_run_delayed_refs = 2".
In other cases, even we queue a work into helper worker to do async
delayed refs processing, __btrfs_end_transaction() still waits there.
Since it's a 50% speedup, it looks like at least 50% of __btrfs_end_transaction()
are waiting for other trans's queued delayed refs, can we do the check
a little bit earlier, in btrfs_async_run_delayed_refs()?
>
> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> ---
> fs/btrfs/ctree.h | 2 +-
> fs/btrfs/extent-tree.c | 15 ++++++++++++---
> fs/btrfs/inode.c | 1 +
> fs/btrfs/transaction.c | 3 ++-
> 4 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 55a24c5..4222936 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3505,7 +3505,7 @@ void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
> int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
> struct btrfs_root *root, unsigned long count);
> int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> - unsigned long count, int wait);
> + unsigned long count, u64 transid, int wait);
> int btrfs_lookup_data_extent(struct btrfs_root *root, u64 start, u64 len);
> int btrfs_lookup_extent_info(struct btrfs_trans_handle *trans,
> struct btrfs_root *root, u64 bytenr,
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 4b5a517..f23f426 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2839,6 +2839,7 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
>
> struct async_delayed_refs {
> struct btrfs_root *root;
> + u64 transid;
> int count;
> int error;
> int sync;
> @@ -2854,9 +2855,16 @@ static void delayed_ref_async_start(struct btrfs_work *work)
>
> async = container_of(work, struct async_delayed_refs, work);
>
> - trans = btrfs_join_transaction(async->root);
> + trans = btrfs_attach_transaction(async->root);
> if (IS_ERR(trans)) {
> - async->error = PTR_ERR(trans);
> + if (PTR_ERR(trans) != -ENOENT)
> + async->error = PTR_ERR(trans);
> + goto done;
> + }
> +
> + /* Don't bother flushing if we got into a different transaction */
> + if (trans->transid != async->transid) {
> + btrfs_end_transaction(trans, async->root);
Interesting, btrfs_end_transaction will also issue a work to do
delayed_ref_async_start, and it doesn't wait.
Thanks,
-liubo
> goto done;
> }
>
> @@ -2880,7 +2888,7 @@ done:
> }
>
> int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> - unsigned long count, int wait)
> + unsigned long count, u64 transid, int wait)
> {
> struct async_delayed_refs *async;
> int ret;
> @@ -2892,6 +2900,7 @@ int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> async->root = root->fs_info->tree_root;
> async->count = count;
> async->error = 0;
> + async->transid = transid;
> if (wait)
> async->sync = 1;
> else
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 723e4bb..e6dd4cc 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -4534,6 +4534,7 @@ delete:
> BUG_ON(ret);
> if (btrfs_should_throttle_delayed_refs(trans, root))
> btrfs_async_run_delayed_refs(root,
> + trans->transid,
> trans->delayed_ref_updates * 2, 0);
> if (be_nice) {
> if (truncate_space_check(trans, root,
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 43885e5..7c7671d 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -817,6 +817,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
> {
> struct btrfs_transaction *cur_trans = trans->transaction;
> struct btrfs_fs_info *info = root->fs_info;
> + u64 transid = trans->transid;
> unsigned long cur = trans->delayed_ref_updates;
> int lock = (trans->type != TRANS_JOIN_NOLOCK);
> int err = 0;
> @@ -904,7 +905,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>
> kmem_cache_free(btrfs_trans_handle_cachep, trans);
> if (must_run_delayed_refs) {
> - btrfs_async_run_delayed_refs(root, cur,
> + btrfs_async_run_delayed_refs(root, cur, transid,
> must_run_delayed_refs == 1);
> }
> return err;
> --
> 2.5.0
>
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