On Wed, Apr 27, 2016 at 09:59:38AM -0400, Chris Mason wrote:
> 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,
> >
> > Signed-off-by: Josef Bacik <jba...@fb.com>
> > ---
> > 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;
> > + }
>
> This ends up deadlocking because btrfs_attach_transaction waits in ways
> that join does not. The differences between these two are really
> subtle, and we manage to make this mistake every year or so.
>
> Subject: [PATCH] btrfs: fix deadlock in delayed_ref_async_start
>
> "Btrfs: track transid for delayed ref flushing" was deadlocking on
> btrfs_attach_transaction because its not safe to call from the async
> delayed ref start code. This commit brings back btrfs_join_transaction
> instead and checks for a blocked commit.
Are we going to take this patch?
Thanks,
-liubo
>
> Signed-off-by: Josef Bacik <jba...@fb.com>
> Signed-off-by: Chris Mason <c...@fb.com>
> ---
> fs/btrfs/extent-tree.c | 20 +++++++++++---------
> 1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 6ce5b6c..44da4ac 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2845,16 +2845,13 @@ static void delayed_ref_async_start(struct btrfs_work
> *work)
>
> async = container_of(work, struct async_delayed_refs, work);
>
> - trans = btrfs_attach_transaction(async->root);
> - if (IS_ERR(trans)) {
> - if (PTR_ERR(trans) != -ENOENT)
> - async->error = PTR_ERR(trans);
> + /* if the commit is already started, we don't need to wait here */
> + if (btrfs_transaction_blocked(async->root->fs_info))
> goto done;
> - }
>
> - /* Don't bother flushing if we got into a different transaction */
> - if (trans->transid != async->transid) {
> - btrfs_end_transaction(trans, async->root);
> + trans = btrfs_join_transaction(async->root);
> + if (IS_ERR(trans)) {
> + async->error = PTR_ERR(trans);
> goto done;
> }
>
> @@ -2863,10 +2860,15 @@ static void delayed_ref_async_start(struct btrfs_work
> *work)
> * wait on delayed refs
> */
> trans->sync = true;
> +
> + /* Don't bother flushing if we got into a different transaction */
> + if (trans->transid > async->transid)
> + goto end;
> +
> ret = btrfs_run_delayed_refs(trans, async->root, async->count);
> if (ret)
> async->error = ret;
> -
> +end:
> ret = btrfs_end_transaction(trans, async->root);
> if (ret && !async->error)
> async->error = ret;
> --
> 2.8.0.rc2
>
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