Re: [PATCH][v2] btrfs: track odirect bytes in flight

[David Sterba <dsterba@suse.cz>]

On Thu, Apr 25, 2019 at 04:52:47PM -0400, Josef Bacik wrote:
> When diagnosing a slowdown of generic/224 I noticed we were not doing
> anything when calling into shrink_delalloc().  This is because all
> writes in 224 are O_DIRECT, not delalloc, and thus our delalloc_bytes
> counter is 0, which short circuits most of the work inside of
> shrink_delalloc().  However O_DIRECT writes still consume metadata
> resources and generate ordered extents, which we can still wait on.
> 
> Fix this by tracking outstandingn odirect write bytes, and use this as
> well as the delalloc byts counter to decide if we need to lookup and
> wait on any ordered extents.  If we have more odirect writes than
> delalloc bytes we'll go ahead and wait on any ordered extents
> irregardless of our flush state as flushing delalloc is likely to not
> gain us anything.

Ok, that helps.

> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> v1->v2:
> - updated the changelog
> 
>  fs/btrfs/ctree.h        |  1 +
>  fs/btrfs/disk-io.c      | 15 ++++++++++++++-
>  fs/btrfs/extent-tree.c  | 17 +++++++++++++++--
>  fs/btrfs/ordered-data.c |  9 ++++++++-
>  4 files changed, 38 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 7e774d48c48c..e293d74b2ead 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1016,6 +1016,7 @@ struct btrfs_fs_info {
>  	/* used to keep from writing metadata until there is a nice batch */
>  	struct percpu_counter dirty_metadata_bytes;
>  	struct percpu_counter delalloc_bytes;
> +	struct percpu_counter odirect_bytes;

I've changed odirect to dio everywhere, as this is the common reference
to direct IO kernel code, O_DIRECT is for userspace.