Re: [PATCH RFC 2/4] xfs: defer agfl block frees when dfops is available

[Dave Chinner <david@fromorbit.com>]

On Thu, Dec 07, 2017 at 01:58:08PM -0500, Brian Foster wrote:
> The AGFL fixup code executes before every block allocation/free and
> rectifies the AGFL based on the current, dynamic allocation
> requirements of the fs. The AGFL must hold a minimum number of
> blocks to satisfy a worst case split of the free space btrees caused
> by the impending allocation operation. The AGFL is also updated to
> maintain the implicit requirement for a minimum number of free slots
> to satisfy a worst case join of the free space btrees.
> 
> Since the AGFL caches individual blocks, AGFL reduction typically
> involves multiple, single block frees. We've had reports of
> transaction overrun problems during certain workloads that boil down
> to AGFL reduction freeing multiple blocks and consuming more space
> in the log than was reserved for the transaction.
> 
> Since the objective of freeing AGFL blocks is to ensure free AGFL
> free slots are available for the upcoming allocation, one way to
> address this problem is to release surplus blocks from the AGFL
> immediately but defer the free of those blocks (similar to how
> file-mapped blocks are unmapped from the file in one transaction and
> freed via a deferred operation) until the transaction is rolled.
> This turns AGFL reduction into an operation with predictable log
> reservation consumption.
> 
> Add the capability to defer AGFL block frees when a deferred ops
> list is handed to the AGFL fixup code. Deferring AGFL frees is a
> conditional behavior based on whether the caller has populated the
> new dfops field of the xfs_alloc_arg structure. A bit of
> customization is required to handle deferred completion processing
> because AGFL blocks are accounted against a separate reservation
> pool and AGFL are not inserted into the extent busy list when freed
> (they are inserted when used and released back to the AGFL). Reuse
> the majority of the existing deferred extent free infrastructure and
> customize it appropriately to handle AGFL blocks.

Ok, so it uses the EFI/EFD to make sure that the block freeing is
logged and replayed. So my question is:

> +/*
> + * AGFL blocks are accounted differently in the reserve pools and are not
> + * inserted into the busy extent list.
> + */
> +STATIC int
> +xfs_agfl_free_finish_item(
> +	struct xfs_trans		*tp,
> +	struct xfs_defer_ops		*dop,
> +	struct list_head		*item,
> +	void				*done_item,
> +	void				**state)
> +{

How does this function get called by log recovery when processing
the EFI as there is no flag in the EFI that says this was a AGFL
block?

That said, I haven't traced through whether this matters or not,
but I suspect it does because freelist frees use XFS_AG_RESV_AGFL
and that avoids accounting the free to the superblock counters
because the block is already accounted as free space....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com