Re: [PATCH v2 2/2] xfs: kick extra large ioends to completion workqueue

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Oct 05, 2020 at 11:21:02AM -0400, Brian Foster wrote:
> We've had reports of soft lockup warnings in the iomap ioend
> completion path due to very large bios and/or bio chains. Divert any
> ioends with 256k or more pages to process to the workqueue so
> completion occurs in non-atomic context and can reschedule to avoid
> soft lockup warnings.

Hmmmm... is there any way we can just make end_page_writeback faster?

TBH it still strikes me as odd that we'd cap ioends this way just to
cover for the fact that we have to poke each and every page.

(Also, those 'bool atomic' in the other patch make me kind of nervous --
how do we make sure (from a QA perspective) that nobody gets that wrong?)

--D

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
> 
> v2:
> - Fix type in macro.
> 
>  fs/xfs/xfs_aops.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 3e061ea99922..c00cc0624986 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -30,6 +30,13 @@ XFS_WPC(struct iomap_writepage_ctx *ctx)
>  	return container_of(ctx, struct xfs_writepage_ctx, ctx);
>  }
>  
> +/*
> + * Kick extra large ioends off to the workqueue. Completion will process a lot
> + * of pages for a large bio or bio chain and a non-atomic context is required to
> + * reschedule and avoid soft lockup warnings.
> + */
> +#define XFS_LARGE_IOEND	(262144ULL << PAGE_SHIFT)
> +
>  /*
>   * Fast and loose check if this write could update the on-disk inode size.
>   */
> @@ -239,7 +246,8 @@ static inline bool xfs_ioend_needs_workqueue(struct iomap_ioend *ioend)
>  {
>  	return ioend->io_private ||
>  		ioend->io_type == IOMAP_UNWRITTEN ||
> -		(ioend->io_flags & IOMAP_F_SHARED);
> +		(ioend->io_flags & IOMAP_F_SHARED) ||
> +		(ioend->io_size >= XFS_LARGE_IOEND);
>  }
>  
>  STATIC void
> -- 
> 2.25.4
>