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

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

On Fri, Oct 02, 2020 at 11:33:57AM -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.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  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..84ee917014f1 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	(262144 << PAGE_SHIFT)

Hm, shouldn't that 262144 have to be annoated with a 'ULL' so that a
dumb compiler won't turn that into a u32 and shift that all the way to
zero?

I still kind of wonder about the letting the limit hit 16G on power with
64k pages, but I guess the number of pages we have to whack is ... not
that high?

I dunno, if you fire up a 64k-page system with fantastical IO
capabilities, attach a realtime volume, fallocate a 32G file and then
try to write to that, will it actually turn that into one gigantic IO?

> +
>  /*
>   * 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
>