Re: [PATCH 11/11] xfs: reduce exclusive locking on unaligned dio

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jan 22, 2021 at 05:20:43PM +0100, Christoph Hellwig wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Attempt shared locking for unaligned DIO, but only if the the
> underlying extent is already allocated and in written state. On
> failure, retry with the existing exclusive locking.
> 
> Test case is fio randrw of 512 byte IOs using AIO and an iodepth of
> 32 IOs.
> 
> Vanilla:
> 
>   READ: bw=4560KiB/s (4670kB/s), 4560KiB/s-4560KiB/s (4670kB/s-4670kB/s), io=134MiB (140MB), run=30001-30001msec
>   WRITE: bw=4567KiB/s (4676kB/s), 4567KiB/s-4567KiB/s (4676kB/s-4676kB/s), io=134MiB (140MB), run=30001-30001msec
> 
> Patched:
>    READ: bw=37.6MiB/s (39.4MB/s), 37.6MiB/s-37.6MiB/s (39.4MB/s-39.4MB/s), io=1127MiB (1182MB), run=30002-30002msec
>   WRITE: bw=37.6MiB/s (39.4MB/s), 37.6MiB/s-37.6MiB/s (39.4MB/s-39.4MB/s), io=1128MiB (1183MB), run=30002-30002msec
> 
> That's an improvement from ~18k IOPS to a ~150k IOPS, which is
> about the IOPS limit of the VM block device setup I'm testing on.
> 
> 4kB block IO comparison:
> 
>    READ: bw=296MiB/s (310MB/s), 296MiB/s-296MiB/s (310MB/s-310MB/s), io=8868MiB (9299MB), run=30002-30002msec
>   WRITE: bw=296MiB/s (310MB/s), 296MiB/s-296MiB/s (310MB/s-310MB/s), io=8878MiB (9309MB), run=30002-30002msec
> 
> Which is ~150k IOPS, same as what the test gets for sub-block
> AIO+DIO writes with this patch.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> [hch: rebased, split unaligned from nowait]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Brian Foster <bfoster@redhat.com>

Looks good to me now,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_file.c  | 58 +++++++++++++++++++++++++++++++++++-----------
>  fs/xfs/xfs_iomap.c | 29 ++++++++++++++++-------
>  2 files changed, 66 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index c60ff7b5dd829e..39695b59dfcc92 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -544,10 +544,13 @@ xfs_file_dio_write_aligned(
>   * to do sub-block zeroing and that requires serialisation against other direct
>   * I/O to the same block.  In this case we need to serialise the submission of
>   * the unaligned I/O so that we don't get racing block zeroing in the dio layer.
> + * In the case where sub-block zeroing is not required, we can do concurrent
> + * sub-block dios to the same block successfully.
>   *
> - * This means that unaligned dio writes always block. There is no "nowait" fast
> - * path in this code - if IOCB_NOWAIT is set we simply return -EAGAIN up front
> - * and we don't have to worry about that anymore.
> + * Optimistically submit the I/O using the shared lock first, but use the
> + * IOMAP_DIO_OVERWRITE_ONLY flag to tell the lower layers to return -EAGAIN
> + * if block allocation or partial block zeroing would be required.  In that case
> + * we try again with the exclusive lock.
>   */
>  static noinline ssize_t
>  xfs_file_dio_write_unaligned(
> @@ -555,13 +558,28 @@ xfs_file_dio_write_unaligned(
>  	struct kiocb		*iocb,
>  	struct iov_iter		*from)
>  {
> -	int			iolock = XFS_IOLOCK_EXCL;
> +	size_t			isize = i_size_read(VFS_I(ip));
> +	size_t			count = iov_iter_count(from);
> +	int			iolock = XFS_IOLOCK_SHARED;
> +	unsigned int		flags = IOMAP_DIO_OVERWRITE_ONLY;
>  	ssize_t			ret;
>  
> -	/* unaligned dio always waits, bail */
> -	if (iocb->ki_flags & IOCB_NOWAIT)
> -		return -EAGAIN;
> -	xfs_ilock(ip, iolock);
> +	/*
> +	 * Extending writes need exclusivity because of the sub-block zeroing
> +	 * that the DIO code always does for partial tail blocks beyond EOF, so
> +	 * don't even bother trying the fast path in this case.
> +	 */
> +	if (iocb->ki_pos > isize || iocb->ki_pos + count >= isize) {
> +retry_exclusive:
> +		if (iocb->ki_flags & IOCB_NOWAIT)
> +			return -EAGAIN;
> +		iolock = XFS_IOLOCK_EXCL;
> +		flags = IOMAP_DIO_FORCE_WAIT;
> +	}
> +
> +	ret = xfs_ilock_iocb(iocb, iolock);
> +	if (ret)
> +		return ret;
>  
>  	/*
>  	 * We can't properly handle unaligned direct I/O to reflink files yet,
> @@ -578,15 +596,29 @@ xfs_file_dio_write_unaligned(
>  		goto out_unlock;
>  
>  	/*
> -	 * If we are doing unaligned I/O, this must be the only I/O in-flight.
> -	 * Otherwise we risk data corruption due to unwritten extent conversions
> -	 * from the AIO end_io handler.  Wait for all other I/O to drain first.
> +	 * If we are doing exclusive unaligned I/O, this must be the only I/O
> +	 * in-flight.  Otherwise we risk data corruption due to unwritten extent
> +	 * conversions from the AIO end_io handler.  Wait for all other I/O to
> +	 * drain first.
>  	 */
> -	inode_dio_wait(VFS_I(ip));
> +	if (flags & IOMAP_DIO_FORCE_WAIT)
> +		inode_dio_wait(VFS_I(ip));
>  
>  	trace_xfs_file_direct_write(iocb, from);
>  	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> -			   &xfs_dio_write_ops, IOMAP_DIO_FORCE_WAIT);
> +			   &xfs_dio_write_ops, flags);
> +
> +	/*
> +	 * Retry unaligned I/O with exclusive blocking semantics if the DIO
> +	 * layer rejected it for mapping or locking reasons. If we are doing
> +	 * nonblocking user I/O, propagate the error.
> +	 */
> +	if (ret == -EAGAIN && !(iocb->ki_flags & IOCB_NOWAIT)) {
> +		ASSERT(flags & IOMAP_DIO_OVERWRITE_ONLY);
> +		xfs_iunlock(ip, iolock);
> +		goto retry_exclusive;
> +	}
> +
>  out_unlock:
>  	if (iolock)
>  		xfs_iunlock(ip, iolock);
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 7b9ff824e82d48..ef76f775fabf11 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -784,15 +784,28 @@ xfs_direct_write_iomap_begin(
>  		goto allocate_blocks;
>  
>  	/*
> -	 * NOWAIT IO needs to span the entire requested IO with a single map so
> -	 * that we avoid partial IO failures due to the rest of the IO range not
> -	 * covered by this map triggering an EAGAIN condition when it is
> -	 * subsequently mapped and aborting the IO.
> +	 * NOWAIT and OVERWRITE I/O needs to span the entire requested I/O with
> +	 * a single map so that we avoid partial IO failures due to the rest of
> +	 * the I/O range not covered by this map triggering an EAGAIN condition
> +	 * when it is subsequently mapped and aborting the I/O.
>  	 */
> -	if ((flags & IOMAP_NOWAIT) &&
> -	    !imap_spans_range(&imap, offset_fsb, end_fsb)) {
> +	if (flags & (IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY)) {
>  		error = -EAGAIN;
> -		goto out_unlock;
> +		if (!imap_spans_range(&imap, offset_fsb, end_fsb))
> +			goto out_unlock;
> +	}
> +
> +	/*
> +	 * For overwrite only I/O, we cannot convert unwritten extents without
> +	 * requiring sub-block zeroing.  This can only be done under an
> +	 * exclusive IOLOCK, hence return -EAGAIN if this is not a written
> +	 * extent to tell the caller to try again.
> +	 */
> +	if (flags & IOMAP_OVERWRITE_ONLY) {
> +		error = -EAGAIN;
> +		if (imap.br_state != XFS_EXT_NORM &&
> +	            ((offset | length) & mp->m_blockmask))
> +			goto out_unlock;
>  	}
>  
>  	xfs_iunlock(ip, lockmode);
> @@ -801,7 +814,7 @@ xfs_direct_write_iomap_begin(
>  
>  allocate_blocks:
>  	error = -EAGAIN;
> -	if (flags & IOMAP_NOWAIT)
> +	if (flags & (IOMAP_NOWAIT | IOMAP_OVERWRITE_ONLY))
>  		goto out_unlock;
>  
>  	/*
> -- 
> 2.29.2
>