Re: [PATCH] xfs: force buffer writeback before blocking on the ilock in inode reclaim

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Sun, Nov 20, 2011 at 02:23:34AM -0500, Christoph Hellwig wrote:
> If we are doing synchronous inode reclaim we block the VM from making
> progress in memory reclaim.  So if we encouter a flush locked inode
> make sure we force out all delayed buffers ASAP to speed up the wait
> for it to be unlocked.  Without this we can get hangs of up to 30
> seconds during workloads hitting synchronous inode reclaim.

I don't think we need to push out all delayed buffers - that's an
awfully big sledge hammer to get a single buffer moving. Indeed, we
already have a mechanism for dealing with this problem -
xfs_buf_delwri_promote() - when we hit it during AIL flushing.

IOWs, we only need to promote the buffer the inode sits in and kick
xfsbufd. that is, something like:

        bp = xfs_incore(ip->i_mount->m_ddev_targp, iip->ili_format.ilf_blkno,
                        iip->ili_format.ilf_len, XBF_TRYLOCK);
        if (bp && XFS_BUF_ISDELAYWRITE(bp)) {
                xfs_buf_delwri_promote(bp);
                wake_up_process(mp->m_ddev_targp->bt_task);
        }
        if (bp)
                xfs_buf_relse(bp);


Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx