Re: [PATCH 00/24] xfs: rework inode flushing to make inode reclaim fully asynchronous

Dave Chinner
2020-05-22T04:04:16Z

FWIW, I forgot to put it in the original description - the series
can be pulled from my git tree here:

git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs.git xfs-async-inode-reclaim

Cheers,

Dave.

On Fri, May 22, 2020 at 01:50:05PM +1000, Dave Chinner wrote:
> Hi folks,
>
> Inode flushing requires that we first lock an inode, then check it,
> then lock the underlying buffer, flush the inode to the buffer and
> finally add the inode to the buffer to be unlocked on IO completion.
> We then walk all the other cached inodes in the buffer range and
> optimistically lock and flush them to the buffer without blocking.
>
> This cluster write effectively repeats the same code we do with the
> initial inode, except now it has to special case that initial inode
> that is already locked. Hence we have multiple copies of very
> similar code, and it is a result of inode cluster flushing being
> based on a specific inode rather than grabbing the buffer and
> flushing all available inodes to it.
>
> The problem with this at the moment is that we we can't look up the
> buffer until we have guaranteed that an inode is held exclusively
> and it's not going away while we get the buffer through an imap
> lookup. Hence we are kinda stuck locking an inode before we can look
> up the buffer.
>
> This is also a result of inodes being detached from the cluster
> buffer except when IO is being done. This has the further problem
> that the cluster buffer can be reclaimed from memory and then the
> inode can be dirtied. At this point cleaning the inode requires a
> read-modify-write cycle on the cluster buffer. If we then are put
> under memory pressure, cleaning that dirty inode to reclaim it
> requires allocating memory for the cluster buffer and this leads to
> all sorts of problems.
>
> We used synchronous inode writeback in reclaim as a throttle that
> provided a forwards progress mechanism when RMW cycles were required
> to clean inodes. Async writeback of inodes (e.g. via the AIL) would
> immediately exhaust remaining memory reserves trying to allocate
> inode cluster after inode cluster. The synchronous writeback of an
> inode cluster allowed reclaim to release the inode cluster and have
> it freed almost immediately which could then be used to allocate the
> next inode cluster buffer. Hence the IO based throttling mechanism
> largely guaranteed forwards progress in inode reclaim. By removing
> the requirement for require memory allocation for inode writeback
> filesystem level, we can issue writeback asynchrnously and not have
> to worry about the memory exhaustion anymore.
>
> Another issue is that if we have slow disks, we can build up dirty
> inodes in memory that can then take hours for an operation like
> unmount to flush. A RMW cycle per inode on a slow RAID6 device can
> mean we only clean 50 inodes a second, and when there are hundreds
> of thousands of dirty inodes that need to be cleaned this can take a
> long time. PInning the cluster buffers will greatly speed up inode
> writeback on slow storage systems like this.
>
> These limitations all stem from the same source: inode writeback is
> inode centric, And they are largely solved by the same architectural
> change: make inode writeback cluster buffer centric. This series is
> makes that architectural change.
>
> Firstly, we start by pinning the inode backing buffer in memory
> when an inode is marked dirty (i.e. when it is logged). By tracking
> the number of dirty inodes on a buffer as a counter rather than a
> flag, we avoid the problem of overlapping inode dirtying and buffer
> flushing racing to set/clear the dirty flag. Hence as long as there
> is a dirty inode in memory, the buffer will not be able to be
> reclaimed. We can safely do this inode cluster buffer lookup when we
> dirty an inode as we do not hold the buffer locked - we merely take
> a reference to it and then release it - and hence we don't cause any
> new lock order issues.
>
> When the inode is finally cleaned, the reference to the buffer can
> be removed from the inode log item and the buffer released. This is
> done from the inode completion callbacks that are attached to the
> buffer when the inode is flushed.
>
> Pinning the cluster buffer in this way immediately avoids the RMW
> problem in inode writeback and reclaim contexts by moving the memory
> allocation and the blocking buffer read into the transaction context
> that dirties the inode. This inverts our dirty inode throttling
> mechanism - we now throttle the rate at which we can dirty inodes to
> rate at which we can allocate memory and read inode cluster buffers
> into memory rather than via throttling reclaim to rate at which we
> can clean dirty inodes.
>
> Hence if we are under memory pressure, we'll block on memory
> allocation when trying to dirty the referenced inode, rather than in
> the memory reclaim path where we are trying to clean unreferenced
> inodes to free memory. Hence we no longer have to guarantee
> forwards progress in inode reclaim as we aren't doing memory
> allocation, and that means we can remove inode writeback from the
> XFS inode shrinker completely without changing the system tolerance
> for low memory operation.
>
> Tracking the buffers via the inode log item also allows us to
> completely rework the inode flushing mechanism. While the inode log
> item is in the AIL, it is safe for the AIL to access any member of
> the log item. Hence the AIL push mechanisms can access the buffer
> attached to the inode without first having to lock the inode.
>
> This means we can essentially lock the buffer directly and then
> call xfs_iflush_cluster() without first going through xfs_iflush()
> to find the buffer. Hence we can remove xfs_iflush() altogether,
> because the two places that call it - the inode item push code and
> inode reclaim - no longer need to flush inodes directly.
>
> This can be further optimised by attaching the inode to the cluster
> buffer when the inode is dirtied. i.e. when we add the buffer
> reference to the inode log item, we also attach the inode to the
> buffer for IO processing. This leads to the dirty inodes always
> being attached to the buffer and hence we no longer need to add them
> when we flush the inode and remove them when IO completes. Instead
> the inodes are attached when the node log item is dirtied, and
> removed when the inode log item is cleaned.
>
> With this structure in place, we no longer need to do
> lookups to find the dirty inodes in the cache to attach to the
> buffer in xfs_iflush_cluster() - they are already attached to the
> buffer. Hence when the AIL pushes an inode, we just grab the buffer
> from the log item, and then walk the buffer log item list to lock
> and flush the dirty inodes attached to the buffer.
>
> This greatly simplifies inode writeback, and removes another memory
> allocation from the inode writeback path (the array used for the
> radix tree gang lookup). And while the radix tree lookups are fast,
> walking the linked list of dirty inodes is faster.
>
> There is followup work I am doing that uses the inode cluster buffer
> as a replacement in the AIL for tracking dirty inodes. This part of
> the series is not ready yet as it has some intricate locking
> requirements. That is an optimisation, so I've left that out because
> solving the inode reclaim blocking problems is the important part of
> this work.
>
> In short, this series simplifies inode writeback and fixes the long
> standing inode reclaim blocking issues without requiring any changes
> to the memory reclaim infrastructure.
>
> Note: dquots should probably be converted to cluster flushing in a
> similar way, as they have many of the same issues as inode flushing.
>
> Thoughts, comments and improvemnts welcome.
>
> -Dave.
>
>
>
> Dave Chinner (24):
> xfs: remove logged flag from inode log item
> xfs: add an inode item lock
> xfs: mark inode buffers in cache
> xfs: mark dquot buffers in cache
> xfs: mark log recovery buffers for completion
> xfs: call xfs_buf_iodone directly
> xfs: clean up whacky buffer log item list reinit
> xfs: fold xfs_istale_done into xfs_iflush_done
> xfs: use direct calls for dquot IO completion
> xfs: clean up the buffer iodone callback functions
> xfs: get rid of log item callbacks
> xfs: pin inode backing buffer to the inode log item
> xfs: make inode reclaim almost non-blocking
> xfs: remove IO submission from xfs_reclaim_inode()
> xfs: allow multiple reclaimers per AG
> xfs: don't block inode reclaim on the ILOCK
> xfs: remove SYNC_TRYLOCK from inode reclaim
> xfs: clean up inode reclaim comments
> xfs: attach inodes to the cluster buffer when dirtied
> xfs: xfs_iflush() is no longer necessary
> xfs: rename xfs_iflush_int()
> xfs: rework xfs_iflush_cluster() dirty inode iteration
> xfs: factor xfs_iflush_done
> xfs: remove xfs_inobp_check()
>
> fs/xfs/libxfs/xfs_inode_buf.c | 27 +-
> fs/xfs/libxfs/xfs_inode_buf.h | 6 -
> fs/xfs/libxfs/xfs_trans_inode.c | 108 +++++--
> fs/xfs/xfs_buf.c | 44 ++-
> fs/xfs/xfs_buf.h | 49 +--
> fs/xfs/xfs_buf_item.c | 205 +++++--------
> fs/xfs/xfs_buf_item.h | 8 +-
> fs/xfs/xfs_buf_item_recover.c | 5 +-
> fs/xfs/xfs_dquot.c | 32 +-
> fs/xfs/xfs_dquot.h | 1 +
> fs/xfs/xfs_dquot_item_recover.c | 2 +-
> fs/xfs/xfs_file.c | 9 +-
> fs/xfs/xfs_icache.c | 293 +++++-------------
> fs/xfs/xfs_inode.c | 515 +++++++++++---------------------
> fs/xfs/xfs_inode.h | 2 +-
> fs/xfs/xfs_inode_item.c | 281 ++++++++---------
> fs/xfs/xfs_inode_item.h | 9 +-
> fs/xfs/xfs_inode_item_recover.c | 2 +-
> fs/xfs/xfs_log_recover.c | 5 +-
> fs/xfs/xfs_mount.c | 4 -
> fs/xfs/xfs_mount.h | 1 -
> fs/xfs/xfs_trans.h | 3 -
> fs/xfs/xfs_trans_buf.c | 15 +-
> fs/xfs/xfs_trans_priv.h | 12 +-
> 24 files changed, 680 insertions(+), 958 deletions(-)
>
> --
> 2.26.2.761.g0e0b3e54be
>
>

--
Dave Chinner
david@fromorbit.com