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@xxxxxxxxxxxxx
