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linux/fs/xfs/xfs_drain.h

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xfs: allow queued AG intents to drain before scrubbing When a writer thread executes a chain of log intent items, the AG header buffer locks will cycle during a transaction roll to get from one intent item to the next in a chain. Although scrub takes all AG header buffer locks, this isn't sufficient to guard against scrub checking an AG while that writer thread is in the middle of finishing a chain because there's no higher level locking primitive guarding allocation groups. When there's a collision, cross-referencing between data structures (e.g. rmapbt and refcountbt) yields false corruption events; if repair is running, this results in incorrect repairs, which is catastrophic. Fix this by adding to the perag structure the count of active intents and make scrub wait until it has both AG header buffer locks and the intent counter reaches zero. One quirk of the drain code is that deferred bmap updates also bump and drop the intent counter. A fundamental decision made during the design phase of the reverse mapping feature is that updates to the rmapbt records are always made by the same code that updates the primary metadata. In other words, callers of bmapi functions expect that the bmapi functions will queue deferred rmap updates. Some parts of the reflink code queue deferred refcount (CUI) and bmap (BUI) updates in the same head transaction, but the deferred work manager completely finishes the CUI before the BUI work is started. As a result, the CUI drops the intent count long before the deferred rmap (RUI) update even has a chance to bump the intent count. The only way to keep the intent count elevated between the CUI and RUI is for the BUI to bump the counter until the RUI has been created. A second quirk of the intent drain code is that deferred work items must increment the intent counter as soon as the work item is added to the transaction. When a BUI completes and queues an RUI, the RUI must increment the counter before the BUI decrements it. The only way to accomplish this is to require that the counter be bumped as soon as the deferred work item is created in memory. In the next patches we'll improve on this facility, but this patch provides the basic functionality. Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Dave Chinner <dchinner@redhat.com>
2023-04-12 01:59:58 +00:00
// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Copyright (C) 2022-2023 Oracle. All Rights Reserved.
* Author: Darrick J. Wong <djwong@kernel.org>
*/
#ifndef XFS_DRAIN_H_
#define XFS_DRAIN_H_
struct xfs_perag;
#ifdef CONFIG_XFS_DRAIN_INTENTS
/*
* Passive drain mechanism. This data structure tracks a count of some items
* and contains a waitqueue for callers who would like to wake up when the
* count hits zero.
*/
struct xfs_defer_drain {
/* Number of items pending in some part of the filesystem. */
atomic_t dr_count;
/* Queue to wait for dri_count to go to zero */
struct wait_queue_head dr_waiters;
};
void xfs_defer_drain_init(struct xfs_defer_drain *dr);
void xfs_defer_drain_free(struct xfs_defer_drain *dr);
xfs: minimize overhead of drain wakeups by using jump labels To reduce the runtime overhead even further when online fsck isn't running, use a static branch key to decide if we call wake_up on the drain. For compilers that support jump labels, the call to wake_up is replaced by a nop sled when nobody is waiting for intents to drain. From my initial microbenchmarking, every transition of the static key between the on and off states takes about 22000ns to complete; this is paid entirely by the xfs_scrub process. When the static key is off (which it should be when fsck isn't running), the nop sled adds an overhead of approximately 0.36ns to runtime code. The post-atomic lockless waiter check adds about 0.03ns, which is basically free. For the few compilers that don't support jump labels, runtime code pays the cost of calling wake_up on an empty waitqueue, which was observed to be about 30ns. However, most architectures that have sufficient memory and CPU capacity to run XFS also support jump labels, so this is not much of a worry. Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Dave Chinner <dchinner@redhat.com>
2023-04-12 01:59:59 +00:00
void xfs_drain_wait_disable(void);
void xfs_drain_wait_enable(void);
xfs: allow queued AG intents to drain before scrubbing When a writer thread executes a chain of log intent items, the AG header buffer locks will cycle during a transaction roll to get from one intent item to the next in a chain. Although scrub takes all AG header buffer locks, this isn't sufficient to guard against scrub checking an AG while that writer thread is in the middle of finishing a chain because there's no higher level locking primitive guarding allocation groups. When there's a collision, cross-referencing between data structures (e.g. rmapbt and refcountbt) yields false corruption events; if repair is running, this results in incorrect repairs, which is catastrophic. Fix this by adding to the perag structure the count of active intents and make scrub wait until it has both AG header buffer locks and the intent counter reaches zero. One quirk of the drain code is that deferred bmap updates also bump and drop the intent counter. A fundamental decision made during the design phase of the reverse mapping feature is that updates to the rmapbt records are always made by the same code that updates the primary metadata. In other words, callers of bmapi functions expect that the bmapi functions will queue deferred rmap updates. Some parts of the reflink code queue deferred refcount (CUI) and bmap (BUI) updates in the same head transaction, but the deferred work manager completely finishes the CUI before the BUI work is started. As a result, the CUI drops the intent count long before the deferred rmap (RUI) update even has a chance to bump the intent count. The only way to keep the intent count elevated between the CUI and RUI is for the BUI to bump the counter until the RUI has been created. A second quirk of the intent drain code is that deferred work items must increment the intent counter as soon as the work item is added to the transaction. When a BUI completes and queues an RUI, the RUI must increment the counter before the BUI decrements it. The only way to accomplish this is to require that the counter be bumped as soon as the deferred work item is created in memory. In the next patches we'll improve on this facility, but this patch provides the basic functionality. Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Dave Chinner <dchinner@redhat.com>
2023-04-12 01:59:58 +00:00
/*
* Deferred Work Intent Drains
* ===========================
*
* When a writer thread executes a chain of log intent items, the AG header
* buffer locks will cycle during a transaction roll to get from one intent
* item to the next in a chain. Although scrub takes all AG header buffer
* locks, this isn't sufficient to guard against scrub checking an AG while
* that writer thread is in the middle of finishing a chain because there's no
* higher level locking primitive guarding allocation groups.
*
* When there's a collision, cross-referencing between data structures (e.g.
* rmapbt and refcountbt) yields false corruption events; if repair is running,
* this results in incorrect repairs, which is catastrophic.
*
* The solution is to the perag structure the count of active intents and make
* scrub wait until it has both AG header buffer locks and the intent counter
* reaches zero. It is therefore critical that deferred work threads hold the
* AGI or AGF buffers when decrementing the intent counter.
*
* Given a list of deferred work items, the deferred work manager will complete
* a work item and all the sub-items that the parent item creates before moving
* on to the next work item in the list. This is also true for all levels of
* sub-items. Writer threads are permitted to queue multiple work items
* targetting the same AG, so a deferred work item (such as a BUI) that creates
* sub-items (such as RUIs) must bump the intent counter and maintain it until
* the sub-items can themselves bump the intent counter.
*
* Therefore, the intent count tracks entire lifetimes of deferred work items.
* All functions that create work items must increment the intent counter as
* soon as the item is added to the transaction and cannot drop the counter
* until the item is finished or cancelled.
*/
struct xfs_perag *xfs_perag_intent_get(struct xfs_mount *mp,
xfs_agnumber_t agno);
void xfs_perag_intent_put(struct xfs_perag *pag);
void xfs_perag_intent_hold(struct xfs_perag *pag);
void xfs_perag_intent_rele(struct xfs_perag *pag);
int xfs_perag_intent_drain(struct xfs_perag *pag);
bool xfs_perag_intent_busy(struct xfs_perag *pag);
#else
struct xfs_defer_drain { /* empty */ };
#define xfs_defer_drain_free(dr) ((void)0)
#define xfs_defer_drain_init(dr) ((void)0)
#define xfs_perag_intent_get(mp, agno) xfs_perag_get((mp), (agno))
#define xfs_perag_intent_put(pag) xfs_perag_put(pag)
static inline void xfs_perag_intent_hold(struct xfs_perag *pag) { }
static inline void xfs_perag_intent_rele(struct xfs_perag *pag) { }
#endif /* CONFIG_XFS_DRAIN_INTENTS */
#endif /* XFS_DRAIN_H_ */
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