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466c525d6d
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>
167 lines
3.9 KiB
C
167 lines
3.9 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* Copyright (C) 2022-2023 Oracle. All Rights Reserved.
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* Author: Darrick J. Wong <djwong@kernel.org>
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*/
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#include "xfs.h"
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#include "xfs_fs.h"
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#include "xfs_shared.h"
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#include "xfs_format.h"
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#include "xfs_trans_resv.h"
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#include "xfs_mount.h"
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#include "xfs_ag.h"
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#include "xfs_trace.h"
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/*
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* Use a static key here to reduce the overhead of xfs_drain_rele. If the
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* compiler supports jump labels, the static branch will be replaced by a nop
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* sled when there are no xfs_drain_wait callers. Online fsck is currently
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* the only caller, so this is a reasonable tradeoff.
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*
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* Note: Patching the kernel code requires taking the cpu hotplug lock. Other
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* parts of the kernel allocate memory with that lock held, which means that
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* XFS callers cannot hold any locks that might be used by memory reclaim or
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* writeback when calling the static_branch_{inc,dec} functions.
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*/
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static DEFINE_STATIC_KEY_FALSE(xfs_drain_waiter_gate);
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void
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xfs_drain_wait_disable(void)
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{
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static_branch_dec(&xfs_drain_waiter_gate);
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}
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void
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xfs_drain_wait_enable(void)
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{
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static_branch_inc(&xfs_drain_waiter_gate);
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}
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void
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xfs_defer_drain_init(
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struct xfs_defer_drain *dr)
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{
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atomic_set(&dr->dr_count, 0);
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init_waitqueue_head(&dr->dr_waiters);
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}
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void
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xfs_defer_drain_free(struct xfs_defer_drain *dr)
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{
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ASSERT(atomic_read(&dr->dr_count) == 0);
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}
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/* Increase the pending intent count. */
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static inline void xfs_defer_drain_grab(struct xfs_defer_drain *dr)
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{
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atomic_inc(&dr->dr_count);
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}
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static inline bool has_waiters(struct wait_queue_head *wq_head)
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{
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/*
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* This memory barrier is paired with the one in set_current_state on
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* the waiting side.
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*/
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smp_mb__after_atomic();
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return waitqueue_active(wq_head);
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}
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/* Decrease the pending intent count, and wake any waiters, if appropriate. */
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static inline void xfs_defer_drain_rele(struct xfs_defer_drain *dr)
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{
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if (atomic_dec_and_test(&dr->dr_count) &&
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static_branch_unlikely(&xfs_drain_waiter_gate) &&
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has_waiters(&dr->dr_waiters))
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wake_up(&dr->dr_waiters);
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}
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/* Are there intents pending? */
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static inline bool xfs_defer_drain_busy(struct xfs_defer_drain *dr)
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{
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return atomic_read(&dr->dr_count) > 0;
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}
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/*
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* Wait for the pending intent count for a drain to hit zero.
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*
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* Callers must not hold any locks that would prevent intents from being
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* finished.
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*/
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static inline int xfs_defer_drain_wait(struct xfs_defer_drain *dr)
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{
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return wait_event_killable(dr->dr_waiters, !xfs_defer_drain_busy(dr));
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}
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/*
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* Get a passive reference to an AG and declare an intent to update its
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* metadata.
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*/
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struct xfs_perag *
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xfs_perag_intent_get(
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struct xfs_mount *mp,
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xfs_agnumber_t agno)
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{
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struct xfs_perag *pag;
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pag = xfs_perag_get(mp, agno);
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if (!pag)
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return NULL;
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xfs_perag_intent_hold(pag);
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return pag;
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}
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/*
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* Release our intent to update this AG's metadata, and then release our
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* passive ref to the AG.
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*/
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void
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xfs_perag_intent_put(
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struct xfs_perag *pag)
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{
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xfs_perag_intent_rele(pag);
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xfs_perag_put(pag);
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}
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/*
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* Declare an intent to update AG metadata. Other threads that need exclusive
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* access can decide to back off if they see declared intentions.
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*/
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void
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xfs_perag_intent_hold(
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struct xfs_perag *pag)
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{
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trace_xfs_perag_intent_hold(pag, __return_address);
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xfs_defer_drain_grab(&pag->pag_intents_drain);
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}
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/* Release our intent to update this AG's metadata. */
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void
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xfs_perag_intent_rele(
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struct xfs_perag *pag)
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{
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trace_xfs_perag_intent_rele(pag, __return_address);
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xfs_defer_drain_rele(&pag->pag_intents_drain);
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}
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/*
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* Wait for the intent update count for this AG to hit zero.
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* Callers must not hold any AG header buffers.
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*/
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int
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xfs_perag_intent_drain(
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struct xfs_perag *pag)
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{
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trace_xfs_perag_wait_intents(pag, __return_address);
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return xfs_defer_drain_wait(&pag->pag_intents_drain);
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}
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/* Has anyone declared an intent to update this AG? */
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bool
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xfs_perag_intent_busy(
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struct xfs_perag *pag)
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{
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return xfs_defer_drain_busy(&pag->pag_intents_drain);
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}
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