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d1b222c6be
Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs takes advantage of unrelated grace periods, thus reducing the memory footprint in the face of floods of call_rcu() invocations. However, the ->cblist field is a more-complex rcu_segcblist structure which must be protected via locking. Even though there are only three entities which can acquire this lock (the CPU invoking call_rcu(), the no-CBs grace-period kthread, and the no-CBs callbacks kthread), the contention on this lock is excessive under heavy stress. This commit therefore greatly reduces contention by provisioning an rcu_cblist structure field named ->nocb_bypass within the rcu_data structure. Each no-CBs CPU is permitted only a limited number of enqueues onto the ->cblist per jiffy, controlled by a new nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to about 16 enqueues per millisecond (16 * 1000 / HZ). When that limit is exceeded, the CPU instead enqueues onto the new ->nocb_bypass. The ->nocb_bypass is flushed into the ->cblist every jiffy or when the number of callbacks on ->nocb_bypass exceeds qhimark, whichever happens first. During call_rcu() floods, this flushing is carried out by the CPU during the course of its call_rcu() invocations. However, a CPU could simply stop invoking call_rcu() at any time. The no-CBs grace-period kthread therefore carries out less-aggressive flushing (every few jiffies or when the number of callbacks on ->nocb_bypass exceeds (2 * qhimark), whichever comes first). This means that the no-CBs grace-period kthread cannot be permitted to do unbounded waits while there are callbacks on ->nocb_bypass. A ->nocb_bypass_timer is used to provide the needed wakeups. [ paulmck: Apply Coverity feedback reported by Colin Ian King. ] Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
128 lines
4.3 KiB
C
128 lines
4.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0+ */
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/*
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* RCU segmented callback lists, internal-to-rcu header file
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*
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* Copyright IBM Corporation, 2017
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*
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* Authors: Paul E. McKenney <paulmck@linux.ibm.com>
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*/
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#include <linux/rcu_segcblist.h>
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/* Return number of callbacks in the specified callback list. */
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static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
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{
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return READ_ONCE(rclp->len);
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}
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/*
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* Account for the fact that a previously dequeued callback turned out
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* to be marked as lazy.
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*/
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static inline void rcu_cblist_dequeued_lazy(struct rcu_cblist *rclp)
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{
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rclp->len_lazy--;
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}
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void rcu_cblist_init(struct rcu_cblist *rclp);
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void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
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void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
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struct rcu_cblist *srclp,
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struct rcu_head *rhp);
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struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);
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/*
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* Is the specified rcu_segcblist structure empty?
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*
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* But careful! The fact that the ->head field is NULL does not
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* necessarily imply that there are no callbacks associated with
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* this structure. When callbacks are being invoked, they are
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* removed as a group. If callback invocation must be preempted,
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* the remaining callbacks will be added back to the list. Either
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* way, the counts are updated later.
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*
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* So it is often the case that rcu_segcblist_n_cbs() should be used
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* instead.
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*/
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static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp)
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{
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return !READ_ONCE(rsclp->head);
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}
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/* Return number of callbacks in segmented callback list. */
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static inline long rcu_segcblist_n_cbs(struct rcu_segcblist *rsclp)
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{
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#ifdef CONFIG_RCU_NOCB_CPU
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return atomic_long_read(&rsclp->len);
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#else
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return READ_ONCE(rsclp->len);
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#endif
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}
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/* Return number of lazy callbacks in segmented callback list. */
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static inline long rcu_segcblist_n_lazy_cbs(struct rcu_segcblist *rsclp)
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{
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return rsclp->len_lazy;
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}
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/* Return number of lazy callbacks in segmented callback list. */
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static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
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{
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return rcu_segcblist_n_cbs(rsclp) - rsclp->len_lazy;
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}
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/*
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* Is the specified rcu_segcblist enabled, for example, not corresponding
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* to an offline CPU?
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*/
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static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
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{
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return rsclp->enabled;
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}
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/* Is the specified rcu_segcblist offloaded? */
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static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
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{
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return rsclp->offloaded;
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}
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/*
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* Are all segments following the specified segment of the specified
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* rcu_segcblist structure empty of callbacks? (The specified
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* segment might well contain callbacks.)
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*/
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static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
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{
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return !READ_ONCE(*READ_ONCE(rsclp->tails[seg]));
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}
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void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
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void rcu_segcblist_init(struct rcu_segcblist *rsclp);
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void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
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void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
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bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);
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bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp);
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struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp);
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struct rcu_head *rcu_segcblist_first_pend_cb(struct rcu_segcblist *rsclp);
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bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp);
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void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
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struct rcu_head *rhp, bool lazy);
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bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
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struct rcu_head *rhp, bool lazy);
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void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
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struct rcu_cblist *rclp);
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void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq);
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bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq);
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void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
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struct rcu_segcblist *src_rsclp);
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