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Now that the (de-)offloading process can only apply to offline CPUs, there is no more concurrency between rcu_core and nocb kthreads. Also the mutation now happens on empty queues. Therefore the state machine can be reduced to a single bit called SEGCBLIST_OFFLOADED. Simplify the transition as follows: * Upon offloading: queue the rdp to be added to the rcuog list and wait for the rcuog kthread to set the SEGCBLIST_OFFLOADED bit. Unpark rcuo kthread. * Upon de-offloading: Park rcuo kthread. Queue the rdp to be removed from the rcuog list and wait for the rcuog kthread to clear the SEGCBLIST_OFFLOADED bit. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
213 lines
11 KiB
C
213 lines
11 KiB
C
/* SPDX-License-Identifier: GPL-2.0+ */
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/*
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* RCU segmented callback lists
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*
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* This seemingly RCU-private file must be available to SRCU users
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* because the size of the TREE SRCU srcu_struct structure depends
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* on these definitions.
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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.net.ibm.com>
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*/
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#ifndef __INCLUDE_LINUX_RCU_SEGCBLIST_H
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#define __INCLUDE_LINUX_RCU_SEGCBLIST_H
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#include <linux/types.h>
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#include <linux/atomic.h>
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/* Simple unsegmented callback lists. */
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struct rcu_cblist {
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struct rcu_head *head;
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struct rcu_head **tail;
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long len;
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};
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#define RCU_CBLIST_INITIALIZER(n) { .head = NULL, .tail = &n.head }
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/* Complicated segmented callback lists. ;-) */
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/*
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* Index values for segments in rcu_segcblist structure.
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*
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* The segments are as follows:
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*
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* [head, *tails[RCU_DONE_TAIL]):
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* Callbacks whose grace period has elapsed, and thus can be invoked.
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* [*tails[RCU_DONE_TAIL], *tails[RCU_WAIT_TAIL]):
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* Callbacks waiting for the current GP from the current CPU's viewpoint.
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* [*tails[RCU_WAIT_TAIL], *tails[RCU_NEXT_READY_TAIL]):
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* Callbacks that arrived before the next GP started, again from
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* the current CPU's viewpoint. These can be handled by the next GP.
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* [*tails[RCU_NEXT_READY_TAIL], *tails[RCU_NEXT_TAIL]):
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* Callbacks that might have arrived after the next GP started.
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* There is some uncertainty as to when a given GP starts and
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* ends, but a CPU knows the exact times if it is the one starting
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* or ending the GP. Other CPUs know that the previous GP ends
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* before the next one starts.
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*
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* Note that RCU_WAIT_TAIL cannot be empty unless RCU_NEXT_READY_TAIL is also
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* empty.
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*
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* The ->gp_seq[] array contains the grace-period number at which the
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* corresponding segment of callbacks will be ready to invoke. A given
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* element of this array is meaningful only when the corresponding segment
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* is non-empty, and it is never valid for RCU_DONE_TAIL (whose callbacks
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* are already ready to invoke) or for RCU_NEXT_TAIL (whose callbacks have
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* not yet been assigned a grace-period number).
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*/
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#define RCU_DONE_TAIL 0 /* Also RCU_WAIT head. */
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#define RCU_WAIT_TAIL 1 /* Also RCU_NEXT_READY head. */
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#define RCU_NEXT_READY_TAIL 2 /* Also RCU_NEXT head. */
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#define RCU_NEXT_TAIL 3
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#define RCU_CBLIST_NSEGS 4
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/*
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* ==NOCB Offloading state machine==
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*
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*
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* ----------------------------------------------------------------------------
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* | SEGCBLIST_RCU_CORE |
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* | |
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* | Callbacks processed by rcu_core() from softirqs or local |
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* | rcuc kthread, without holding nocb_lock. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* ----------------------------------------------------------------------------
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* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED |
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* | |
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* | Callbacks processed by rcu_core() from softirqs or local |
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* | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* ----------------------------------------------------------------------------
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* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED |
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* | + unparked CB kthread |
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* | |
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* | CB kthread got unparked and processes callbacks concurrently with |
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* | rcu_core(), holding nocb_lock. |
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* ---------------------------------------------------------------------------
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* |
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* v
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* ---------------------------------------------------------------------------|
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* | SEGCBLIST_RCU_CORE | |
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* | SEGCBLIST_LOCKING | |
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* | SEGCBLIST_OFFLOADED | |
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* | SEGCBLIST_KTHREAD_GP |
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* | + unparked CB kthread |
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* | |
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* | GP kthread woke up and acknowledged nocb_lock. |
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* ---------------------------------------- -----------------------------------
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* |
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* v
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* |--------------------------------------------------------------------------|
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* | SEGCBLIST_LOCKING | |
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* | SEGCBLIST_OFFLOADED | |
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* | SEGCBLIST_KTHREAD_GP | |
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* | + unparked CB kthread |
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* | |
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* | Kthreads handle callbacks holding nocb_lock, local rcu_core() stops |
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* | handling callbacks. Enable bypass queueing. |
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* ----------------------------------------------------------------------------
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*/
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/*
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* ==NOCB De-Offloading state machine==
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*
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*
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* |--------------------------------------------------------------------------|
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* | SEGCBLIST_LOCKING | |
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* | SEGCBLIST_OFFLOADED | |
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* | SEGCBLIST_KTHREAD_GP |
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* | + unparked CB kthread |
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* | |
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* | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core() |
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* | ignores callbacks. Bypass enqueue is enabled. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* |--------------------------------------------------------------------------|
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* | SEGCBLIST_RCU_CORE | |
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* | SEGCBLIST_LOCKING | |
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* | SEGCBLIST_OFFLOADED | |
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* | SEGCBLIST_KTHREAD_GP |
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* | + unparked CB kthread |
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* | |
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* | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core() |
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* | handles callbacks concurrently. Bypass enqueue is disabled. |
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* | Invoke RCU core so we make sure not to preempt it in the middle with |
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* | leaving some urgent work unattended within a jiffy. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* |--------------------------------------------------------------------------|
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* | SEGCBLIST_RCU_CORE | |
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* | SEGCBLIST_LOCKING | |
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* | SEGCBLIST_KTHREAD_GP |
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* | + unparked CB kthread |
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* | |
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* | CB/GP kthreads and local rcu_core() handle callbacks concurrently |
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* | holding nocb_lock. Wake up GP kthread if necessary. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* |--------------------------------------------------------------------------|
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* | SEGCBLIST_RCU_CORE | |
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* | SEGCBLIST_LOCKING | |
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* | + unparked CB kthread |
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* | |
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* | GP kthread woke up and acknowledged the fact that SEGCBLIST_OFFLOADED |
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* | got cleared. The callbacks from the target CPU will be ignored from the|
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* | GP kthread loop. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* ----------------------------------------------------------------------------
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* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING |
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* | + parked CB kthread |
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* | |
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* | CB kthread is parked. Callbacks processed by rcu_core() from softirqs or |
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* | local rcuc kthread, while holding nocb_lock. |
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* ----------------------------------------------------------------------------
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* |
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* v
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* ----------------------------------------------------------------------------
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* | SEGCBLIST_RCU_CORE |
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* | |
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* | Callbacks processed by rcu_core() from softirqs or local |
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* | rcuc kthread, without holding nocb_lock. |
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* ----------------------------------------------------------------------------
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*/
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#define SEGCBLIST_ENABLED BIT(0)
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#define SEGCBLIST_OFFLOADED BIT(1)
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struct rcu_segcblist {
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struct rcu_head *head;
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struct rcu_head **tails[RCU_CBLIST_NSEGS];
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unsigned long gp_seq[RCU_CBLIST_NSEGS];
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#ifdef CONFIG_RCU_NOCB_CPU
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atomic_long_t len;
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#else
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long len;
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#endif
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long seglen[RCU_CBLIST_NSEGS];
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u8 flags;
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};
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#define RCU_SEGCBLIST_INITIALIZER(n) \
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{ \
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.head = NULL, \
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.tails[RCU_DONE_TAIL] = &n.head, \
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.tails[RCU_WAIT_TAIL] = &n.head, \
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.tails[RCU_NEXT_READY_TAIL] = &n.head, \
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.tails[RCU_NEXT_TAIL] = &n.head, \
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}
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#endif /* __INCLUDE_LINUX_RCU_SEGCBLIST_H */
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