The Journey to RCU


RCU, cond_resched(), and performance regressions
Using Quiescent States to Reclaim Memory
A Tour Through RCU’s Requirements
A Critical RCU Safety Property is… Ease of Use!
What is RCU?
What is RCU? – “Read, Copy, Update”
Requirements for RCU part 1: the fundamentals


2001 Read-Copy Update on ols
As of March 2013: Simplifying RCU
RCU dissertation.2004.07
RCU part 3: the RCU API, 2008 edition
The RCU API, 2010 Edition
The RCU API, 2014 Edition
The RCU API, 2019 edition

RCU Talk by Joel Fernandes

RCU in 2019
Kernel Recipes 2019 - RCU in 2019


Verification of the Tree-Based Hierarchical Read-Copy Update in the Linux Kernel:

The problems - refrence

The fundermental problems underlying RCU is how to make sure if there is a refrence to the data which is going to be reclaim.
So RCU is essentially used as resource-reclamation mechanism. And we know there are 3 popular ways hazard pointer, reference counting, Quiescent state to do rerouce reclamation. RCU use quiescent state.

Quiescent state in Linux kernel

URCU: Any line of code not in an RCU read-side critical section is termed a quiescent state
… after all the CPUs in the system have gone through at least one “quiescent” state (such as context switch, idle loop, or user code)
The rcu and rcu_bh flavors of RCU have different sets of quiescent states.
rcu_flavor_sched_clock_irq rcu_rs


@冯博群 你好,请教一个问题,对于rcu-preempt, CPU report QS的意义是什么? 我理解只有task QS 才不block GP。
冯博群: qs都是CPU的,每个task都要report qs的话,那记录的结构得多复杂; rcu preempt是搞了一个list用来记录block当前qs的task; 当前gp; 过gp的条件就是list为空,且所有的CPU都report过qs;你说得概念上没啥问题,但是实现中不是这样作的.


RCU implementations that avoid unnecessarily awakening dyntick-idle CPUs will mark those CPUs as being in an extended quiescent state from Hierarchical RCU

EQS vs interrupt vs read-side section

Hierarchical RCU

EQS vs interrupt vs read-side section

Heavy and light

Grace period - time to reclaim

URCU: any period of time during which each reader thread resides in at least one quiescent state is called a grace period.
… after each CPU has passed through at least one quiescent state, the RCU grace period ends.

GP vs QS vs read vs updater

GP: rcu_state
QS: rcu_data
data: rcu_head
1. when to start GP
Updater want to free data if no GP is in-progress.

GP vs Data(callback)

continuos Data on GP. Segmented.

Deffered QS

Reporting of a deferred QS reporting (when rcu_read_unlock() could not help).

Memory ordering

A Tour Through TREE_RCU’s Grace-Period Memory Ordering

Start a grapce period

Start a New Grace Period

Expedite grace period

Expedited Grace Period Design

Process data

Callbacks vs GP vs segments

GP is completed, current, next
commit 5127bed588a2f8f3a1f732de2a8a190b7df5dce3
Author: Lai Jiangshan
Date: Sun Jul 6 17:23:59 2008 +0800
rcu classic: new algorithm for callbacks-processing(v2)

Next segment list

Finally, the RCU NEXT TAIL segment contains callbacks that are not yet associated with any grace period. from V..
RCU_NEXT_TAIL: Callbacks that have not yet been associated with a grace period. from design/data-structure.
commit 64db4cfff99c04cd5f550357edcc8780f96b54a2
Refs: v2.6.28-rc8-92-g64db4cfff99c
Author: Paul E. McKenney
AuthorDate: Thu Dec 18 21:55:32 2008 +0100
Commit: Ingo Molnar
CommitDate: Thu Dec 18 21:56:04 2008 +0100
“Tree RCU”: scalable classic RCU implementation
+ * [*nxttail[RCU_NEXT_READY_TAIL], NULL = *nxttail[RCU_NEXT_TAIL]):
+ * Entries that might have arrived after current GP ended
+ * [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
+ * Entries known to have arrived before current GP ended
commit 15fecf89e46a962ccda583d919e25d9da7bf0723
Refs: v4.11-rc2-13-g15fecf89e46a
Author: Paul E. McKenney
AuthorDate: Wed Feb 8 12:36:42 2017 -0800
Commit: Paul E. McKenney
CommitDate: Tue Apr 18 11:38:18 2017 -0700
srcu: Abstract multi-tail callback list handling
+ * [*tails[RCU_NEXT_READY_TAIL], *tails[RCU_NEXT_TAIL]):
+ * Callbacks that might have arrived after the next GP started.

Adavnce segments

If an old grace period has ended,
rcu advance cbs() is invoked to advance all callbacks,
otherwise, rcu accelerate cbs() is invoked to assign a
grace period to any recently arrived callbacks. If a new grace
period has started, ->passed quiesce is set to zero, and if
in addition RCU is waiting for a quiescent state from this
CPU, ->qs pending is set to one, so that a new quiescent
state will be detected for the new grace period.


rcu_segcblist_accelerate - the core function

Batch processing

RCU callbacks offload

Relocating RCU callbacks
API 14:

RCU CPU stall
Decoding Those Inscrutable RCU CPU Stall Warnings
update_process_times rcu_pending print_other_cpu_stall print_cpu_stall_info

RCU and dynticks

Status of Linux dynticks
Full dynticks status - Frederic Weisbecker, Red Hat
RCU and dynticks-idle mode


* Exit an RCU extended quiescent state, which can be either the
* idle loop or adaptive-tickless usermode execution.
* We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
* allow for the possibility of usermode upcalls messing up our count of
* interrupt nesting level during the busy period that is just now starting.
static void noinstr rcu_eqs_exit(bool user)
See desgin/data-structures
However, it turns out that when running in non-idle kernel context, the Linux kernel is fully capable of entering interrupt handlers that never exit and perhaps also vice versa. Therefore, whenever the ->dynticks_nesting field is incremented up from zero, the ->dynticks_nmi_nesting field is set to a large positive number, and whenever the ->dynticks_nesting field is decremented down to zero, the the ->dynticks_nmi_nesting field is set to zero. Assuming that the number of misnested interrupts is not sufficient to overflow the counter, this approach corrects the ->dynticks_nmi_nesting field every time the corresponding CPU enters the idle loop from process context.

Translation for Hierarchical RCU

rcu_enter_nohz -> rcu_idle_enter
RCU idle includes eqs which includes
* Enter an RCU extended quiescent state, which can be either the
* idle loop or adaptive-tickless usermode execution.
* We crowbar the ->dynticks_nmi_nesting field to zero to allow for
* the possibility of usermode upcalls having messed up our count
* of interrupt nesting level during the prior busy period.
static noinstr void rcu_eqs_enter(bool user)
Firo: so extended quiescent state includes non-extended quiescent state.


Special/bottom bit in rcu_data:: dynticks

commit b8c17e6664c461e4aed545a943304c3b32dd309c
Refs: v4.11-rc2-1-gb8c17e6664c4
Author: Paul E. McKenney
AuthorDate: Tue Nov 8 14:25:21 2016 -0800
Commit: Paul E. McKenney
CommitDate: Tue Apr 18 11:19:22 2017 -0700
rcu: Maintain special bits at bottom of ->dynticks counter

rcu_data::dynticks counter? validate by warn on?

Integrating and Validating dynticks and Preemptable RCU??


A GP completed seq A
CBs to process
New GP in-progress
CBs waiting for GP seq B

Tree RCU / Hierarchical RCU

Hierarchical RCU
A Tour Through TREE_RCU’s Data Structures
A Tour Through TREE_RCU’s Expedited Grace Periods
Tree preempt RCU: 3.0 and RCU: what went wrong
Verification of Tree-Based Hierarchical Read-Copy Update in the Linux Kernel

RCU sched

commit 9b06e818985d139fd9e82c28297f7744e1b484e1
Refs: v2.6.12-rc3-362-g9b06e818985d
Author: Paul E. McKenney
AuthorDate: Sun May 1 08:59:04 2005 -0700
Commit: Linus Torvalds
CommitDate: Sun May 1 08:59:04 2005 -0700
[PATCH] Deprecate synchronize_kernel, GPL replacement
The synchronize_kernel() primitive is used for quite a few different purposes:
waiting for RCU readers, waiting for NMIs, waiting for interrupts, and so on.
This makes RCU code harder to read, since synchronize_kernel() might or might
not have matching rcu_read_lock()s. This patch creates a new
synchronize_rcu() that is to be used for RCU readers and a new
synchronize_sched() that is used for the rest. These two new primitives
currently have the same implementation, but this is might well change with
additional real-time support.
+ * synchronize_sched - block until all CPUs have exited any non-preemptive
+ * kernel code sequences.
+ * This means that all preempt_disable code sequences, including NMI and
+ * hardware-interrupt handlers, in progress on entry will have completed
+ * before this primitive returns. However, this does not guarantee that
+ * softirq handlers will have completed, since in some kernels
+ * This primitive provides the guarantees made by the (deprecated)
+ * synchronize_kernel() API. In contrast, synchronize_rcu() only
+ * guarantees that rcu_read_lock() sections will have completed.

Quiescent states for RCU-sched

Verification of Tree-Based Hierarchical Read-Copy Update in the Linux Kernel
The non-preemptible RCU-sched flavor’s quiescent states apply to CPUs, and are user-space execution, context switch, idle, and offline state. Therefore, RCU-sched only needs to track tasks and interrupt handlers that are actually running because blocked and preempted tasks are always in quiescent states. Thus, RCU-sched needs only track CPU states.

RCU-preempt Preemptiable RCU

RCU-preempt: What happens on a context switch
Towards Hard Realtime Response from the Linux Kernel on SMP Hardware
The 1st: Read-copy-update for realtime Papper
The design of preemptible read-copy-update
The RCU implementation for the -rt patchset is unusual in that it permits read-side critical sections to be preempted and to be blocked waiting for locks. However, it does not handle general blocking (for example, via the wait_event() primitive): if you need that, you should instead use SRCU.
Realtime RCU
RCU-preempt: What happens on a context switch


+#define RCU_READ_UNLOCK_NEED_QS (1 << 2) /* RCU core needs CPU response. */

RCU-preempt and sleeping

A recent discussion on LKML clarified to me that “preempted to run something else” not only covers involuntary preemption but also voluntarily sleeping.
The exception is -rt “spinlock” acquisition. If the “spinlock” is held,
the task acquiring it will block, which is legal within an RCU-preempt
read-side critical section.
This exception is why I define bad things in terms of lack of
susceptibility to priority boosting instead of sleeping.

Priority-Boosting RCU

Priority-Boosting RCU Read-Side Critical Sections

RCU bh

rcu-bh design by Joel Fernandes
From kernel doc rcu design

The softirq-disable (AKA “bottom-half”, hence the “_bh” abbreviations) flavor of RCU, or RCU-bh, was developed by Dipankar Sarma to provide a flavor of RCU that could withstand the network-based denial-of-service attacks researched by Robert Olsson. These attacks placed so much networking load on the system that some of the CPUs never exited softirq execution, which in turn prevented those CPUs from ever executing a context switch, which, in the RCU implementation of that time, prevented grace periods from ever ending. The result was an out-of-memory condition and a system hang.
The solution was the creation of RCU-bh, which does local_bh_disable() across its read-side critical sections, and which uses the transition from one type of softirq processing to another as a quiescent state in addition to context switch, idle, user mode, and offline. This means that RCU-bh grace periods can complete even when some of the CPUs execute in softirq indefinitely, thus allowing algorithms based on RCU-bh to withstand network-based denial-of-service attacks.


Sleepable RCU

Tasks RCU

The RCU-tasks subsystem

Tiny RCU / RCU: The Bloatwatch Edition

RCU: The Bloatwatch Edition

Classic RCU - let it die peacefully

commit c17ef45342cc033fdf7bdd5b28615e0090f8d2e7
Author: Paul E. McKenney
Date: Tue Jun 23 17:12:47 2009 -0700
rcu: Remove Classic RCU
Remove Classic RCU, given that the combination of Tree RCU and
the proposed Bloatwatch RCU do everything that Classic RCU can
with fewer bugs.
Brief Overview of Classic RCU Implementation
Linux2.6.11版本:classic RCU的实现


RCU is mapped to either RCU-sched or RCU-preempt depending on configuration.

[RFC,-tip,0/4] RCU cleanups and simplified preemptable RCU
o Rename variables and functions so that RCU-sched is an
underlying definition, along with RCU-bh and (when so
configured) RCU-preempt. RCU then maps to either RCU-sched
or RCU-preempt, depending on configuration.
commit d6714c22b43fbcbead7e7b706ff270e15f04a791
Refs: v2.6.31-rc6-15-gd6714c22b43f
Author: Paul E. McKenney
AuthorDate: Sat Aug 22 13:56:46 2009 -0700
Commit: Ingo Molnar
CommitDate: Sun Aug 23 10:32:37 2009 +0200
rcu: Renamings to increase RCU clarity
Make RCU-sched, RCU-bh, and RCU-preempt be underlying
implementations, with “RCU” defined in terms of one of the
three. Update the outdated rcu_qsctr_inc() names, as these
functions no longer increment anything.