实时Linux内核进化：引入可抢占锁与性能提升

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本文档探讨了实时Linux（Real-Time Linux）的发展历程，特别关注于其在Linux内核中的进化过程。自2004年10月作者在Linux内核邮件列表上公布了一个实时Linux内核原型以来，实时性在Linux系统中的表现有了显著提升。该原型引入了可抢占锁（preemptible locking），使得即使任务在执行关键区域时也能被中断，从而极大地提高了系统的实时响应能力。 Ingo Molnar对这一可抢占锁原型进行了深入评审，并在此基础上快速开发出了基于现有自愿抢占（Voluntary Preemption）项目的可抢占内核。这一成果催生了Real-Time preemption patch，目前用户可以通过以下链接下载使用：[http://people.redhat.com/~mingo/realtime-preempt/](http://people.redhat.com/%7Emingo/realtime-preempt/)。本文的核心内容详述了实时Linux内核的基础构建，包括线程上下文切换处理的优化、以及如何通过替换非抢占式锁（non-preemptible locking）为抢占式互斥锁（preemptible mutex-based locks）来增强系统的实时性能。这些改进对于确保实时性应用在Linux环境下的高效运行至关重要，因为它们允许在保证系统稳定性的同时，实现低延迟和高响应时间，这对于工业控制、网络通信和其他对时间敏感的应用领域来说是极为重要的技术进步。作者Sven-Thorsten Dietrich和Daniel Walker来自MontaVista Software, Inc，他们在文中分享了他们在实时Linux内核开发中的实践经验和理论思考，以便其他开发者和研究人员了解和借鉴这一领域的最新进展。本文为理解和实施实时Linux提供了深入的技术视角，对于Linux内核社区和实时系统研究者来说是一份不可多得的参考资料。

isters, are subject to concurrent read-modify-write

access by multiple threads and interrupt handlers.

Shared data that is operated upon by multiple

independent tasks, must be protected from context

switches such that read-modify-update operations

are not overlapped

In order to guarantee data integrity, transaction-

alized accesses are imposed on all code operating on

Kernel data objects, by means of critical sections.

A critical section estabishes a section of pro-

tected code, wherein only one task is allowed to op-

erate.

Any process executing within the boundaries of

a critical section, must exit the critical section before

another process is allowed to enter.

The critical sections boundaries serialize concur-

rent accesses to shared objects and therefore, guar-

antee that shared data and hardware register opera-

tions remain in a stable state at all times.

There are two basic classes of critical sections in

the kernel:

1. Critical Sections protecting threads from con-

current access by other threads

2. Critical Sections protecting threads from con-

current access by interrupts

3.2 Critical Section Management

Critical section management in a Linux uniproces-

sor environment is the most basic and serves as the

entry point into the discussion. In the UP environ-

ment, only a single thread of execution exists on the

CPU. The running (current) thread executes until it

relinquises the CPU voluntarily, or is interrupted by

a processor exception, such as an interrupt.

3.3 Protection from other Threads

The simplest form of critical section available in the

Linux kernel is known as a preemption-disabled sec-

tion of code. Concurrency protection from other

threads (but not interrupts) is achieved by suppress-

ing task switches while a thread is executing inside

a critical section. Task switches are suppressed by

the preempt

disable function. This Kernel design-

feature guarantees, that a thread can transition the

critical section before another thread is allowed to

execute. The requirements of the critical section

are fulﬁlled, since entry into a simple critical section

using the preempt disable() primitive, establishes a

non-preemptible thread execution state.

#define preempt_disable() \

do { \

inc_preempt_count(); \

barrier(); \

} while (0)

Each Linux task has a preemption counter, pre-

empt

count, in its task control structure. The

preempt disable() function increments the threads

preempt count variable. The preempt count vari-

able is allocated on a per-thread basis in the asm-

<arch>/thread info.h header ﬁle, but the value of

preempt count exerts a global eﬀect upon the sys-

tem:

If preempt count is non-zero, task switches are

prohibited on the CPU. preempt enable() decre-

ments the preempt count variable, via the pre-

empt enable no resched() call.

#define preempt_enable() \

do { \

preempt_enable_no_resched(); \

preempt_check_resched(); \

} while (0)

Preemption may only tke place only if pre-

empt count is equal to zero.

3.3.1 Nesting of Critical Sections

It is possible for critical sections to be nested in-

side the kernel. In this case, each successive critical

section entered by the executing thread, is associ-

ated with another call to preempt disable(). The

preempt count variable tracks the nesting of criti-

cal sections, and must be decremented by a call to

preempt enable(), when a critical section is unlocked.

Only after the last critical section has been unlocked,

does the value of preempt count reach 0, indicat-

ing that the task may be preempted. In order to

achieve proper accounting of critical section nesting,

each call to preempt enable(), must thus be balanced

by a subsequent call to preempt disable().

3.4 Protection from Interrupt Han-

dlers

Critical sections demarcated by preempt disable()

and preempt enable() pairs run with interrupts en-

abled. If an interrupt occurs while a thread is exe-

cuting in a critical section, exception processing pro-

ceeds immediately, and the thread of execution is

always returned to the running thread. The current

threads preempt count is sampled upon return from

the exception, and that state determines, whether

the return from exception passes through the sched-

uler, or continues with the interrupted thread. The

code for handling interrupts while preemption is dis-

abled, is typically found in assembly code in the in-

terrupt dispatch routines located in the architecture-

speciﬁc entry.S ﬁle.

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