Linux kernel-<armv7a-switch_to>

最新推荐文章于 2025-01-02 20:54:20 发布

原创最新推荐文章于 2025-01-02 20:54:20 发布 · 761 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#kernel

Linux kernel 专栏收录该内容

19 篇文章

订阅专栏

本文详细介绍了 ARMv7-M 架构下进程上下文切换的实现方式，包括寄存器的保存与恢复过程。此外，还探讨了 Linux 内核中的通知链机制，解释了原子通知链、阻塞通知链、原始通知链和 SRCU 通知链的不同应用场景及特点。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

1.__switch_to

File：entry_v7m.S
TI_CPU_SAVE 即上下文环境，在thread_info中cpu_context的偏移量

/*
 * Register switch for ARMv7-M processors.
 * r0 = previous task_struct, r1 = previous thread_info, r2 = next thread_info
 * previous and next are guaranteed not to be the same.
 */
ENTRY(__switch_to)
    .fnstart
    .cantunwind
    /* save field */
    add ip, r1, #TI_CPU_SAVE
    stmia   ip!, {r4 - r11}     @ Store most regs on stack
    str sp, [ip], #4
    str lr, [ip], #4
    mov r5, r0
    add r4, r2, #TI_CPU_SAVE
    /* notifier to switch process */
    ldr r0, =thread_notify_head
    mov r1, #THREAD_NOTIFY_SWITCH
    bl  atomic_notifier_call_chain
    mov ip, r4
    mov r0, r5
    /* restore the new process field */
    ldmia   ip!, {r4 - r11}     @ Load all regs saved previously
    ldr sp, [ip]
    ldr pc, [ip, #4]!
    .fnend
ENDPROC(__switch_to)

2.thread_notify_head

File:process.c


ATOMIC_NOTIFIER_HEAD(thread_notify_head);

EXPORT_SYMBOL_GPL(thread_notify_head);

2.1 什么是notifier

* 摘抄源码中的注释*

/*
 * Notifier chains are of four types:
 *
 *  Atomic notifier chains: Chain callbacks run in interrupt/atomic
 *      context. Callouts are not allowed to block.
 *  原子通知链：应用在中断或原子上下文
 *  Blocking notifier chains: Chain callbacks run in process context.
 *      Callouts are allowed to block.
 *  阻塞通知链：应用在进程上下文
 *  Raw notifier chains: There are no restrictions on callbacks,
 *      registration, or unregistration.  All locking and protection
 *      must be provided by the caller.
 *  原始通知链：应用在不是很严格的条件下，比如回调，注册，注销的环境中。其中的锁保护需要由调用者提供。
 *  SRCU notifier chains: A variant of blocking notifier chains, with
 *      the same restrictions.
 *  SRCU 通知链：是阻塞通知链的变体，一样的应用限制。
 * atomic_notifier_chain_register() may be called from an atomic context,
 * but blocking_notifier_chain_register() and srcu_notifier_chain_register()
 * must be called from a process context.  Ditto for the corresponding
 * _unregister() routines.
 *
 * atomic_notifier_chain_unregister(), blocking_notifier_chain_unregister(),
 * and srcu_notifier_chain_unregister() _must not_ be called from within
 * the call chain.
 *
 * SRCU notifier chains are an alternative form of blocking notifier chains.
 * They use SRCU (Sleepable Read-Copy Update) instead of rw-semaphores for
 * protection of the chain links.  This means there is _very_ low overhead
 * in srcu_notifier_call_chain(): no cache bounces and no memory barriers.
 * As compensation, srcu_notifier_chain_unregister() is rather expensive.
 * SRCU notifier chains should be used when the chain will be called very
 * often but notifier_blocks will seldom be removed.  Also, SRCU notifier
 * chains are slightly more difficult to use because they require special
 * runtime initialization.
 * 补充一点，srcu通知链的花销是巨大的。当调用非常频繁，但是通知链的阻塞情况，很少被移除时，最好使用SRCU通知链。不过，用的时候会稍微麻烦一点，因为需要特殊的实时初始化。
 */

2.2 notifier

File: thread_notify.h

/*
 * These are the reason codes for the thread notifier.
 */
#define THREAD_NOTIFY_FLUSH 0
#define THREAD_NOTIFY_EXIT  1
#define THREAD_NOTIFY_SWITCH    2
#define THREAD_NOTIFY_COPY  3