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1.由于在不同的硬件平台上经常遇到usleep不准确的问题，比如usleep(2*1000)，结果sleep了10ms，是不是有点过分，测试代码如下：
#include <stdio.h>#include <stdlib.h>int main(int argc,char **argv){ struct timeval oldTime, newTime; int iStime,i,j; iStime=5; for(i=0;i<60;i++) { for(j=0;j<10;j++) { gettimeofday( &oldTime, NULL ); usleep( iStime * 1000 ); gettimeofday( &newTime, NULL ); printf("iStime:%d,actual time:%lld\n",iStime,((long long)(newTime.tv_sec*1000 + newTime.tv_usec/1000)-(long long)(oldTime.tv_sec*1000 + oldTime.tv_usec/1000))); } iStime++; }}当然为防止出现意外，禁止测试期间设置系统时间。
2. 根据以前的经验，此usleep不准主要是由于Kernel中系统timer的rating值过高引起的。
3. 下面从源码的角度分析一下usleep的实现细节，并进一步分析其原因。以下以Android4.0.1为例进行分析。注此问题主要与Kernel有关，与glibc或bionic无关，因为小弟最近搞Android，所以就以Android为例进行研究。
4. 首先找到usleep的源码：
//位于/bionic/libc/unistd/usleep.c#include <time.h>#include <errno.h>int usleep(unsigned long usec){ struct timespec ts; ts.tv_sec = usec/1000000UL;#ifdef __arm__ /* avoid divisions and modulos on the ARM */ ts.tv_nsec = (usec - ts.tv_sec*1000000UL)*1000;#else ts.tv_nsec = (usec % 1000000UL) * 1000UL;#endif for (;;) { if ( nanosleep( &ts, &ts ) == 0 ) return 0; // We try again if the nanosleep failure is EINTR. // The other possible failures are EINVAL (which we should pass through), // and ENOSYS, which doesn't happen. if ( errno != EINTR ) return -1; }}
它也很懒的，就调用了nanosleep，哪就看看nanasleep的源码吧! 不幸是只找到一个extern int nanosleep(const struct timespec *, struct timespec *); 它位于/bionic/libc/include/sys/linux-unistd.h,并没有找到它的实现。其实看看Linux系统调用，早就知道它是一个系统调用，哪就分析一下是如何进行系统调用的，以前只是讲过原理，并没有实例，在此把它完成了。
5. 寻找系统调用函数
如果这个函数没有实现，哪肯定是不能调用的，就像MIT教授在公开课上所讲的，搞计算机的不像搞别的，做不了假，别人不管你怎么设计的，只看你实现的结果，很有道理。也证明了搞if else的人不能做弊。哪就从它的Android.mk入手吧，看看还Link了什么东东。打开libc的Android.mk发现，其中有一行
include $(LOCAL_PATH)/arch-$(TARGET_ARCH)/syscalls.mk
这就是关键所在，syscalls系统调用，不正是我们要找的吗？进入arch-arm/syscalls.mk一看，其中一大片.s,Search一下，看有没有nanosleep.s，还真有这么一行，真是大快人心：syscall_src += arch-arm/syscalls/nanosleep.S
赶紧去瞧瞧，ARM汇编水平不高，能看懂吗？先把代码贴上再说，不懂就问google.
/* autogenerated by gensyscalls.py */#include <sys/linux-syscalls.h> .text .type nanosleep, #function .globl nanosleep .align 4 .fnstartnanosleep: .save {r4, r7} stmfd sp!, {r4, r7} ldr r7, =__NR_nanosleep swi #0 ldmfd sp!, {r4, r7} movs r0, r0 bxpl lr b __set_syscall_errno .fnend__NR_nanosleep是个什么东东，凭直觉，肯定在sys/linux-syscalls.h中有定义。打开/libc/include/sys/linux-syscalls.h并search __NR_nanosleep, 明白了，它定义了__NR_nanosleep的值为(__NR_SYSCALL_BASE + 162),其实就是定义了其系统调用号。这就与前一文swi连接起来了。上面的代码把系统调用号传递给r7,然后触发了一个软中断，从而进入内核态执行。
6. 软中断处理流程
根据常识，既然是软中断，就一定有一个对应的ISR，打开/kernel/arch/arm/kernel/entry-common.S，发现其中有一个ENTRY(vector_swi),这就是我们要找的ISR,其详细代码如下：
.align 5ENTRY(vector_swi) sub sp, sp, #S_FRAME_SIZE stmia sp, {r0 - r12} @ Calling r0 - r12 ARM( add r8, sp, #S_PC ) ARM( stmdb r8, {sp, lr}^ ) @ Calling sp, lr THUMB( mov r8, sp ) THUMB( store_user_sp_lr r8, r10, S_SP ) @ calling sp, lr mrs r8, spsr @ called from non-FIQ mode, so ok. str lr, [sp, #S_PC] @ Save calling PC str r8, [sp, #S_PSR] @ Save CPSR str r0, [sp, #S_OLD_R0] @ Save OLD_R0 zero_fp /* * Get the system call number. */#if defined(CONFIG_OABI_COMPAT) /* * If we have CONFIG_OABI_COMPAT then we need to look at the swi * value to determine if it is an EABI or an old ABI call. */#ifdef CONFIG_ARM_THUMB tst r8, #PSR_T_BIT movne r10, #0 @ no thumb OABI emulation ldreq r10, [lr, #-4] @ get SWI instruction#else ldr r10, [lr, #-4] @ get SWI instruction A710( and ip, r10, #0x0f000000 @ check for SWI ) A710( teq ip, #0x0f000000 ) A710( bne .Larm710bug )#endif#ifdef CONFIG_CPU_ENDIAN_BE8 rev r10, r10 @ little endian instruction#endif#elif defined(CONFIG_AEABI) /* * Pure EABI user space always put syscall number into scno (r7). */ A710( ldr ip, [lr, #-4] @ get SWI instruction ) A710( and ip, ip, #0x0f000000 @ check for SWI ) A710( teq ip, #0x0f000000 ) A710( bne .Larm710bug )#elif defined(CONFIG_ARM_THUMB) /* Legacy ABI only, possibly thumb mode. */ tst r8, #PSR_T_BIT @ this is SPSR from save_user_regs addne scno, r7, #__NR_SYSCALL_BASE @ put OS number in ldreq scno, [lr, #-4]#else /* Legacy ABI only. */ ldr scno, [lr, #-4] @ get SWI instruction A710( and ip, scno, #0x0f000000 @ check for SWI ) A710( teq ip, #0x0f000000 ) A710( bne .Larm710bug )#endif#ifdef CONFIG_ALIGNMENT_TRAP ldr ip, __cr_alignment ldr ip, [ip] mcr p15, 0, ip, c1, c0 @ update control register#endif enable_irq get_thread_info tsk adr tbl, sys_call_table @ load syscall table pointer ldr ip, [tsk, #TI_FLAGS] @ check for syscall tracing#if defined(CONFIG_OABI_COMPAT) /* * If the swi argument is zero, this is an EABI call and we do nothing. * * If this is an old ABI call, get the syscall number into scno and * get the old ABI syscall table address. */ bics r10, r10, #0xff000000 eorne scno, r10, #__NR_OABI_SYSCALL_BASE ldrne tbl, =sys_oabi_call_table#elif !defined(CONFIG_AEABI) bic scno, scno, #0xff000000 @ mask off SWI op-code eor scno, scno, #__NR_SYSCALL_BASE @ check OS number#endif stmdb sp!, {r4, r5} @ push fifth and sixth args tst ip, #_TIF_SYSCALL_TRACE @ are we tracing syscalls? bne __sys_trace cmp scno, #NR_syscalls @ check upper syscall limit adr lr, BSYM(ret_fast_syscall) @ return address ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine add r1, sp, #S_OFF2: mov why, #0 @ no longer a real syscall cmp scno, #(__ARM_NR_BASE - __NR_SYSCALL_BASE) eor r0, scno, #__NR_SYSCALL_BASE @ put OS number back bcs arm_syscall b sys_ni_syscall @ not private funcENDPROC(vector_swi)
7. 找与nanosleep对应的处理函数
从上面的代码中可以看出，它将调用sys_call_table中的某个函数。在同一个文件中寻找sys_call_table,其代码如下：
.type sys_call_table, #objectENTRY(sys_call_table)#include "calls.S"
看看linux/arch/arm/kernel/calls.S中的内容：
/* 0 */ CALL(sys_restart_syscall) CALL(sys_exit) CALL(sys_fork_wrapper) CALL(sys_read) CALL(sys_write) .../* 160 */ CALL(sys_sched_get_priority_min) CALL(sys_sched_rr_get_interval) CALL(sys_nanosleep) CALL(sys_mremap) CALL(sys_setresuid16)
原来nanosleep系统调用在Kernel中的函数为sys_nanosleep,现在去分析一下是如何实现高精度的sleep的，是忙等（执行nop指令），还是闲等（让出CPU使用权）呢? 马上就会有答案了。由于小弟知识有限，没哪么简单，我找了2个小时也没有找到答案，惭愧啊!
8. 先看看熟悉的系统调用open吧!
也不幸运，没有sys_open这样的函数。反正知道这个东东在fs/open.c中，基本原理应该是一样的。在此文件中找到了下面这个函数：
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)
linux/syscalls.h定义如下：
asmlinkage long sys_open(const char __user *filename,int flags, int mode); (asmlinkage就是一个extern "C")
这兄弟俩长得太像了，再看看SYSCALL_DEFINE3的定义，看看能不能找到二者的关系。
哈哈哈哈哈哈.....,终于在linux/syscalls.h中找到答案了，SYSCALL_DEFINE3的定义如下：
#define __SYSCALL_DEFINEx(x, name, ...) asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__))#define SYSCALL_DEFINEx(x, sname, ...) __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
把SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)还原就变成了：
asmlinkage long sys_open(const char __user *filename,int flags, int mode);是不是与要找的函数一模一样呢？终于找到如何看这个代码的方法了！
9. 继续找sys_nanosleep的实现代码
先看看linux/kernel/hrtimer.c中的commnets:
* High-resolution kernel timers
*
* In contrast to the low-resolution timeout API implemented in
* kernel/timer.c, hrtimers provide finer resolution and accuracy
* depending on system configuration and capabilities.
*
* These timers are currently used for:
* - itimers
* - POSIX timers
* - nanosleep
* - precise in-kernel timing
看到上面的nanosleep了吗?说明有机会找到了。
SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp, struct timespec __user *, rmtp)这不就是我要找的吗? 由于这是一个宏，在SourceInsight中查找函数nanosleep是找不到的，search字符串nanosleep是可行的。其代码如下：
SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp, struct timespec __user *, rmtp){ struct timespec tu; if (copy_from_user(&tu, rqtp, sizeof(tu))) return -EFAULT; if (!timespec_valid(&tu)) return -EINVAL; return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);}
hrtimer_nanosleep实现如下：
long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp, const enum hrtimer_mode mode, const clockid_t clockid){ struct restart_block *restart; struct hrtimer_sleeper t; int ret = 0; unsigned long slack; slack = current->timer_slack_ns; if (rt_task(current)) slack = 0; hrtimer_init_on_stack(&t.timer, clockid, mode); hrtimer_set_expires_range_ns(&t.timer, timespec_to_ktime(*rqtp), slack); if (do_nanosleep(&t, mode)) goto out; /* Absolute timers do not update the rmtp value and restart: */ if (mode == HRTIMER_MODE_ABS) { ret = -ERESTARTNOHAND; goto out; } if (rmtp) { ret = update_rmtp(&t.timer, rmtp); if (ret <= 0) goto out; } restart = ¤t_thread_info()->restart_block; restart->fn = hrtimer_nanosleep_restart; restart->nanosleep.index = t.timer.base->index; restart->nanosleep.rmtp = rmtp; restart->nanosleep.expires = hrtimer_get_expires_tv64(&t.timer); ret = -ERESTART_RESTARTBLOCK;out: destroy_hrtimer_on_stack(&t.timer); return ret;}static int __sched do_nanosleep(struct hrtimer_sleeper *t, enum hrtimer_mode mode){ hrtimer_init_sleeper(t, current); do { set_current_state(TASK_INTERRUPTIBLE); hrtimer_start_expires(&t->timer, mode); if (!hrtimer_active(&t->timer)) t->task = NULL; if (likely(t->task)) schedule(); hrtimer_cancel(&t->timer); mode = HRTIMER_MODE_ABS; } while (t->task && !signal_pending(current)); __set_current_state(TASK_RUNNING); return t->task == NULL;}
调用流程如下：
nanosleep()--> sys_nanosleep()--> hrtimer_nanosleep()--> do_nanosleep()--> hrtimer_start()--> enqueue_hrtimer() -->hrtimer_enqueue_reprogram()--> hrtimer_reprogram()-->int tick_program_event(ktime_t expires, int force)->
(struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev; 获得clock_event_device)
int tick_dev_program_event(struct clock_event_device *dev, ktime_t expires, int force)->
int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,ktime_t now) ->
dev->set_next_event((unsigned long) clc, dev)<在注册的clock_event_device中提供此函数，其主要功能是设置相关寄存器，以设置此超时事件>
分享一下我老师大神的人工智能教程。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！https://blog.csdn.net/jiangjunshow