首先給大家分享一個巨牛巨牛的人工智能教程，是我無意中發現的。教程不僅零基礎，通俗易懂，而且非常風趣幽默，還時不時有內涵段子，像看小說一樣，哈哈～我正在學習中，覺得太牛了，所以分享給大家！點這里可以跳轉到教程

1.由于在不同的硬件平臺上經常遇到usleep不準確的問題，比如usleep(2*1000)，結果sleep了10ms，是不是有點過分，測試代碼如下：

#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++;??? }}

當然為防止出現意外，禁止測試期間設置系統時間。

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2. 根據以前的經驗，此usleep不準主要是由于Kernel中系統timer的rating值過高引起的。

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3. 下面從源碼的角度分析一下usleep的實現細節，并進一步分析其原因。以下以Android4.0.1為例進行分析。注此問題主要與Kernel有關，與glibc或bionic無關，因為小弟最近搞Android，所以就以Android為例進行研究。

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4. 首先找到usleep的源碼：

//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;? }}

它也很懶的，就調用了nanosleep，哪就看看nanasleep的源碼吧! 不幸是只找到一個extern int? nanosleep(const struct timespec *, struct timespec *); 它位于/bionic/libc/include/sys/linux-unistd.h,并沒有找到它的實現。其實看看Linux系統調用，早就知道它是一個系統調用，哪就分析一下是如何進行系統調用的，以前只是講過原理，并沒有實例，在此把它完成了。

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5. 尋找系統調用函數

如果這個函數沒有實現，哪肯定是不能調用的，就像MIT教授在公開課上所講的，搞計算機的不像搞別的，做不了假，別人不管你怎么設計的，只看你實現的結果，很有道理。也證明了搞if else的人不能做弊。哪就從它的Android.mk入手吧，看看還Link了什么東東。打開libc的Android.mk發現，其中有一行

include $(LOCAL_PATH)/arch-$(TARGET_ARCH)/syscalls.mk

這就是關鍵所在，syscalls系統調用，不正是我們要找的嗎？進入arch-arm/syscalls.mk一看，其中一大片.s,Search一下，看有沒有nanosleep.s，還真有這么一行，真是大快人心：syscall_src += arch-arm/syscalls/nanosleep.S

趕緊去瞧瞧，ARM匯編水平不高，能看懂嗎？先把代碼貼上再說，不懂就問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是個什么東東，憑直覺，肯定在sys/linux-syscalls.h中有定義。打開/libc/include/sys/linux-syscalls.h并search __NR_nanosleep, 明白了，它定義了__NR_nanosleep的值為(__NR_SYSCALL_BASE + 162),其實就是定義了其系統調用號。這就與前一文swi連接起來了。上面的代碼把系統調用號傳遞給r7,然后觸發了一個軟中斷，從而進入內核態執行。

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6. 軟中斷處理流程

根據常識，既然是軟中斷，就一定有一個對應的ISR，打開/kernel/arch/arm/kernel/entry-common.S，發現其中有一個ENTRY(vector_swi),這就是我們要找的ISR,其詳細代碼如下：

.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. 找與nanosleep對應的處理函數

從上面的代碼中可以看出，它將調用sys_call_table中的某個函數。在同一個文件中尋找sys_call_table,其代碼如下：

.type sys_call_table, #objectENTRY(sys_call_table)#include "calls.S"

看看linux/arch/arm/kernel/calls.S中的內容：

/* 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)

原來nanosleep系統調用在Kernel中的函數為sys_nanosleep,現在去分析一下是如何實現高精度的sleep的，是忙等（執行nop指令），還是閑等（讓出CPU使用權）呢? 馬上就會有答案了。由于小弟知識有限，沒哪么簡單，我找了2個小時也沒有找到答案，慚愧啊!

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8. 先看看熟悉的系統調用open吧!

也不幸運，沒有sys_open這樣的函數。反正知道這個東東在fs/open.c中，基本原理應該是一樣的。在此文件中找到了下面這個函數：

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)

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linux/syscalls.h定義如下：

asmlinkage long sys_open(const char __user *filename,int flags, int mode); (asmlinkage就是一個extern "C")

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這兄弟倆長得太像了，再看看SYSCALL_DEFINE3的定義，看看能不能找到二者的關系。

哈哈哈哈哈哈.....,終于在linux/syscalls.h中找到答案了，SYSCALL_DEFINE3的定義如下：

#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)還原就變成了：

asmlinkage long sys_open(const char __user *filename,int flags, int mode);是不是與要找的函數一模一樣呢？終于找到如何看這個代碼的方法了！

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9. 繼續找sys_nanosleep的實現代碼

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了嗎?說明有機會找到了。

SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp,??struct timespec __user *, rmtp)這不就是我要找的嗎? 由于這是一個宏，在SourceInsight中查找函數nanosleep是找不到的，search字符串nanosleep是可行的。其代碼如下：

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實現如下：

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;}

調用流程如下：

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; 獲得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)<在注冊的clock_event_device中提供此函數，其主要功能是設置相關寄存器，以設置此超時事件>

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