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
.fnstart
nanosleep:
.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 5
ENTRY(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_OFF
2: 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 func
ENDPROC(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, #object
ENTRY(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>