Posix timers clock_gettime 分析

原文地址:点击打开链接

int clock_getres(clockid_t clk_id, struct timespec *res)

这个函数就是根据 clk_id 返回相应的 time:

CLOCK_REALTIME     real_time clock 系统绝对时间
CLOCK_MONOTONIC    单调时间

关于这个函数更详细的介绍请参考 man 手册

 

首先假设在用户态

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struct timespec now;
 
clock_gettime(CLOCK_MONOTONIC, &now)

 

走起:

首先要从初始化开始:
kernel/posix-timers.c

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__initcall(init_posix_timers);
 
#define __initcall(fn) device_initcall(fn)
 
#define device_initcall(fn)             __define_initcall("6",fn,6)

 

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static __init int init_posix_timers( void )
{     
         struct k_clock clock_realtime = {
                 .clock_getres = hrtimer_get_res,
         };
         struct k_clock clock_monotonic = {
                 .clock_getres = hrtimer_get_res,
                 .clock_get = posix_ktime_get_ts,
                 .clock_set = do_posix_clock_nosettime,
         };
         struct k_clock clock_monotonic_raw = {
                 .clock_getres = hrtimer_get_res,
                 .clock_get = posix_get_monotonic_raw,
                 .clock_set = do_posix_clock_nosettime,
                 .timer_create = no_timer_create,
                 .nsleep = no_nsleep,
         };
         struct k_clock clock_realtime_coarse = {
                 .clock_getres = posix_get_coarse_res,
                 .clock_get = posix_get_realtime_coarse,
                 .clock_set = do_posix_clock_nosettime,
                 .timer_create = no_timer_create,
                 .nsleep = no_nsleep,
         };
         struct k_clock clock_monotonic_coarse = {
                 .clock_getres = posix_get_coarse_res,
                 .clock_get = posix_get_monotonic_coarse,
                 .clock_set = do_posix_clock_nosettime,
                 .timer_create = no_timer_create,
                 .nsleep = no_nsleep,
         };
        
         register_posix_clock(CLOCK_REALTIME, &clock_realtime);
         register_posix_clock(CLOCK_MONOTONIC, &clock_monotonic);
         // 注册
         register_posix_clock(CLOCK_MONOTONIC_RAW, &clock_monotonic_raw);
         register_posix_clock(CLOCK_REALTIME_COARSE, &clock_realtime_coarse);
         register_posix_clock(CLOCK_MONOTONIC_COARSE, &clock_monotonic_coarse);
 
         posix_timers_cache = kmem_cache_create( "posix_timers_cache" ,
                                         sizeof ( struct k_itimer), 0, SLAB_PANIC,
                                         NULL);
         idr_init(&posix_timers_id);
         return 0;
}

 

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void register_posix_clock( const clockid_t clock_id, struct k_clock *new_clock)
{
         if ((unsigned) clock_id >= MAX_CLOCKS) {
                 printk( "POSIX clock register failed for clock_id %d\n" ,
                        clock_id);
                 return ;
         }
 
         posix_clocks[clock_id] = *new_clock;
}
 
// 这段代码真的很简单,不需要过多的解释
 
static struct k_clock posix_clocks[MAX_CLOCKS];
 
#define CLOCK_REALTIME                  0
#define CLOCK_MONOTONIC                 1
 
#define CLOCK_SGI_CYCLE                 10
#define MAX_CLOCKS                      16
#define CLOCKS_MASK                     (CLOCK_REALTIME | CLOCK_MONOTONIC)
#define CLOCKS_MONO                     CLOCK_MONOTONIC

 

来了

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SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,
                 struct timespec __user *,tp)
{
         struct timespec kernel_tp;
         int error;
 
         if (invalid_clockid(which_clock))
                 return -EINVAL;
         // 首先检查 clock_id 是否 valid,此时是 CLOCK_MONOTONIC
 
         error = CLOCK_DISPATCH(which_clock, clock_get,
                                (which_clock, &kernel_tp));
         // posix_ktime_get_ts(CLOCK_MONOTONIC, &kernel_tp);
 
         if (!error && copy_to_user(tp, &kernel_tp, sizeof (kernel_tp)))
                 error = -EFAULT;
 
         return error;
 
}

 

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static inline int invalid_clockid( const clockid_t which_clock)
{
         if (which_clock < 0)    /* CPU clock, posix_cpu_* will check it */
                 return 0;
         if ((unsigned) which_clock >= MAX_CLOCKS)
                 return 1;
         if (posix_clocks[which_clock].clock_getres != NULL)
                 return 0;
         if (posix_clocks[which_clock].res != 0)
                 return 0;
         return 1;
}

 

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#define CLOCK_DISPATCH(clock, call, arglist) \
         (( clock ) < 0 ? posix_cpu_##call arglist : \
          (posix_clocks[ clock ].call != NULL \
           ? (*posix_clocks[ clock ].call) arglist : common_##call arglist))
 
CLOCK_DISPATCH(CLOCK_MONOTONIC,clock_get,(CLOCK_MONOTONIC, &kernel_tp)) \
(( clock ) <  0 ? posix_cpu_##call arglist : (posix_clocks[ clock ].clock_get != NULL ? (*posix_clocks[ clock ].call) arglist : common_##call arglist))
 
// 其实最终就是这个样子
posix_ktime_get_ts(CLOCK_MONOTONIC, &kernel_tp);

 

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static int posix_ktime_get_ts(clockid_t which_clock, struct timespec *tp)
{
         ktime_get_ts(tp);
         return 0;
}
 
 
void ktime_get_ts( struct timespec *ts)
{
         struct timespec tomono; unsigned int seq; s64 nsecs;
 
         WARN_ON(timekeeping_suspended);
 
         do {
                 seq = read_seqbegin(&xtime_lock);
                 // 加读锁
 
                 *ts = xtime;
                 // 当前时间,内核时间(UTC 时间)
 
                 tomono = wall_to_monotonic;
                 nsecs = timekeeping_get_ns();
                 // 得到距离上一次得到时间中间走过的时间(纳秒)
 
         } while (read_seqretry(&xtime_lock, seq));
 
         set_normalized_timespec(ts, ts->tv_sec + tomono.tv_sec,
                                 ts->tv_nsee + tomono.tv_nsec + nsecs);
}

这个函数和 getnstimeofday 很像,细节可以参考 gettimeofday

 

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void set_normalized_timespec( struct timespec *ts, time_t sec, s64 nsec)
{     
         while (nsec >= NSEC_PER_SEC) {
                 /*
                  * The following asm() prevents the compiler from
                  * optimising this loop into a modulo operation. See
                  * also __iter_div_u64_rem() in include/linux/time.h
                  */
                 asm( "" : "+rm" (nsec));
                 nsec -= NSEC_PER_SEC;
                 ++sec;
         }
         while (nsec < 0) {
                 asm( "" : "+rm" (nsec));
                 nsec += NSEC_PER_SEC;
                 --sec;
         }
         ts->tv_sec = sec;
         ts->tv_nsec = nsec;
}

其实原理就是记录一下开始的时间在 timekeeping_init() 中,然后用当前时间减去开始的时间就是经过的时间也就是单调时间~

 

但是 wall_to_monotonic 这个东东就不得不说一下了,他到底是神马玩意儿呢? 关于它还得追溯到 timekeeping_init() 中

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struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
 
void __init timekeeping_init( void )
{     
         struct clocksource * clock ;
         unsigned long flags;
         struct timespec now, boot;
 
         read_persistent_clock(&now);
         // 读取 RTC chip,get the UTC time
  
         read_boot_clock(&boot);
 
         ......
  
         xtime.tv_sec = now.tv_sec;
         xtime.tv_nsec = now.tv_nsec;
         raw_time.tv_sec = 0;
         raw_time.tv_nsec = 0;
         if (boot.tv_sec == 0 && boot.tv_nsec == 0) {
                 boot.tv_sec = xtime.tv_sec;
                 boot.tv_nsec = xtime.tv_nsec;
         }
 
         set_normalized_timespec(&wall_to_monotonic,
                                 -boot.tv_sec, -boot.tv_nsec);
 
         ......
}

首先读取 RTC 获得 UTC 时间,保存到 xtime 中,最后记录到 wall_to_monotonic,只不过是个负值。其实就是记录一下系统启动时候的 real time.

 


clock_gettime() 函数 基本上也就说完了,这个函数的实现比较简单,但是仍然有几点需要说明:

1. 它和 gettimeofday() 的区别是什么 ?

当 clock_gettime() clock_id 指定为 CLOCK_REALTIME 时,它与 gettimeofday 完全一样,只不过它返回的是纳秒,而 gettimeofday 返回的是微秒。

struct timespec ts;
struct timeval tv;

clock_gettime(CLOCK_REALTIME,&ts);
gettimeofday(&tv,NULL);

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SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,
                 struct timespec __user *,tp)
{
         ......
 
         error = CLOCK_DISPATCH(which_clock, clock_get,
                                (which_clock, &kernel_tp));
         // 其实就是调用 common_clock_get(CLOCK_REALTIME, &kernel_tp);
         
 
         ......
 
}
 
static int common_clock_get(clockid_t which_clock, struct timespec *tp)
{     
         ktime_get_real_ts(tp);
         return 0;
}
         
#define ktime_get_real_ts(ts)   getnstimeofday(ts)
 
// 就是调用 getnstimeofday(),同理再 gettimeofday() 中也是调用 getnstimeofday() ,关于这个函数细节参考下:gettimeofday

 

2.CLOCK_MONOTONIC 单调时间,那究竟什么是单调时间呢?

CLOCK_MONOTONIC 表示单调时间,此时间会一直增加,不会受系统时间改变的影响。其实就是表示系统启动了多长时间,可通过 uptime 查看.

比如我们用 settimeofday 往回设置时间,假设 current 20:00:00 往回设置 10 s,这样当用 gettimeofday 获得当前时间时是获得的绝对的时间,为 19:50:00,也就是说 gettimeofday 获得的”值”会比没设置前小。但是当用 clock_getime 并指定 clock_id 为 CLOCK_MONOTONIC 时,在时间设置前后其“值”都是递增的。因为在 do_settimeofday 中 wall_to_monotonic 也会被设置,会被往回设置 10 s.

举个例子:
比如系统启动时的 UTC 时间是 20:00:00 ,此时 wall_to_monotonic 记录着这个时间,假设当前时间为 21:00:00,那么系统启动了 1h,通过 clock_gettime(CLOCK_MONOTONIC, …. ) 即可得到这个 1h.好现在往回设置时间,设置到 20:30:00,那么 相应 wall_to_monotonic 的值也会往回设置,为 19:30:00,xtime 回在这个基础上增加,这就是为什么当我们用 clock_gettime 获得单调时间时它的值一直增加了:

下面的代码说明了这一点:

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int do_settimeofday( struct timespec *tv)
{
         ......
 
         // tv 是要设置的时间
         ts_delta.tv_sec = tv->tv_sec - xtime.tv_sec;
         ts_delta.tv_nsec = tv->tv_nsec - xtime.tv_nsec;
         wall_to_monotonic = timespec_sub(wall_to_monotonic, ts_delta);
 
         ......
}
 
static inline struct timespec timespec_sub( struct timespec lhs,
                                                 struct timespec rhs)
{
         struct timespec ts_delta;
         set_normalized_timespec(&ts_delta, lhs.tv_sec - rhs.tv_sec,
                                 lhs.tv_nsec - rhs.tv_nsec);
         return ts_delta;
}

 


写个测试程序来验证这一点:

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#include
#include
#include
 
int main( int argc, const char *argv[])
{     
         struct timeval tv1,tv2;
         struct timespec ts1,ts2;
         struct timeval temp;
 
 
         gettimeofday(&tv1,NULL);
         clock_gettime(CLOCK_MONOTONIC,&ts1);
 
         temp = tv1;
         temp.tv_sec -= 10;
 
         settimeofday(&temp,NULL);
 
         gettimeofday(&tv2,NULL);
         clock_gettime(CLOCK_MONOTONIC,&ts2);
 
         printf ( "gettimeofday start = %ld.%6ld,end = %ld.%6ld, \n\t => diff = %f \n" ,tv1.tv_sec, tv1.tv_usec, tv2.tv_sec, tv2.tv_usec, ((tv2.tv_sec * 1000000 + tv2.tv_usec) - (tv1.tv_sec * 1000000 + tv1.tv_usec))/1000000.0);
 
         printf ( "clock_gettime start = %ld.%9ld,end = %ld.%9ld, \n\t => diff = %f \n" ,ts1.tv_sec, ts1.tv_nsec, ts2.tv_sec, ts2.tv_nsec, ((ts2.tv_sec * 1000000000 + ts2.tv_nsec) - (ts1.tv_sec * 1000000000 + ts1.tv_nsec))/1000000000.0);
 
         return 0;
}

 

# ./clock_gettime_gettimeofday
gettimeofday start = 1345788337.265370,end = 1345788327.265383,  << end 值明显变小
         => diff = -9.999987
clock_gettime start = 2595.332710380,end = 2595.332726631,      << end 值增大
         => diff = 0.000016

最后 clock_gettime 就大致 BB 完了,还有几个与时间有关的 posix 函数(clock_getres,clock_settime),这几个函数的实现与 clock_gettime 大致相同

This entry was posted in  kernel,  timer and tagged  2.6.32,  kernel,  timer.

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