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void getnstimeofday(struct timespec *ts)

{

    unsigned long seq;

    s64 nsecs;

    WARN_ON(timekeeping_suspended);

do {

    // 下面代码执行过程中xtime可能会被更改，这里通过持有一个序号来避免显示加锁，如果该代码执行完毕之后，seq并未改变，说明xtime未被更改，此次执行成功，否则重试；无论这里重试与否，CPU都会一直干活；

    seq = read_seqbegin(&xtime_lock);

        *ts = xtime;

        nsecs = timekeeping_get_ns(); //从当前时钟源取更细致的时间精度

        /* If arch requires, add in gettimeoffset() */

        nsecs += arch_gettimeoffset(); // 中断从发出到执行有时间消耗，可能需要做补偿

    } while (read_seqretry(&xtime_lock, seq));

    timespec_add_ns(ts, nsecs); //时间转换

}

/* Timekeeper helper functions. */

static inline s64 timekeeping_get_ns(void)

{

    cycle_t cycle_now, cycle_delta;

    struct clocksource *clock;

    /* read clocksource: */

clock = timekeeper.clock;

// 使用系统注册的时钟源[2]来读取，当前情况下，一般tsc是默认时钟源

// /sys/devices/system/clocksource/clocksource0/current_clocksource

// 下面的调用最终执行native_read_tsc, 里面就是一条汇编指令rdtsc

    cycle_now = clock->read(clock);

    /* calculate the delta since the last update_wall_time: */

    cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;

    /* return delta convert to nanoseconds using ntp adjusted mult. */

    return clocksource_cyc2ns(cycle_delta, timekeeper.mult,

                  timekeeper.shift);

}

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#include <sys/time.h>

#include <iostream>

#include <time.h>

using namespace std;

uint64_t now()

{

    struct timeval tv;

    gettimeofday(&tv, NULL);

    return tv.tv_sec * 1000000 + tv.tv_usec;

}

void* func_gettimeofday(void* p)

{

    int32_t c = *(int32_t*)p;

    uint64_t start = now();

    uint64_t us = 0;

    int i = 0;

    while (i++ < c) {

        struct timeval tv;

        gettimeofday(&tv, NULL);

        us += tv.tv_usec; // avoid optimize

    }

    cout << "gettimeofday(" << us << ") , times : " << c << endl;

    cout << "thread " << pthread_self() << " consume " << now() - start << " us" << endl;

    return 0;

}

void* func_clockgettime(void* p)

{

    int32_t c = *(int32_t*)p;

    uint64_t start = now();

    uint64_t us = 0;

    int i = 0;

    while (i++ < c) {

        struct timespec tp;

        clock_gettime(CLOCK_REALTIME, &tp);

        us += tp.tv_nsec;

    }

    cout << "clock_gettime(" << us << ") , times : " << c << endl;

    cout << "thread " << pthread_self() << " consume " << now() - start << " us" << endl;

    return 0;

}

int main(int argc, char** argv)

{

    if (argc != 4) {

        cout << " [gettimeofday/clock_gettime] thread_number loop_count" << endl;

        exit(-1);

    }

    string mode = string(argv[1]);

    int n = atoi(argv[2]);

    int loop = atoi(argv[3]);

    pthread_t* ts = new pthread_t[n];

    for (int i = 0; i < n; i++) {

        if (mode == "gettimeofday") {

            pthread_create(ts+i, NULL, func_gettimeofday, &loop);

        } else {

            pthread_create(ts+i, NULL, func_clockgettime, &loop);

        }

    }

    for (int i = 0; i < n; i++) {

        pthread_join(ts[i], NULL);

    }

    delete [] ts;

    return 0;

}