ktime_set

ktime_set是Linux内核中的一个函数，用于设置一个ktime_t类型的时间值。ktime_t是内核中用于表示时间的结构体类型，在内核中使用纳秒作为时间单位。

该函数的定义如下：

static inline ktime_t ktime_set(const s64 secs, const unsigned long nsecs)
{
    return (ktime_t) { .tv64 = secs * NSEC_PER_SEC + nsecs };
}

其中，secs表示秒数，nsecs表示纳秒数，函数返回一个ktime_t类型的时间值，其tv64成员表示时间的纳秒数。

例如，要设置一个时间值为1秒500毫秒的ktime_t变量kt，则可以使用以下代码：

ktime_t kt = ktime_set(1, 500000000);

相关问题

High Resolution Timer 的使用

High Resolution Timer（HRT）是Linux内核提供的一种高精度计时器，可以提供纳秒级别的计时精度。下面是HRT的使用步骤：

1. 定义一个hrtimer结构体变量，该结构体变量用于表示一个HRT计时器。

2. 使用hrtimer_init()函数来初始化hrtimer结构体变量，设置HRT计时器的回调函数以及计时器的模式（相对或绝对时间）。

3. 使用hrtimer_start()函数来启动HRT计时器，设置计时器的超时时间。

4. 在HRT计时器超时时，内核会自动调用设置的回调函数。

5. 在回调函数中，可以进行需要高精度计时的操作，例如测量函数执行时间、执行周期性任务等。

6. 若需要取消已经启动的HRT计时器，可以使用hrtimer_cancel()函数来取消计时器。

下面是一个简单的HRT计时器示例代码，用于测量函数执行时间：

#include <linux/hrtimer.h>
#include <linux/ktime.h>

static struct hrtimer my_hrtimer;
static ktime_t start_time;

enum hrtimer_restart my_hrtimer_callback(struct hrtimer *timer)
{
    ktime_t end_time = ktime_get();
    printk(KERN_INFO "Function execution time: %lld ns\n", ktime_to_ns(ktime_sub(end_time, start_time)));
    return HRTIMER_NORESTART;
}

void my_function()
{
    start_time = ktime_get();
    hrtimer_init(&my_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
    my_hrtimer.function = my_hrtimer_callback;
    hrtimer_start(&my_hrtimer, ktime_set(1, 0), HRTIMER_MODE_REL);
    // do something here
    hrtimer_cancel(&my_hrtimer);
}

在上述代码中，my_function()函数调用时会启动一个HRT计时器，并在计时器超时时自动调用my_hrtimer_callback()函数，该函数会输出函数执行时间。

how to calculate RTT in linux kernel

In Linux kernel, you can calculate Round Trip Time (RTT) using the timestamp of the packet when it is sent and received. The steps to calculate RTT are as follows:

1. Set a timestamp in the packet when it is sent.
2. When the packet is received, retrieve the timestamp from the packet and calculate the time difference between the current time and the timestamp.
3. This time difference gives you the RTT.

In the Linux kernel, you can get the timestamp using the function `ktime_get()` which returns the current time in kernel-specific format. You can also use the function `ktime_to_us()` to convert the kernel-specific time to microseconds.

When the packet is received, you can retrieve the timestamp using the function `skb_tstamp_tx()` which returns the timestamp that was set when the packet was transmitted. Then, you can calculate the time difference between the current time and the timestamp using the following code:

struct timespec64 now;
ktime_get_real_ts64(&now);
u64 rtt = ktime_to_us(ktime_sub(now, skb_tstamp_tx(skb)));

The `ktime_sub()` function calculates the difference between two kernel-specific times, and `ktime_to_us()` converts the result to microseconds.

You can use this method to calculate RTT in the Linux kernel.