# Linux Project3
### 組員名單
* 110525015 劉松靄
* 111525013 何雋永
* 109502547 楊晴方
### 系統環境
* 作業系統： ubuntu 18.04
* 原本 Kernel 版本：5.4.0-131-generic
* 更新 Kernel 版本 : 6.0.6
### Q1
#### kernel code

#include <linux/kernel.h>
#include <linux/syscalls.h>

SYSCALL_DEFINE0(getCpuNumber){
        return (current->thread_info).cpu;
}

由於 CONFIG_THREAD_INFO_IN_TASK = Y，所以 thread_info 在 task_struct 中，圖示如下。

#### user code

#include <linux/kernel.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <stdio.h>

int main(){
        unsigned cpu, node;

        printf("get_cpu_number: %u\n", syscall(338));
        syscall(SYS_getcpu, &cpu, &node, NULL);

        printf("Default function: %u\n", cpu);
        return 0;
}

#### Result

jack@jack-ASUSPRO-D840MB-M840MB:~$ ./p3
get_cpu_number: 0
Default function: 0
jack@jack-ASUSPRO-D840MB-M840MB:~$ ./p3
get_cpu_number: 0
Default function: 0
jack@jack-ASUSPRO-D840MB-M840MB:~$ ./p3
get_cpu_number: 4
Default function: 4

### Q2
#### kernel code
* ##### task_struct

#ifdef CONFIG_RV
        /*
         * Per-task RV monitor. Nowadays fixed in RV_PER_TASK_MONITORS.
         * If we find justification for more monitors, we can think
         * about adding more or developing a dynamic method. So far,
         * none of these are justified.
         */
        union rv_task_monitor           rv[RV_PER_TASK_MONITORS];
#endif

        int context_switch_count; // <---這行是新增的
        /*
         * New fields for task_struct should be added above here, so that
         * they are included in the randomized portion of task_struct.
         */
        randomized_struct_fields_end

        /* CPU-specific state of this task: */
        struct thread_struct            thread;

        /*
         * WARNING: on x86, 'thread_struct' contains a variable-sized
         * structure.  It *MUST* be at the end of 'task_struct'.
         *
         * Do not put anything below here!
         */
};

* ##### context_switch

        rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);

        prepare_lock_switch(rq, next, rf);

        /* Here we just switch the register state and the stack. */
        prev->context_switch_count++; // <---這行是新增的
        next->context_switch_count++; // <---這行是新增的
        switch_to(prev, next, prev);
        barrier();

        return finish_task_switch(prev);
}


計算由 running 轉成 ready(把 cpu 給別人)和從 ready 轉 running(把 cpu 拿回來)的數目
* ##### syscall

#include <linux/kernel.h>
#include <linux/syscalls.h>

SYSCALL_DEFINE0(start_to_count_number_of_process_switches){
        current->context_switch_count = 0;
        return 0;
}

SYSCALL_DEFINE0(stop_to_count_number_of_process_switches){
        return current->context_switch_count;
}

#### user code
* ##### IO_bound

#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <linux/kernel.h>
#include <unistd.h>
#include <sys/time.h>

#define ON 1
#define OFF 0
#define start_to_count_number_of_process_switches 339
#define stop_to_count_number_of_process_switches 340
int main(){
        int cnt = 0, b = 0;
        int __switch = ON;
        long start_sec, end_sec;
        struct timeval tv;

        syscall(start_to_count_number_of_process_switches);
        gettimeofday(&tv, NULL);
        start_sec = tv.tv_sec;

        while(__switch==ON){
                sleep(0.01);
                printf("[%d ]", b++);

                gettimeofday(&tv, NULL);
                end_sec = tv.tv_sec;

                if(end_sec-start_sec >= 120)
                        __switch = OFF;
        }
        cnt = syscall(stop_to_count_number_of_process_switches);
        printf("\nDuring the past 2 minutes the process makes %d times process switches.\n", cnt);
        return 0;
}

* ##### Results

[2025675 ][2025676 ][2025677 ][2025678 ][2025679 ][2025680 ]
During the past 2 minutes the process makes 4050978 times process switches.

* ##### CPU_bound

#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <linux/kernel.h>
#include <unistd.h>
#include <sys/time.h>

#define ON 1
#define OFF 0
#define start_to_count_number_of_process_switches 339
#define stop_to_count_number_of_process_switches 340
int main(){
        int cnt = 0, b = 0;
        int __switch = ON;
        long start_sec, end_sec;
        struct timeval tv;

        syscall(start_to_count_number_of_process_switches);
        gettimeofday(&tv, NULL);
        start_sec = tv.tv_sec;

        while(__switch==ON){
                b = b+1;

                gettimeofday(&tv, NULL);
                end_sec = tv.tv_sec;

                if(end_sec-start_sec >= 120)
                        __switch = OFF;
        }
        cnt = syscall(stop_to_count_number_of_process_switches);
        printf("\nDuring the past 2 minutes the process makes %d times process switches.\n", cnt);
        return 0;
}

* ##### Results

During the past 2 minutes the process makes 2182 times process switches.

### 補充的知識點
#### 1. fork vs. vfork vs. clone
fork

#ifdef __ARCH_WANT_SYS_FORK
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
	struct kernel_clone_args args = {
		.exit_signal = SIGCHLD,
	};

	return kernel_clone(&args);
#else
	/* can not support in nommu mode */
	return -EINVAL;
#endif
}
#endif

vfork

#ifdef __ARCH_WANT_SYS_VFORK
SYSCALL_DEFINE0(vfork)
{
	struct kernel_clone_args args = {
		.flags		= CLONE_VFORK | CLONE_VM,
		.exit_signal	= SIGCHLD,
	};

	return kernel_clone(&args);
}
#endif

clone

SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
		 int __user *, parent_tidptr,
		 int __user *, child_tidptr,
		 unsigned long, tls)
{
	struct kernel_clone_args args = {
		.flags		= (lower_32_bits(clone_flags) & ~CSIGNAL),
		.pidfd		= parent_tidptr,
		.child_tid	= child_tidptr,
		.parent_tid	= parent_tidptr,
		.exit_signal	= (lower_32_bits(clone_flags) & CSIGNAL),
		.stack		= newsp,
		.tls		= tls,
	};

	return kernel_clone(&args);
}

#### 2. pid 0 1 2 的 process
* pid 0 是 swapper process 是 pid 1、2 的 parent
* pid 1 是 init process，是所有 user space process 的祖先
* pid 2 是 kthreadd process，是所有 kernel space process 的祖先

所有的 process 只有 pid 0 是由靜態生成的，其他皆透過 kernel_clone 產生 (早期的版本是 do_fork)
#### 3. do while(0)
##### 如下所示，是 switch_to 的 macro，do while(0) 就是只執行一次，那幹嘛多此一舉 (?

#define switch_to(prev, next, last)					\
do {									\
	((last) = __switch_to_asm((prev), (next)));			\
} while (0)

##### 考慮以下情形

只寫重點

void a(){
    cout << "a\n";
}

void b(){
    cout << "b\n";
}

#define hello a();b();

int main() {
    if(false)
        hello
    return 0;
}

##### 期望輸出

甚麼都沒有

##### 實際輸出

b

##### 原因如下

在擴展的時候 main 會變成下面那樣，所以無論如何 b() 都會執行到

if(false)
    a();
b();
return 0;

##### 那如果我們把 code 框起來 {} 呢

#define hello {a();b();}

那 main 會變成下面那樣

if(false) {
    a();
    b();
}
return 0;

##### ok 看起來可以正確運行，接著考慮分號

#define hello {a();b();}

有 else 就出事，因為平常寫程式會加分號。

if(false)
    hello <- 這邊就不能有分號QQ

##### 最終版本 do while(0)

#define hello 
do {
    a();b();
} while(0)
這樣就一定要加分號，不加還會報錯。