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Month: 九月 2015 (Page 1 of 3)

Linux kernel 笔记 (23)——安装Suse kernel代码

安装SLES(Suse Linux Enterprise Server)时,在选择安装软件界面时,选择“C/C++ Compiler and Tools”:

Capture

就会自动安装这个版本的Suse kernel代码:

Capture1

 

Lua笔记(23)—— pairs和ipairs

在这篇帖子中,提到pairsipairs的区别:

pairs() returns an iterator that will return the key and value of every key in the table, in no particular order. Usually, k and v are used to hold the key and value it returns. This is used to perform actions on each item in a table in turn, like when printing the contents of each value in the table.

ipairs() is very similar to pairs(), except that it will start at table[1] and iterate through all numerically indexed entries until the first nil value. It does this in order, which can be useful when you’re looking for the first item in a list that meets certain criterion.

pairs遍历任意table,返回的keyvalue没有什么顺序。而ipairs只会返回key是数字的元素。ipairs通常用在访问数组上。

参考下面这个程序:

t = {hello = 100, 200, 300}

print("k, v in pairs:")
for k, v in pairs(t) do
    print(k, v)
end

print("k, v in ipairs:")
for k, v in ipairs(t) do
    print(k, v)
end

执行结果如下:

k, v in pairs:
1   200
2   300
hello   100
k, v in ipairs:
1   200
2   300

可以看到,pairs可以遍历所有的key-value对,而ipairs只会访问key是数字的元素。

 

“/dev/tty”,“/dev/console”和“/dev/tty0”的区别

这篇笔记来自于stackoverflow的一篇帖子,答案如下:

From the documentation(http://www.kernel.org/doc/Documentation/devices.txt):

    /dev/tty        Current TTY device
    /dev/console    System console
    /dev/tty0       Current virtual console

In the good old days /dev/console was System Administrator console. And TTYs were users' serial devices attached to a server.
Now /dev/console and /dev/tty0 represent current display and usually are the same. You can override it for example by adding console=ttyS0 to grub.conf. After that your /dev/tty0 is a monitor and /dev/console is /dev/ttyS0.

An exercise to show the difference between /dev/tty and /dev/tty0:

Switch to the 2nd console by pressing Ctrl+Alt+F2. Login as root. Type "sleep 5; echo tty0 > /dev/tty0". Press Enter and switch to the 3rd console by pressing Alt+F3.
Now switch back to the 2nd console by pressing Alt+F2. Type "sleep 5; echo tty > /dev/tty", press Enter and switch to the 3rd console.

You can see that "tty" is the console where process starts, and "tty0" is a always current console.

早些时候,/dev/console是系统管理员控制台,而TTYs则代表用户连接服务器的串行设备。而现在,/dev/console/dev/tty0均指当前的显示设备,并且通常情况下是一样的。你可以修改/dev/console所关联的设备。举个例子,在grub.conf中加入console=ttyS0。则现在,/dev/tty0所关联的是显示器,而dev/console则关联/dev/ttyS0

/dev/tty是当前进程控制的tty设备,而tty0则是当前的控制台。当你在一个终端执行“sleep 5; echo tty0 > /dev/tty0”命令后,切换到其它终端,则tty0会在你切换后的终端显示。而执行“sleep 5; echo tty > /dev/tty”命令后,无论切换到那个终端,tty始终会在输入命令的终端显示。

 

Linux kernel 笔记 (22)——一个简单的模块Makefile

LDD3Compiling and Loading一节的编译模块的Makefile为例:

# If KERNELRELEASE is defined, we've been invoked from the
# kernel build system and can use its language.
ifneq ($(KERNELRELEASE),)
    obj-m := hello.o 

# Otherwise we were called directly from the command
# line; invoke the kernel build system.
else

    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    PWD  := $(shell pwd)

default:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

endif

当在命令行执行make命令时(当前工作目录即模块源文件所在目录),因为当前模块所在目录里没有定义KERNELRELEASE,所以执行else部分,即把KERNELDIRPWD变量赋值。

接下来执行“$(MAKE) -C $(KERNELDIR) M=$(PWD) modules”命令。-C选项的含义是把目录切换到KERNELDIR目录下,然后读取KERNELDIR目录下的MakefileM选项是在编译modules再切换回模块所在目录。此时由于KERNELRELEASE变量已经定义,即可以得知需要编译obj-m

参考资料:
Understanding a make file for making .ko files

 

*NIX & Hacking —— 第9期

做一本我感兴趣的杂志,就这么简单!

Assembler

Assembler relaxation

GDB

GDB dashboard

Go

Best practices for a new Go developer
On Go, Portability, and System Interfaces

Kernel

A Toure of Bootloading
GRUB 2 bootloader – Full tutorial
How I ended up writing new real-time kernel
Kernel bypass
Linux Kernel Crash Book

Network

TCP in 30 instructions

RMS

Interviews: RMS Answers Your Questions

Rust

Why Rust?

Tracing

Dynamic Tracing with DTrace & SystemTap

Linux kernel 笔记 (21)——per-CPU变量

per-CPU变量顾名思义,即当你声明一个per-CPU变量时,当前系统上的每个CPU都会有一份当前变量的copy。使用per-CPU变量好处是访问它几乎不需要加锁,因为每个CPU都有一份copy。此外,CPU可以把这个变量放在自己的cache里,访问起来会特别快。定义per-CPU变量方法如下:

DEFINE_PER_CPU(type, name);  

如果per-CPU变量是数组,则定义方式如下:

DEFINE_PER_CPU(type[length], array); 

per-CPU变量可以导出,供其它模块使用:

EXPORT_PER_CPU_SYMBOL(per_cpu_var);
EXPORT_PER_CPU_SYMBOL_GPL(per_cpu_var);

要在其它模块使用per-CPU变量,则需要声明:

DECLARE_PER_CPU(type, name);

访问per-CPU变量可以使用get_cpu_var(var)set_cpu_var(var)这两个macro

/* <linux/percpu.h>*/

/*
 * Must be an lvalue. Since @var must be a simple identifier,
 * we force a syntax error here if it isn't.
 */
#define get_cpu_var(var) (*({               \
    preempt_disable();              \
    &__get_cpu_var(var); }))

/*
 * The weird & is necessary because sparse considers (void)(var) to be
 * a direct dereference of percpu variable (var).
 */
#define put_cpu_var(var) do {               \
    (void)&(var);                   \
    preempt_enable();               \
} while (0)

因为kernel线程是允许preemption的,所以在get_cpu_var中需要调用preempt_disable,并且要和put_cpu_var配对使用。

访问另一个CPUper-CPU变量:

per_cpu(variable, int cpu_id);

参考资料:
Driver porting: per-CPU variables;
Per-CPU Variables

 

Linux kernel 笔记 (20)——设备的major和minor号

/dev目录下执行ls -lt命令:

 

Capture 上面红框框起来的部分就是设备号,前面是major,后面是minor major号表示设备所使用的驱动,而minor号则表示具体的设备。在上图中,tty的驱动都是driver 4,而利用minor号区别不同的tty设备。 另外,通过/proc/devices文件也可以看到设备所使用的驱动,即major号:

linux-a21w:/dev # cat /proc/devices
Character devices:
  1 mem
  4 /dev/vc/0
  4 tty
  4 ttyS
  5 /dev/tty
  5 /dev/console
  5 /dev/ptmx
  7 vcs
......

关于dev_tmajorminor号定义如下(kernel版本是4.0):

/* <linux/types.h>: */
typedef __u32 __kernel_dev_t;
typedef __kernel_dev_t      dev_t;

/* <linux/kdev_t.h> */
#define MINORBITS   20
#define MINORMASK   ((1U << MINORBITS) - 1)

#define MAJOR(dev)  ((unsigned int) ((dev) >> MINORBITS))
#define MINOR(dev)  ((unsigned int) ((dev) & MINORMASK))
#define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))

dev_t32 bit长,其中高12位是major,低20位是minor

获取设备号的两种方法:

(1)预先指定设备号:

int register_chrdev_region(dev_t from, unsigned count, const char *name) 

from包含majorminor,通常情况下minor指定为0count指定连续设备号的数量,name指定设备的名字。register_chrdev_region实现如下:

/**
 * register_chrdev_region() - register a range of device numbers
 * @from: the first in the desired range of device numbers; must include
 *        the major number.
 * @count: the number of consecutive device numbers required
 * @name: the name of the device or driver.
 *
 * Return value is zero on success, a negative error code on failure.
 */
int register_chrdev_region(dev_t from, unsigned count, const char *name)
{
    struct char_device_struct *cd;
    dev_t to = from + count;
    dev_t n, next;

    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        if (next > to)
            next = to;
        cd = __register_chrdev_region(MAJOR(n), MINOR(n),
                   next - n, name);
        if (IS_ERR(cd))
            goto fail;
    }
    return 0;
fail:
    to = n;
    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
    }
    return PTR_ERR(cd);
}

可以看到register_chrdev_region即是把from开始连续count个设备号(dev_t类型,包含majorminor)都注册。

举个例子(/drivers/tty/tty_io.c):

register_chrdev_region(MKDEV(TTYAUX_MAJOR, 1), 1, "/dev/console")

(2)动态分配设备号(推荐使用):

int alloc_chrdev_region(dev_t *dev, unsigned int firstminor, unsigned int count, char *name);

dev是传出参数,为动态获得的设备号;firstminor指定第一个minorcountnameregister_chrdev_region的参数定义。alloc_chrdev_region实现如下:

/**
 * alloc_chrdev_region() - register a range of char device numbers
 * @dev: output parameter for first assigned number
 * @baseminor: first of the requested range of minor numbers
 * @count: the number of minor numbers required
 * @name: the name of the associated device or driver
 *
 * Allocates a range of char device numbers.  The major number will be
 * chosen dynamically, and returned (along with the first minor number)
 * in @dev.  Returns zero or a negative error code.
 */
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
            const char *name)
{
    struct char_device_struct *cd;
    cd = __register_chrdev_region(0, baseminor, count, name);
    if (IS_ERR(cd))
        return PTR_ERR(cd);
    *dev = MKDEV(cd->major, cd->baseminor);
    return 0;
}

举个例子(/drivers/watchdog/watchdog_dev.c):

alloc_chrdev_region(&watchdog_devt, 0, MAX_DOGS, "watchdog");

释放设备号:

void unregister_chrdev_region(dev_t first, unsigned int count);

 

Linux kernel 笔记 (19)——“soft lockup – CPU# stuck …”bug

soft lockup - CPU# stuck ...bugkernel log类似这样:

[   28.124107] BUG: soft lockup - CPU#0 stuck for 23s! [init:1]
[   28.124720] Modules linked in:
[   28.125247] Supported: Yes
[   28.125763] Modules linked in:
[   28.126277] Supported: Yes
[   28.126774] 
[   28.127264] Pid: 1, comm: init Not tainted 3.0.101-63-xen #1  
[   28.127765] EIP: 0061:[<c00ded0a>] EFLAGS: 00000202 CPU: 0
[   28.128002] EIP is at handle_mm_fault+0x18a/0x2b0
[   28.128002] EAX: 0002bfc1 EBX: 00000000 ECX: 00000000 EDX: 00000000
[   28.128002] ESI: 2bfc1067 EDI: 00000000 EBP: ebfc6200 ESP: ebc35d48
[   28.128002]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: e021
[   28.128002] Process init (pid: 1, ti=ebc08000 task=ebc32ce0 task.ti=ebc34000)
[   28.128002] Stack:
[   28.128002]  ebfc1778 ebfc6200 00000029 0002bfc1 00000000 080efc90 ebfc2570 ebfc9e40
[   28.128002]  ec7bd000 ebc35dfc 00000003 ebfc2570 080efc90 c0350ad4 00000029 00000100
[   28.128002]  00000008 00000003 ebfc9e78 ebc32ce0 ebfc9e40 00000000 00000029 00000003
[   28.128002] Call Trace:
[   28.128002]  [<c0350ad4>] do_page_fault+0x1f4/0x4b0
[   28.128002]  [<c034df54>] error_code+0x30/0x38
[   28.128002]  [<c01da35f>] clear_user+0x2f/0x50   
[   28.128002]  [<c01480d4>] load_elf_binary+0xae4/0xc30    
[   28.128002]  [<c01094d1>] search_binary_handler+0x1e1/0x2e0  
[   28.128002]  [<c01097b4>] do_execve_common+0x1e4/0x280   
[   28.128002]  [<c000a9c2>] sys_execve+0x52/0x80   
[   28.128002]  [<c035443e>] ptregs_execve+0x12/0x18    
[   28.128002]  [<c034dc3d>] syscall_call+0x7/0x7   
[   28.128002]  [<c000933f>] kernel_execve+0x1f/0x30    
[   28.128002]  [<c000424e>] init_post+0xde/0x130   
[   28.128002]  [<c057d638>] kernel_init+0x160/0x18f    
[   28.128002]  [<c0354526>] kernel_thread_helper+0x6/0x10  
[   28.128002] Code: 89 f2 89 f8 81 e2 00 f0 ff ff 25 ff 0f 00 00 89 54 24 0c 89 44 24 10 8b 44 24 0c 8b 54 24 10 0f ac d0 0c 89 44 24 0c 8b 44 24 0c <c1> ea 0c 89 54 24 10 c1 e0 05 03 44 24 20 e8 b3 90 ff ff 8b 54 
......

这个Bug背后的原理是这样的:

Linux kernel针对每个CPU都有一个watchdog进程。使用ps -ef | grep watctdog可以看到:

[nan@longriver ~]$ ps -ef | grep watchdog
root         6     2  0 Apr20 ?        00:00:16 [watchdog/0]
root        10     2  0 Apr20 ?        00:00:11 [watchdog/1]
root        14     2  0 Apr20 ?        00:00:10 [watchdog/2]
root        18     2  0 Apr20 ?        00:00:09 [watchdog/3]
nan   6726  4608  0 17:28 pts/28   00:00:00 grep watchdog

watchdog进程会搜集所监控的CPU的关于使用时间的信息([watchdog/X]中的X代表监控的CPU ID),并把这些信息存在kernel中。kernel中有专门的interrupt函数会调用softlockup计数器,并把当前的时间与之前kernel中存储的时间值进行比较。如果相差超过一个门限值,则就认为watchdog进程没有获得足够的执行时间用来更新kernel中的信息,也就是CPU一直被其它task占据着。这会被kernel认为是一种不正常的现象,就会打印出如上所示的call traceregister等等信息。

参考资料:
Linux Kernel BUG: soft lockup CPU#1 stuck

 

Luajit笔记(2)——FFI库(1)

Luajit提供的FFI库(ffi.*)允许Lua代码调用外部的C函数和使用C数据结构。但是默认情况下,FFI库不会被加载和初始化。因此建议在每个使用FFI库的Lua文件开头加载:

local ffi = require("ffi")

看下面这个例子:

local ffi = require("ffi")

ffi.cdef[[
int printf(const char *format, ...);
]]

ffi.C.printf("Hello world!\n")

执行结果如下:

Hello world!

(1)
ffi.cdef的定义如下:

ffi.cdef(def)

def必须是一个Lua字符串,建议使用“[[...]]”这种格式。ffi.cdef包含的是对C语言类型的定义和外部符号(变量和函数)的声明(仅仅是声明,并没有和实际的内存地址进行绑定,实际的绑定是通过C library namespace)。要注意对C类型的声明不会经过C预处理器,除了#pragma pack以外,包括#define在内的指令都要进行处理替换,比如使用enum等等。

(2)
ffi.C是默认的C library namespace。它同编译器有些类似,但不用显示地声明链接库。在POSIX系统上,ffi.C会在defaultglobalnamespace上绑定符号。包括libclibm,等等。还有Luajit自身提供的API

 

“devel”包是什么?

RHEL上安装package时,经常看到同样名字的package有两个:分别是带和不带devel后缀的。例如:

elfutils-libelf.x86_64 : Library to read and write ELF files
elfutils-libelf-devel.x86_64 : Development support for libelf

两者区别是:不带devel后缀的package,通常只包含能让程序运行的动态库和配置文件。而带devel后缀的package,则包含使用这个package开发程序的所有的必需文件。比如头文件,等等。有时devel package还包含静态库。

参考资料:
What are *-devel packages?

 

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