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社区首页 >专栏 >MySQL DBA如何利用strace/pstack/gdb来定位问题

MySQL DBA如何利用strace/pstack/gdb来定位问题

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老叶茶馆
发布于 2020-06-24 17:22:07
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发布于 2020-06-24 17:22:07
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文章被收录于专栏：MySQL修行 | 老叶茶馆MySQL修行 | 老叶茶馆

strace简介

strace是Linux环境下的一款程序调试工具，用来监察一个应用程序所使用的系统调用。 Strace是一个简单的跟踪系统调用执行的工具。在其最简单的形式中，它可以从开始到结束跟踪二进制的执行，并在进程的生命周期中输出一行具有系统调用名称，每个系统调用的参数和返回值的文本行。

常用选项

  • -T：strace输出显示时间
  • -t：可以在每行的输出之前添加时间戳，-tt可以精确到微妙级别，-ttt也可以精确到微妙级，但是它并不是打印当前时间，而是显示自从epoch以来的所经过的秒数
  • -s：指定输出的字符串的最大长度，默认为32，如果输出到文件的话会全部输出
  • -o：指定将strace输出到文件
  • -ff：如果提供-o filename，则所有进程的跟踪结果输出到相应的filename.pid中，pid是各进程的进程号.
  • -e：指定跟踪某个行为，例如-e trace=open指定只跟踪open行为
  • -y：将文件句柄用文件路径代替显示

正确姿势

简单使用
代码语言：javascript
复制
strace -T -tt -o /tmp/strace.log CMD

strace -T -tt CMD 2>&1 |tee /tmp/strace.log

strace -T -tt -s 100 -o /tmp/strace.log CMD

strace -T -tt -s 100 -ff -o /tmp/strace.log CMD

strace -T -tt -s 100 -e trace=XXXX -o /tmp/strace.log CMD
使用案例

利用strace观察客户端client执行SQL

代码语言：javascript
复制
#通过sys.processlist表中pid可以知道客户端连接pid
MySQL [sys]> select thd_id,conn_id,user,pid,program_name,command,current_statement from processlist where conn_id>0 and pid>0;
+--------+---------+------------------+------+--------------+---------+-------------------+
| thd_id | conn_id | user             | pid  | program_name | command | current_statement |
+--------+---------+------------------+------+--------------+---------+-------------------+
|     78 |      22 | xucl@172.17.0.11 | 8656 | mysql        | Sleep   | select * from t1  |
+--------+---------+------------------+------+--------------+---------+-------------------+
1 row in set (0.05 sec)

#session1
[root@VM_0_11_centos ~]# strace -T -tt -s 100 -o /tmp/strace.log -p 5841
strace: Process 5841 attached
^Cstrace: Process 5841 detached

#session进行数据查询 
MySQL [(none)]> use xucl
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MySQL [xucl]> select * from t1;
+----+----+
| id | c1 |
+----+----+
|  1 |    |
+----+----+
1 row in set (0.00 sec)

需要提醒一下的是从客户端连接到MySQL的时候需要注意客户端的版本，例如我用mariadb的客户端连接官方版本8.0就无法获取到pid

查看strace文件

代码语言：javascript
复制
....
20:54:16.901980 poll([{fd=3, events=POLLIN|POLLPRI}], 1, 0) = 0 (Timeout) <0.000006>
20:54:16.902001 write(3, "\21\0\0\0\3select * from t1", 21) = 21 <0.000011>
20:54:16.902026 read(3, "\1\0\0\1\2\"\0\0\2\3def\4xucl\2t1\2t1\2id\2id\f?\0\v\0\0\0\3\3P\0\0\0\"\0\0\3\3def\4xucl\2t1\2t1\2c1\2c1\f!\0\36\0\0\0\375\t@\0\0\0\5\0\0\4\376\0\0\2\0\3\0\0\5\0011\0\5\0\0"..., 16384) = 106 <0.000355>
20:54:16.902405 times({tms_utime=0, tms_stime=0, tms_cutime=0, tms_cstime=0}) = 429630696 <0.000005>
20:54:16.902441 write(1, "+----+----+\n", 12) = 12 <0.000026>
20:54:16.902482 write(1, "| id | c1 |\n", 12) = 12 <0.000018>
20:54:16.902514 write(1, "+----+----+\n", 12) = 12 <0.000018>
20:54:16.902549 write(1, "|  1 |    |\n", 12) = 12 <0.000019>
20:54:16.902582 write(1, "+----+----+\n", 12) = 12 <0.000019>
20:54:16.902631 write(1, "\33(B\33[0;1m1 row in set (0.00 sec)\n", 33) = 33 <0.000019>
20:54:16.902666 write(1, "\33(B\33[m\33(B\33[0;1m\n", 16) = 16 <0.000019>
20:54:16.902702 stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=528, ...}) = 0 <0.000006>
20:54:16.902752 rt_sigprocmask(SIG_BLOCK, [INT], [], 8) = 0 <0.000004>
20:54:16.902771 ioctl(0, TIOCGWINSZ, {ws_row=42, ws_col=163, ws_xpixel=0, ws_ypixel=0}) = 0 <0.000005>
20:54:16.902789 ioctl(0, TIOCSWINSZ, {ws_row=42, ws_col=163, ws_xpixel=0, ws_ypixel=0}) = 0 <0.000004>
20:54:16.902806 ioctl(0, TCGETS, {B38400 opost isig icanon echo ...}) = 0 <0.000005>
20:54:16.902824 ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig -icanon -echo ...}) = 0 <0.000006>
20:54:16.902841 write(1, "\33(B\33[m", 6) = 6 <0.000022>
20:54:16.902877 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 <0.000005>
20:54:16.902895 rt_sigprocmask(SIG_BLOCK, [INT QUIT ALRM TSTP TTIN TTOU], [], 8) = 0 <0.000005>
20:54:16.902912 rt_sigaction(SIGINT, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {0x55e19fee21d0, [INT], SA_RESTORER|SA_RESTART, 0x7fc3e63585f0}, 8) = 0 <0.000005>
20:54:16.902931 rt_sigaction(SIGTERM, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc3e63585f0}, 8) = 0 <0.000005>
20:54:16.902948 rt_sigaction(SIGQUIT, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {0x55e19fee1fa0, [QUIT], SA_RESTORER|SA_RESTART, 0x7fc3e63585f0}, 8) = 0 <0.000005>
20:54:16.902966 rt_sigaction(SIGALRM, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc3e63585f0}, 8) = 0 <0.000005>
20:54:16.902983 rt_sigaction(SIGTSTP, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc3e63585f0}, 8) = 0 <0.000005>
20:54:16.903001 rt_sigaction(SIGTTOU, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc3e63585f0}, 8) = 0 <0.000004>
20:54:16.903018 rt_sigaction(SIGTTIN, {0x55e19ff48a00, [], SA_RESTORER, 0x7fc3e63585f0}, {SIG_DFL, [], SA_RESTORER, 0x7fc3e63585f0}, 8) = 0 <0.000006>
20:54:16.903037 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 <0.000005>
20:54:16.903055 rt_sigaction(SIGWINCH, {0x55e19ff481a0, [], SA_RESTORER|SA_RESTART, 0x7fc3e63585f0}, {0x55e19fee0760, [WINCH], SA_RESTORER|SA_RESTART, 0x7fc3e63585f0}, 8) = 0 <0.000006>
20:54:16.903088 write(1, "MySQL [xucl]> ", 14) = 14 <0.000032>
20:54:16.903139 read(0,  <detached ...>
利用strace观察server端执行
代码语言：javascript
复制
#命令
strace -o /tmp/strace.log -T -tt -f  -p `pidof mysqld`
#假设你想看跟IO相关的行为，可以用-e trace来指定
# mysql相关的IO操作：
# mysql: read, write, open
# innodb: pread64, pwrite64
strace -o /tmp/strace.log -T -tt -f -e trace=read,open,write,pwrite64,pread64 -p `pidof mysqld`

示例：

代码语言：javascript
复制
strace -o /tmp/strace.log -T -tt -ff  -p `pidof mysqld`
#然后在本地通过socket登录执行命令

MySQL [performance_schema]> use xucl;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MySQL [xucl]> select * from t1;
+------+
| id   |
+------+
|    1 |
|    2 |
|    4 |
|    3 |
|    5 |
|    6 |
+------+
6 rows in set (0.01 sec)
#查看本线程对应的LWP
MySQL [xucl]> show processlist;
+----+------+-----------+------+---------+------+----------+------------------+
| Id | User | Host      | db   | Command | Time | State    | Info             |
+----+------+-----------+------+---------+------+----------+------------------+
| 28 | root | localhost | xucl | Query   |    0 | starting | show processlist |
+----+------+-----------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)

MySQL [xucl]> select * from performance_schema.threads where processlist_id=28\G
*************************** 1. row ***************************
          THREAD_ID: 62
               NAME: thread/sql/one_connection
               TYPE: FOREGROUND
     PROCESSLIST_ID: 28
   PROCESSLIST_USER: root
   PROCESSLIST_HOST: localhost
     PROCESSLIST_DB: xucl
PROCESSLIST_COMMAND: Query
   PROCESSLIST_TIME: 0
  PROCESSLIST_STATE: Sending data
   PROCESSLIST_INFO: select * from performance_schema.threads where processlist_id=28
   PARENT_THREAD_ID: NULL
               ROLE: NULL
       INSTRUMENTED: YES
            HISTORY: YES
    CONNECTION_TYPE: Socket
       THREAD_OS_ID: 8208
1 row in set (0.00 sec)

strace会在/tmp文件夹下生成strace.log.xxx，xxx对应的就是THREAD_OS_ID，我们直接看strace.log.8208文件

代码语言：javascript
复制
t@VM_0_9_centos tmp]# cat strace.log.8208
16:44:14.447709 restart_syscall(<... resuming interrupted restart_syscall ...>) = 1 <3.268331>
16:44:17.716076 recvfrom(134<socket:[12555437]>, "\22\0\0\0", 4, MSG_DONTWAIT, NULL, NULL) = 4 <0.000015>
16:44:17.716179 recvfrom(134<socket:[12555437]>, "\3SELECT DATABASE()", 18, MSG_DONTWAIT, NULL, NULL) = 18 <0.000017>
16:44:17.716318 sendto(134<socket:[12555437]>, "\1\0\0\1\1 \0\0\2\3def\0\0\0\nDATABASE()\0\f!\0f"..., 68, MSG_DONTWAIT, NULL, 0) = 68 <0.000033>
16:44:17.716420 recvfrom(134<socket:[12555437]>, "\5\0\0\0", 4, MSG_DONTWAIT, NULL, NULL) = 4 <0.000015>
16:44:17.716476 recvfrom(134<socket:[12555437]>, "\2xucl", 5, MSG_DONTWAIT, NULL, NULL) = 5 <0.000017>
16:44:17.716538 access("./xucl", F_OK)  = 0 <0.000016>
16:44:17.716604 sendto(134<socket:[12555437]>, "\7\0\0\1\0\0\0\2\0\0\0", 11, MSG_DONTWAIT, NULL, 0) = 11 <0.000090>
16:44:17.716738 recvfrom(134<socket:[12555437]>, 0x7f06ec009fe0, 4, MSG_DONTWAIT, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) <0.000014>
16:44:17.716793 poll([{fd=134<socket:[12555437]>, events=POLLIN|POLLPRI}], 1, 600000) = 1 ([{fd=134, revents=POLLIN}]) <1.326139>
16:44:19.042985 recvfrom(134<socket:[12555437]>, "\21\0\0\0", 4, MSG_DONTWAIT, NULL, NULL) = 4 <0.000014>
16:44:19.043048 recvfrom(134<socket:[12555437]>, "\3select * from t1", 17, MSG_DONTWAIT, NULL, NULL) = 17 <0.000017>
16:44:19.043256 sendto(134<socket:[12555437]>, "\1\0\0\1\1\"\0\0\2\3def\4xucl\2t1\2t1\2id\2id\f?"..., 97, MSG_DONTWAIT, NULL, 0) = 97 <0.000137>
16:44:19.043448 recvfrom(134<socket:[12555437]>, 0x7f06ec009fe0, 4, MSG_DONTWAIT, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) <0.000014>
16:44:19.043501 poll([{fd=134<socket:[12555437]>, events=POLLIN|POLLPRI}], 1, 600000) = 1 ([{fd=134, revents=POLLIN}]) <11.279983>
16:44:30.323556 recvfrom(134<socket:[12555437]>, "\21\0\0\0", 4, MSG_DONTWAIT, NULL, NULL) = 4 <0.000050>
16:44:30.323682 recvfrom(134<socket:[12555437]>, "\3show processlist", 17, MSG_DONTWAIT, NULL, NULL) = 17 <0.000017>
16:44:30.323788 sendto(134<socket:[12555437]>, "\1\0\0\1\10\30\0\0\2\3def\0\0\0\2Id\0\f?\0\25\0\0\0\10\201\0\0\0"..., 324, MSG_DONTWAIT, NULL, 0) = 324 <0.000221>
16:44:30.324066 recvfrom(134<socket:[12555437]>, 0x7f06ec009fe0, 4, MSG_DONTWAIT, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) <0.000014>
16:44:30.324120 poll([{fd=134<socket:[12555437]>, events=POLLIN|POLLPRI}], 1, 600000) = 1 ([{fd=134, revents=POLLIN}]) <2.943989>
16:44:33.268173 recvfrom(134<socket:[12555437]>, "A\0\0\0", 4, MSG_DONTWAIT, NULL, NULL) = 4 <0.000055>
16:44:33.268309 recvfrom(134<socket:[12555437]>, "\3select * from performance_schem"..., 65, MSG_DONTWAIT, NULL, NULL) = 65 <0.000017>
16:44:33.268592 sendto(134<socket:[12555437]>, "\1\0\0\1\21H\0\0\2\3def\22performance_schema"..., 1606, MSG_DONTWAIT, NULL, 0) = 1606 <0.000142>
16:44:33.268790 recvfrom(134<socket:[12555437]>, 0x7f06ec009fe0, 4, MSG_DONTWAIT, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) <0.000018>
16:44:33.268853 poll([{fd=134<socket:[12555437]>, events=POLLIN|POLLPRI}], 1, 600000 <detached ...>

可以清楚地看到有对应的SQL输出

pstack简介

pstack用来跟踪进程栈，这个命令在排查进程问题时非常有用，比如我们发现一个服务一直处于work状态（如假死状态，好似死循环），使用这个命令就能轻松定位问题所在； 可以在一段时间内，多执行几次pstack，若发现代码栈总是停在同一个位置，那个位置就需要重点关注，很可能就是出问题的地方；

正确使用姿势

pstack使用起来非常简单，直接pstack -pid即可

代码语言：javascript
复制
[root@VM_0_9_centos mysql3306]# netstat -lntp|grep 3306
tcp6       0      0 :::33060                :::*                    LISTEN      13499/mysqld
tcp6       0      0 :::3306                 :::*                    LISTEN      13499/mysqld
[root@VM_0_9_centos mysql3306]# ps -Lf 13499
UID        PID  PPID   LWP  C NLWP STIME TTY      STAT   TIME CMD
mysql    13499 11316 13499  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13506  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13507  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13508  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13509  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13510  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
mysql    13499 11316 13511  0   53 20:31 pts/0    Sl     0:00 /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my3306.cnf --user=mysql
....

mysqld的pid为13499，fork了几十个线程，有的是用户连接，有的是MySQL自己的线程，例如read_thread、write_thread等等，这里的LWP对应的就是performance_schema.threads表的THREAD_OS_ID。

全部抓取

假如发生了mysqld hang住的情况，可以直接pstack mysqldpid来抓取所有的线程堆栈，这也是通常使用的方式，例如

代码语言：javascript
复制
pstack 13499 >> /tmp/pstack.log

这里简单随便看下抓取到的pstack

代码语言：javascript
复制
Thread 27 (Thread 0x7ff1bc29f700 (LWP 13535)):
#0  0x00007ff1f6368da2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000020ace25 in os_event::timed_wait(timespec const*) () at ../../../mysql-8.0.19/storage/innobase/include/sync0types.h:540
#2  0x00000000020ad9e1 in os_event::wait_time_low (this=0x7ff1dc022458, time_in_usec=<optimized out>, reset_sig_count=1) at ../../../mysql-8.0.19/storage/innobase/os/os0event.cc:495
#3  0x000000000215fe0c in srv_error_monitor_thread() () at ../../../mysql-8.0.19/storage/innobase/srv/srv0srv.cc:1835
#4  0x0000000002076da5 in __invoke_impl<void, void (*&)()> (__f=<synthetic pointer>) at /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/invoke.h:89
#5  __invoke<void (*&)()> (__fn=<synthetic pointer>) at /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/invoke.h:95
#6  __call<void> (__args=..., this=<synthetic pointer>) at /opt/rh/devtoolset-8/root/usr/include/c++/8/functional:400
#7  operator()<> (this=<synthetic pointer>) at /opt/rh/devtoolset-8/root/usr/include/c++/8/functional:484
#8  operator()<void (*)()> (f=@0x7ff1dc4aec18: 0x215fd20 <srv_error_monitor_thread()>, this=0x7ff1dc4aec20) at ../../../mysql-8.0.19/storage/innobase/include/os0thread-create.h:101
#9  __invoke_impl<void, Runnable, void (*)()> (__f=...) at /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/invoke.h:60
#10 __invoke<Runnable, void (*)()> (__fn=...) at /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/invoke.h:95
#11 _M_invoke<0, 1> (this=0x7ff1dc4aec18) at /opt/rh/devtoolset-8/root/usr/include/c++/8/thread:244
#12 operator() (this=0x7ff1dc4aec18) at /opt/rh/devtoolset-8/root/usr/include/c++/8/thread:253
#13 std::thread::_State_impl<std::thread::_Invoker<std::tuple<Runnable, void (*)()> > >::_M_run (this=0x7ff1dc4aec10) at /opt/rh/devtoolset-8/root/usr/include/c++/8/thread:196
#14 0x000000000265d64f in execute_native_thread_routine ()
#15 0x00007ff1f6364e65 in start_thread () from /lib64/libpthread.so.0
#16 0x00007ff1f44a188d in clone () from /lib64/libc.so.6

从堆栈可以看出这其实就是MySQL的error monitor线程

单个抓取

假设你运气好，你现在可以通过performance_schema.threads可以查到对应的线程的thread_os_id，你可以指定单个线程的lwp进行抓取

代码语言：javascript
复制
MySQL [performance_schema]> show processlist;
+----+-----------------+-----------+--------------------+---------+------+------------------------+---------------------------+
| Id | User            | Host      | db                 | Command | Time | State                  | Info                      |
+----+-----------------+-----------+--------------------+---------+------+------------------------+---------------------------+
|  4 | event_scheduler | localhost | NULL               | Daemon  | 4319 | Waiting on empty queue | NULL                      |
| 23 | root            | localhost | performance_schema | Query   |    0 | starting               | show processlist          |
| 24 | root            | localhost | xucl               | Query   |   22 | User sleep             | select sleep(600) from t1 |
+----+-----------------+-----------+--------------------+---------+------+------------------------+---------------------------+
3 rows in set (0.00 sec)

MySQL [performance_schema]> select THREAD_OS_ID from threads where processlist_id=24\G
*************************** 1. row ***************************
THREAD_OS_ID: 15020
1 row in set (0.00 sec)

你可以这样操作

代码语言：javascript
复制
pstack 15020 > /tmp/single_pstack.log

查看pstack结果，只抓取了15020线程的堆栈

代码语言：javascript
复制
Thread 1 (process 15020):
#0  0x00007ff1f6368da2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x0000000001189b53 in native_cond_timedwait (abstime=0x7ff1b810d7f0, mutex=0x3b96760 <LOCK_item_func_sleep>, cond=0x7ff1b810d8e0) at ../../mysql-8.0.19/include/thr_cond.h:149
#2  my_cond_timedwait (abstime=0x7ff1b810d7f0, mp=0x3b96760 <LOCK_item_func_sleep>, cond=0x7ff1b810d8e0) at ../../mysql-8.0.19/include/thr_cond.h:149
#3  inline_mysql_cond_timedwait (src_file=0x2de5538 "../../mysql-8.0.19/sql/item_func.cc", src_line=4786, abstime=0x7ff1b810d7f0, mutex=0x3b96760 <LOCK_item_func_sleep>, that=0x7ff1b810d8e0) at ../../mysql-8.0.19/include/mysql/psi/mysql_cond.h:217
#4  Interruptible_wait::wait (this=this@entry=0x7ff1b810d890, cond=cond@entry=0x7ff1b810d8e0, mutex=mutex@entry=0x3b96760 <LOCK_item_func_sleep>) at ../../mysql-8.0.19/sql/item_func.cc:4786
#5  0x0000000001189e26 in Item_func_sleep::val_int (this=<optimized out>) at ../../mysql-8.0.19/sql/item_func.cc:5435
#6  0x00000000010ff567 in Item::send (this=0x7ff1d45d9380, protocol=0x7ff1d4392240, buffer=<optimized out>) at ../../mysql-8.0.19/sql/item.cc:6638
#7  0x0000000000e3d36b in THD::send_result_set_row(List<Item>*) () at ../../mysql-8.0.19/sql/sql_class.cc:2530
#8  0x000000000127e8f1 in Query_result_send::send_data (this=<optimized out>, thd=0x7ff1d457b7b0, items=...) at ../../mysql-8.0.19/sql/query_result.cc:93
#9  0x0000000000f5d29a in SELECT_LEX_UNIT::ExecuteIteratorQuery(THD*) () at ../../mysql-8.0.19/sql/sql_union.cc:1530
#10 0x0000000000f5f3f9 in SELECT_LEX_UNIT::execute(THD*) () at ../../mysql-8.0.19/sql/sql_union.cc:1588
#11 0x0000000000eedf8b in Sql_cmd_dml::execute_inner(THD*) () at ../../mysql-8.0.19/sql/sql_select.cc:910
#12 0x0000000000ef7418 in Sql_cmd_dml::execute (this=0x7ff1d45d9a38, thd=0x7ff1d457b7b0) at ../../mysql-8.0.19/sql/sql_select.cc:715
#13 0x0000000000eab3a9 in mysql_execute_command(THD*, bool) () at ../../mysql-8.0.19/sql/sql_parse.cc:4478
#14 0x0000000000ead0cc in mysql_parse (thd=thd@entry=0x7ff1d457b7b0, parser_state=parser_state@entry=0x7ff1b810f610) at ../../mysql-8.0.19/sql/sql_parse.cc:5288
#15 0x0000000000eaeb6b in dispatch_command(THD*, COM_DATA const*, enum_server_command) () at ../../mysql-8.0.19/sql/sql_parse.cc:1777
#16 0x0000000000eb0104 in do_command (thd=thd@entry=0x7ff1d457b7b0) at ../../mysql-8.0.19/sql/sql_parse.cc:1275
#17 0x0000000000fc1a08 in handle_connection (arg=arg@entry=0x43cae90) at ../../mysql-8.0.19/sql/conn_handler/connection_handler_per_thread.cc:302
#18 0x00000000023ffdec in pfs_spawn_thread (arg=0x44c2a30) at ../../../mysql-8.0.19/storage/perfschema/pfs.cc:2854
#19 0x00007ff1f6364e65 in start_thread () from /lib64/libpthread.so.0
#20 0x00007ff1f44a188d in clone () from /lib64/libc.so.6

gdb简介

GDB 是 linux 环境下的一般功能强大的调试器，用来调试 C 或 C++ 写的程序。它可以做这些事情

  • Start your program, specifying anything that might affect its behavior.
  • Make your program stop on specified conditions.
  • Examine what has happened, when your program has stopped.
  • Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

常用命令

  • info threads：查看全部线程
  • thread n：指定某个线程
  • b：在某处打断点
  • c：继续往下走
  • s：执行一行代码，如果代码函数调用，则进入函数
  • n：执行一行代码，函数调用不进入
  • p：打印某个变量值
  • list：打印代码的文本信息
  • bt：查看某个线程的栈帧
  • info b：查看当前所有断点信息

实际案例

进入gdb界面

代码语言：javascript
复制
[root@VM_0_9_centos ~]# gdb /usr/local/mysql5.7/bin/mysqld

进入调试

代码语言：javascript
复制
run --defaults-file=/data/mysql/mysql3308/my3308.cnf --user=mysql --gdb

通过info thread查看所有线程，这个时候我从另外一个会话连接到了MySQL，我从pfs.threads查寻到这个会话的LWP值为513，那么我可以通过thread 41切换到这个线程

代码语言：javascript
复制
(gdb) info thread
  Id   Target Id         Frame
  41   Thread 0x7fff819cf700 (LWP 513) "mysqld" 0x00007ffff61abbed in poll () from /lib64/libc.so.6
  40   Thread 0x7fff5affd700 (LWP 416) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  39   Thread 0x7fff81a51700 (LWP 415) "mysqld" 0x00007ffff7bce381 in sigwait () from /lib64/libpthread.so.0
  38   Thread 0x7fff5b7fe700 (LWP 414) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  37   Thread 0x7fff5bfff700 (LWP 413) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  36   Thread 0x7fff6cff9700 (LWP 412) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  35   Thread 0x7fff6d7fa700 (LWP 411) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  34   Thread 0x7fff6dffb700 (LWP 410) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  33   Thread 0x7fff6e7fc700 (LWP 409) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  32   Thread 0x7fff6effd700 (LWP 408) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  31   Thread 0x7fff6f7fe700 (LWP 407) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  30   Thread 0x7fff6ffff700 (LWP 406) "mysqld" 0x00007ffff7bcde5d in nanosleep () from /lib64/libpthread.so.0
  29   Thread 0x7fff8084e700 (LWP 405) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  28   Thread 0x7fff8104f700 (LWP 404) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  27   Thread 0x7fff81850700 (LWP 403) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  24   Thread 0x7fff74c24700 (LWP 398) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  23   Thread 0x7fff75425700 (LWP 397) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  22   Thread 0x7fff75c26700 (LWP 396) "mysqld" 0x00007ffff7bca9f5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  21   Thread 0x7fff76427700 (LWP 395) "mysqld" 0x00007ffff7bcada2 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  20   Thread 0x7fff76c28700 (LWP 394) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  19   Thread 0x7fff77429700 (LWP 393) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  18   Thread 0x7fff77c2a700 (LWP 392) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  17   Thread 0x7fff7842b700 (LWP 391) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  16   Thread 0x7fff78c2c700 (LWP 390) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  15   Thread 0x7fff7942d700 (LWP 389) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  14   Thread 0x7fff79c2e700 (LWP 388) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  13   Thread 0x7fff7a42f700 (LWP 387) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  12   Thread 0x7fff7ac30700 (LWP 386) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  11   Thread 0x7fff7b431700 (LWP 385) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  10   Thread 0x7fff7bc32700 (LWP 384) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  9    Thread 0x7fff7c433700 (LWP 383) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  8    Thread 0x7fff7cc34700 (LWP 382) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  7    Thread 0x7fff7d435700 (LWP 381) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  6    Thread 0x7fff7dc36700 (LWP 380) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  5    Thread 0x7fff7e437700 (LWP 379) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  4    Thread 0x7fff7ec38700 (LWP 378) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  3    Thread 0x7fff7f439700 (LWP 377) "mysqld" 0x00007ffff79bd644 in __io_getevents_0_4 () from /lib64/libaio.so.1
  2    Thread 0x7fffec8c4700 (LWP 376) "mysqld" 0x00007ffff60ef53a in sigwaitinfo () from /lib64/libc.so.6
* 1    Thread 0x7ffff7fe4880 (LWP 371) "mysqld" 0x00007ffff61abbed in poll () from /lib64/libc.so.6
(gdb) thread 41
[Switching to thread 41 (Thread 0x7fff819cf700 (LWP 513))]
#0  0x00007ffff61abbed in poll () from /lib64/libc.so.6

演示一下打断点：

代码语言：javascript
复制
(gdb) b do_command
Breakpoint 1 at 0xc8d060: file /usr/local/src/mysql-5.7.30/sql/sql_parse.cc, line 903.
(gdb) c
Continuing.

(gdb) b do_command
Breakpoint 1 at 0xc8d060: file /usr/local/src/mysql-5.7.30/sql/sql_parse.cc, line 903.
(gdb) n
Single stepping until exit from function poll,
which has no line number information.
vio_io_wait (vio=vio@entry=0x54fd330, event=event@entry=VIO_IO_EVENT_READ, timeout=<optimized out>) at /usr/local/src/mysql-5.7.30/vio/viosocket.c:815
815       MYSQL_END_SOCKET_WAIT(locker, 0);
(gdb) n
817     }
(gdb) n
vio_socket_io_wait (vio=vio@entry=0x54fd330, event=event@entry=VIO_IO_EVENT_READ) at /usr/local/src/mysql-5.7.30/vio/viosocket.c:92
92          ret= -1;
(gdb) n
105     }
(gdb) n
vio_read (vio=0x54fd330, buf=0x54e3260 "\001", size=4) at /usr/local/src/mysql-5.7.30/vio/viosocket.c:134
134       while ((ret= mysql_socket_recv(vio->mysql_socket, (SOCKBUF_T *)buf, size, flags)) == -1)
(gdb) n
148     }
(gdb) n
net_read_raw_loop (net=net@entry=0x54841e8, count=count@entry=4) at /usr/local/src/mysql-5.7.30/sql/net_serv.cc:697
697         if (recvcnt == VIO_SOCKET_ERROR)
(gdb) n
706         else if (!recvcnt)
(gdb) n
715         thd_increment_bytes_received(recvcnt);
(gdb) n
712         count-= recvcnt;
(gdb) n
713         buf+= recvcnt;
(gdb) n
715         thd_increment_bytes_received(recvcnt);
(gdb) n
692       while (count)
(gdb) n
737     }
(gdb) n
736       return MY_TEST(count);
(gdb) n
737     }
(gdb) n
net_read_packet_header (net=0x54841e8) at /usr/local/src/mysql-5.7.30/sql/net_serv.cc:779
779         server_extension->m_after_header(net, user_data, count, rc);
(gdb) n
778         rc= net_read_raw_loop(net, count);
(gdb) n
779         server_extension->m_after_header(net, user_data, count, rc);
(gdb) n
787       if (rc)
(gdb) n
792       pkt_nr= net->buff[net->where_b + 3];
(gdb) n
798       if (pkt_nr != (uchar) net->pkt_nr)
(gdb) n
818       net->pkt_nr++;
(gdb) n
net_read_packet (net=0x54841e8, complen=0x7fff819cec00) at /usr/local/src/mysql-5.7.30/sql/net_serv.cc:850
850       if (net->compress)
(gdb) n
844       if (net_read_packet_header(net))
(gdb) n
847       net->compress_pkt_nr= net->pkt_nr;
(gdb) n
850       if (net->compress)
(gdb) n
868       pkt_len= uint3korr(net->buff+net->where_b);
(gdb) n
871       if (!pkt_len)
(gdb) n
868       pkt_len= uint3korr(net->buff+net->where_b);
(gdb) n
871       if (!pkt_len)
(gdb) p pkt_len
$1 = 22
(gdb) n
874       pkt_data_len = max(pkt_len, *complen) + net->where_b;
(gdb) n
877       if ((pkt_data_len >= net->max_packet) && net_realloc(net, pkt_data_len))
(gdb) n
881       if (net_read_raw_loop(net, pkt_len))
(gdb) n
868       pkt_len= uint3korr(net->buff+net->where_b);
(gdb) n
886       return pkt_len;
(gdb) n
891     }
(gdb) n
my_net_read (net=0x54841e8) at /usr/local/src/mysql-5.7.30/sql/net_serv.cc:922
922         if (len == MAX_PACKET_LENGTH)
(gdb) n
925           ulong save_pos = net->where_b;
(gdb) n
922         if (len == MAX_PACKET_LENGTH)
(gdb) n
937         net->read_pos = net->buff + net->where_b;
(gdb) n
938         if (len != packet_error)
(gdb) n
937         net->read_pos = net->buff + net->where_b;
(gdb) n
938         if (len != packet_error)
(gdb) n
939           net->read_pos[len]=0;             /* Safeguard for mysql_use_result */
(gdb) n
1056      MYSQL_NET_READ_DONE(0, len);
(gdb) n
1058    }
(gdb) n
Protocol_classic::read_packet (this=0x5483a88) at /usr/local/src/mysql-5.7.30/sql/protocol_classic.cc:820
820         raw_packet= m_thd->net.read_pos;
(gdb) n
819         bad_packet= false;
(gdb) n
820         raw_packet= m_thd->net.read_pos;
(gdb) n
829     }
(gdb) n
820         raw_packet= m_thd->net.read_pos;
(gdb) n
829     }
(gdb) n
Protocol_classic::get_command (this=0x5483a88, com_data=0x7fff819cecc0, cmd=0x7fff819cecb0) at /usr/local/src/mysql-5.7.30/sql/protocol_classic.cc:983
983       if (packet_length == 0)                       /* safety */
(gdb) n
990       raw_packet[packet_length]= '\0';                  /* safety */
(gdb) n
1002      return parse_packet(com_data, *cmd);
(gdb) n
990       raw_packet[packet_length]= '\0';                  /* safety */
(gdb) n
992       *cmd= (enum enum_server_command) raw_packet[0];
(gdb) n
994       if (*cmd >= COM_END)
(gdb) n
992       *cmd= (enum enum_server_command) raw_packet[0];
(gdb) n
1002      return parse_packet(com_data, *cmd);
(gdb) n
999       packet_length--;
(gdb) n
1000      raw_packet++;
(gdb) n
1002      return parse_packet(com_data, *cmd);
(gdb) n
1003    }
(gdb) n
do_command (thd=thd@entry=0x5482a30) at /usr/local/src/mysql-5.7.30/sql/sql_parse.cc:974
974       if (rc)
(gdb) s
971       rc= thd->get_protocol()->get_command(&com_data, &command);
(gdb) bt
#0  do_command (thd=thd@entry=0x5482a30) at /usr/local/src/mysql-5.7.30/sql/sql_parse.cc:971
#1  0x0000000000d4abf8 in handle_connection (arg=arg@entry=0x5418ff0) at /usr/local/src/mysql-5.7.30/sql/conn_handler/connection_handler_per_thread.cc:313
#2  0x000000000120a451 in pfs_spawn_thread (arg=0x5452410) at /usr/local/src/mysql-5.7.30/storage/perfschema/pfs.cc:2197
#3  0x00007ffff7bc6e65 in start_thread () from /lib64/libpthread.so.0
#4  0x00007ffff61b688d in clone () from /lib64/libc.so.6
(gdb) s
972       thd->m_server_idle= false;
(gdb) s
974       if (rc)
(gdb) s
1027          thd->variables.net_buffer_length);
(gdb) s
Protocol_classic::get_packet (this=0x5483a88) at /usr/local/src/mysql-5.7.30/sql/protocol_classic.cc:807
807     {
(gdb) bt
#0  Protocol_classic::get_packet (this=0x5483a88) at /usr/local/src/mysql-5.7.30/sql/protocol_classic.cc:807
#1  0x0000000000c8d1b3 in do_command (thd=thd@entry=0x5482a30) at /usr/local/src/mysql-5.7.30/sql/sql_parse.cc:1027
#2  0x0000000000d4abf8 in handle_connection (arg=arg@entry=0x5418ff0) at /usr/local/src/mysql-5.7.30/sql/conn_handler/connection_handler_per_thread.cc:313
#3  0x000000000120a451 in pfs_spawn_thread (arg=0x5452410) at /usr/local/src/mysql-5.7.30/storage/perfschema/pfs.cc:2197
#4  0x00007ffff7bc6e65 in start_thread () from /lib64/libpthread.so.0
#5  0x00007ffff61b688d in clone () from /lib64/libc.so.6

我们可以在调试过程中随时打印某个变量的值，例如：

代码语言：javascript
复制
(gdb) p pkt_len
$1 = 22

甚至gdb可以在紧急情况下救你一命，例如，当MySQL数据库连接打满又没有后台线程可以连接到MySQL的时候，你可以通过gdb来修改MySQL的连接数，例如

代码语言：javascript
复制
gdb -p $(pidof mysqld) -ex "set max_connections=1500" -batch

总结一下：

  • 本文内容比较多，给大家介绍了几个MySQL DBA（可能其他岗位同样适用）应该掌握的必备工具
  • strace可以用来跟踪某个线程的调用情况，例如可以适用strace跟踪客户端SQL执行情况，如果开发说有很多慢SQL，而MySQL却没有任何记录，那么就可以用strace来跟踪把锅甩给开发（案例来自有赞王航威）
  • pstack是平时用的比较多的工具，尤其是诊断MySQL hang住的情况，例如主从延迟特别高等
  • gdb可能用的不是太多，但是我认为是MySQL DBA进阶必会的技能之一，尤其是抽丝剥茧某些疑难case的时候非常有用
参考文章
  1. https://ke.qq.com/course/384101
  2. 《深入主从复制》专栏第29节
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原始发表：2020-06-01，如有侵权请联系 cloudcommunity@tencent.com 删除
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目录
  • strace简介
    • 常用选项
      • 正确姿势
        • 简单使用
        • 使用案例
        • 利用strace观察server端执行
    • pstack简介
      • 正确使用姿势
        • 全部抓取
        • 单个抓取
    • gdb简介
      • 常用命令
        • 实际案例
          • 参考文章
      • 总结一下：
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