offcputime no output

1. environment
os:
```
root@leixia:/usr/share/bcc/tools# uname -a
Linux leixia 4.13.0-38-generic #43-Ubuntu SMP Wed Mar 14 15:20:44 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
root@leixia:/usr/share/bcc/tools#
```
1. Mysql is correct
```
root@leixia:/usr/share/bcc/tools# ps aux|grep mysql
root 6616 0.0 0.0 4592 1688 pts/4 S 13:42 0:00 /bin/sh ./bin/mysqld_safe --defaults-file=/usr/local/mysql/my.cnf
mysql 6789 0.0 2.2 1387340 183492 pts/4 Sl 13:42 0:04 /usr/local/mysql/bin/mysqld --defaults-file=/usr/local/mysql/my.cnf --basedir=/usr/local/mysql --datadir=/usr/local/mysql/data --plugin-dir=/usr/local/mysql/lib/plugin --user=mysql --log-error=leixia.err --pid-file=leixia.pid --port=3306
```

1. Execute the command
```
./offcputime -df -p `pgrep -nx mysqld` 30 > out.stacks
```
but out.stacks is empty

Reproduce ./offcputime -df -p with the reported mysqld PID and 30-second duration on the stated Linux environment. Start by inspecting the offcputime tool's handling of PID filtering and stack output, then verify that out.stacks contains the expected off-CPU stacks or document why no output is produced.

Written by the indexing model from the issue text.