Linux Device Drivers, Second Edition

Debugging by Querying

The previous section described how printk works and how it can be used. What it didn’t talk about are its disadvantages.

A massive use of printk can slow down the system noticeably, because syslogd keeps syncing its output files; thus, every line that is printed causes a disk operation. This is the right implementation from syslogd’s perspective. It tries to write everything to disk in case the system crashes right after printing the message; however, you don’t want to slow down your system just for the sake of debugging messages. This problem can be solved by prefixing the name of your log file as it appears in /etc/syslogd.conf with a minus.^[22] The problem with changing the configuration file is that the modification will likely remain there after you are done debugging, even though during normal system operation you do want messages to be flushed to disk as soon as possible. An alternative to such a permanent change is running a program other than klogd (such as cat /proc/kmsg, as suggested earlier), but this may not provide a suitable environment for normal system operation.

More often than not, the best way to get relevant information is to query the system when you need the information, instead of continually producing data. In fact, every Unix system provides many tools for obtaining system information: ps, netstat, vmstat, and so on.

Two main techniques are available to driver developers for querying the system: creating a file in the /proc ...