Understanding the Linux Kernel, 3rd Edition

Updating System Statistics

The kernel, among the other time-related duties, must periodically collect various data used for:

• Checking the CPU resource limit of the running processes

• Updating statistics about the local CPU workload

• Computing the average system load

• Profiling the kernel code

Updating Local CPU Statistics

We have mentioned that the update_process_times( ) function is invoked—either by the global timer interrupt handler on uniprocessor systems or by the local timer interrupt handler in multiprocessor systems—to update some kernel statistics. This function performs the following steps:

1. Checks how long the current process has been running. Depending on whether the current process was running in User Mode or in Kernel Mode when the timer interrupt occurred, invokes either account_user_time( ) or account_system_time( ). Each of these functions performs essentially the following steps:

   1. Updates either the utime field (ticks spent in User Mode) or the stime field (ticks spent in Kernel Mode) of the current process descriptor. Two additional fields called cutime and cstime are provided in the process descriptor to count the number of CPU ticks spent by the process children in User Mode and Kernel Mode, respectively. For reasons of efficiency, these fields are not updated by update_process_times( ), but rather when the parent process queries the state of one of its children (see the section "Destroying Processes" in Chapter 3).

   2. Checks whether the total CPU time limit has been reached; ...