Table of Contents
Naturally, how the overflow interrupts are received is specific to the hardware architecture, unless we are in "timer" mode, where the logging routine is called directly from the standard kernel timer interrupt handler.
On the i386 architecture, the local APIC is programmed such that when a
counter overflows (that is, it receives an event that causes an integer
overflow of the register value to zero), an NMI is generated. This calls
into the general handler do_nmi(); because OProfile
has registered itself as capable of handling NMI interrupts, this will
call into the OProfile driver code in
arch/i386/oprofile. Here, the saved PC value (the
CPU saves the register set at the time of interrupt on the stack
available for inspection) is extracted, and the counters are examined to
find out which one generated the interrupt. Also determined is whether
the system was inside kernel or user space at the time of the interrupt.
These three pieces of information are then forwarded onto the OProfile
core via oprofile_add_sample(). Finally, the
counter values are reset to the chosen count value, to ensure another
interrupt happens after another N events have occurred. Other
architectures behave in a similar manner.