Question

I used OProfile to profiling my Linux box. During the profiling processes, I’ve found that besides “native_safe_halt” function, the “delay_tsc” is the second most significant function consuming cpu cycles (around 10%). It seems delay_tsc() is a busy loop. But who calls it and what is its function?

Answer 1

Nobody calls it directly since it’s a local function inside that piece of source you link to. The way to call it is by the published __delay() function.

When you call __delay(), this will use the delay_fn function pointer (also local to that file) to select one of several delay functions. By default, the one selected is delay_loop(), which uses x86 instructions to try and mark time.

However, if use_tsc_delay() has been called (at boot time), it switches the function pointer to delay_tsc(), which uses the time stamp counter (a CPU counter) to mark time.

It’s called by any kernel code that wants a reasonably reliable, high-resolution delay function. You can see all the code in the kernel that references __delay here (quite a few places).

I think it’s probably pretty safe, in terms of profiling, to ignore the time spent in that function since its intent is to delay. In other words, it’s not useful work that’s taking a long time to perform – if callers didn’t want to delay, they wouldn’t call it.

Some examples from that list:

• A watchdog timer uses it to pace the cores so that their output is not mixed up with each other, by delaying for some multiple of the current core ID.
• The ATI frame buffer driver appears to use it for delays between low-level accesses to the hardware. In fact, it’s used quite a bit for that purpose in many device drivers.
• It’s used during start-up to figure out the relationship between jiffies and the actual hardware speeds.