Re: [RFC] Fast assurate clock readable from user space and NMI handler

[Daniel Walker <dwalker@mvista.com>]

On Mon, 2007-02-26 at 17:14 -0500, Mathieu Desnoyers wrote:


> For kernel and user space tracing, those small jumps are very annoying :
> it can show, in a trace, that a fork() appears on a CPU after the first
> schedule() of the thread on the other CPU : scheduling causality relationship
> can become very hard to follow. This is only a sample case. Inaccuracy and
> periodical modification of the clock time (non monotonic) can cause important
> inaccuracy in performance tests, even on UP systems. A monotonic clock,
> accessible from anywhere in kernel space (including NMI handler) and
> from user space is very useful for performance analysis and, more
> generally, for timestamping data in per cpu buffers so it can be later
> reordered correctly.

What about adding a layer below do_gettimeofday() which just scheds the
adjustment process? That might be reasonable .. The NMI, and userspace
cases aren't very compelling right now, at least I'm not convinced a
whole new timing interface is needed ..

The latency tracing system in the -rt branch modifies the gettimeofday
facilities , I'm not sure of the correctness of it but it gets called
from anyplace in the kernel including NMI's . 

Here's the function,

cycle_t notrace get_monotonic_cycles(void)
{
        cycle_t cycle_now, cycle_delta;

        /* read clocksource: */
        cycle_now = clocksource_read(clock);

        /* calculate the delta since the last update_wall_time: */
        cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;

        return clock->cycle_last + cycle_delta;
}

That looks safe. When converting this to nanoseconds you would still get
the time adjustments but it would be all at once instead of in little
increments ..

Daniel