Re: getnstimeofday stuck for several milliseconds?

[John Stultz]

On 11/12/2012 03:53 PM, John Stultz wrote:
> On 11/05/2012 12:51 AM, David Henningsson wrote:
> > Hi LKML,
> >
> > I'm trying to make audio more useful in everyday low-latency 
> > scenarios such as gaming or VOIP.
> >
> > While doing so, I ran the wakeup_rt tracer, to track the time from 
> > PulseAudio requesting wakeup (through hrtimers), to the thread 
> > actually running.
> >
> > I'm not sure how much overhead added by the wakeup_rt tracer itself, 
> > but I got 9 ms on one machine and 20 ms on another, which I consider 
> > to be quite a lot even for a standard kernel (i e without RT or other 
> > special configuration).
> >
> > The 9 ms example is pastebinned at [1], and here's where we get stuck 
> > for most of the time:
> >
> >   <idle>-0       3d... 1105us : ktime_get_real <-intel_idle
> >   <idle>-0       3d... 1106us!: getnstimeofday <-ktime_get_real
> >   <idle>-0       3d... 7823us : ktime_get_real <-intel_idle
> >
> >   <idle>-0       3d... 7890us : ktime_get_real <-intel_idle
> >   <idle>-0       3d... 7891us!: getnstimeofday <-ktime_get_real
> >   <idle>-0       3d... 9023us : ktime_get_real <-intel_idle
>
> Looking at the trace you posted here: http://pastebin.se/6iMRdDfR
>
> The trace also looks like its the cpuidle to interrupt transition 
> where you're seeing this.  I sort of wonder if its mis-attributing the 
> idle time to the getnstimeofday()? Mainly because you don't seem to 
> spend much time in intel_idle() otherwise.
>
> Or maybe we're both misreading it and its saying there's a delay 
> between the first ktime_get_real() from intel_idle() to the second 
> call of ktime_get_real(), between which we're in deep idle (which 
> would make sense)?
The more I think about it, I'm pretty sure this is the case:
The full context you need is:
    <idle>-0       3d... 7890us : ktime_get_real <-intel_idle
    <idle>-0       3d... 7891us!: getnstimeofday <-ktime_get_real
    <idle>-0       3d... 9023us : ktime_get_real <-intel_idle
    <idle>-0       3d... 9024us : getnstimeofday <-ktime_get_real

Where intel_idle() is calling ktime_get_real twice in a row, and 
inbetween we see a large latency. Looking at intel_idle() the code in 
question is:

    kt_before = ktime_get_real();

    stop_critical_timings();
    if (!need_resched()) {

        __monitor((void *)&current_thread_info()->flags, 0, 0);
        smp_mb();
        if (!need_resched())
            __mwait(eax, ecx);
    }

    start_critical_timings();

    kt_after = ktime_get_real();


Where we're basically timing how long we were in idle for.

So I think the problem is just misreading the trace output.

thanks
-john




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