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 *)¤t_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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