getnstimeofday stuck for several milliseconds?

From: David Henningsson
Date: Mon Nov 05 2012 - 03:50:59 EST


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

It seems to me that sometimes we get stuck for several milliseconds inside the getnstimeofday function - this was seen on both the 9 ms and the 20 ms trace. This looks like a bug to me, and as I'm not sure on how to best debug it further, and therefore I'm asking for help (or a bug fix!) here.

For reference, the 9 ms trace was from a ~2 year old laptop (core i3 cpu) running 3.7rc2 vanilla/mainline kernel, and the 20 ms trace was from an ~1 year old Atom-based machine running the 3.2-ubuntu kernel. While tracing was enabled, I was running a libSDL game for a minute or two.

Thanks in advance for looking into this, and let me know if you need further information, or anything else I can do to help sorting this one out.

--
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic

[1] http://pastebin.se/6iMRdDfR
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/