Re: Trouble with full nohz and lttng

From: Frederic Weisbecker
Date: Mon Apr 15 2013 - 10:10:26 EST


2013/4/15 Mats Liljegren <mats.liljegren@xxxxxxxx>:
> When I use lttng, I keep getting ticks. Without lttng, no ticks. It's the
> same application I run, the same way. The only difference is whether lttng
> logging has been started or not.
>
> The trace says "sched" as reason for not disabled ticks. Using "top" I can't
> see any extra process/thread being created while my application runs, my
> application gets 99-101% of the CPU. There are a couple of kworker threads
> (3), a migration thread and a ksoftirq thread on the same CPU as my
> application, though "top" gives no indication that they do any execution.
>
> Is this a known limitation of lttng, or is there something else going on
> here?

I have no idea. But the "sched" reason means there is more than one
task in the runqueue at some point. It can be a kthread indeed. Or a
user task that gets migrated / woken up in the CPU.

You'll need to check the sched:sched_switch tracepoint.
--
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/