linux-kernel - Re: Trouble with full nohz and lttng

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Message-ID: <CAFTL4hxNw9EJJ3ZvjYR55b6ZF2YQxMA2q80E+obkU21v6QfEow@mail.gmail.com>
Date:	Mon, 15 Apr 2013 16:10:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mats Liljegren <mats.liljegren@...a.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Trouble with full nohz and lttng

2013/4/15 Mats Liljegren <mats.liljegren@...a.com>:
> 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.
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