Re: Audio I/O parameters

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On Tue, 30 Jul 2013, Steven Rostedt wrote:

> On Tue, 2013-07-30 at 11:42 -0400, Alan Stern wrote:
> > On Mon, 29 Jul 2013, James Stone wrote:
> 
> Just an FYI, it is usually better to email my rostedt@xxxxxxxxxxx
> account. I don't always read my redhat email. That's reserved for
> bugzillas assigned to me ;-)

I got the address from the top of Documentation/trace/ftrace.txt (which
I was reading at the time to make sure I understood the trace output
correctly).  Should that file be updated?

> > > # tracer: irqsoff
> > > #
> > > # irqsoff latency trace v1.1.5 on 3.10.0-ver5
> > > # --------------------------------------------------------------------
> > > # latency: 2173 us, #4/4, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
> > > #    -----------------
> > > #    | task: apt-check-3628 (uid:1000 nice:19 policy:0 rt_prio:0)
> > > #    -----------------
> > > #  => started at: perf_event_update_userpage
> > > #  => ended at:   retint_careful
> > > #
> > > #
> > > #                  _------=> CPU#
> > > #                 / _-----=> irqs-off
> > > #                | / _----=> need-resched
> > > #                || / _---=> hardirq/softirq
> > > #                ||| / _--=> preempt-depth
> > > #                |||| /     delay
> > > #  cmd     pid   ||||| time  |   caller
> > > #     \   /      |||||  \    |   /
> > > apt-chec-3628    0d.h.    0us!: local_clock <-perf_event_update_userpage
> 
> perf_event_update_userpage should not be taking 2ms to complete.

How can that be chased down?  And even so, 2 ms is a lot smaller than 7 
ms.

> > This suggests that the IRQ line was disabled or interrupts were
> > disabled on all four CPUs for 7 ms.  However, the irqsoff trace shows
> > that the maximum latency was 2 ms.  Something strange is going on, but
> > I don't know what or how to find it.
> > 
> > Steve, do you have any ideas about this?  Or suggestions for other 
> > people to ask?
> 
> Is there SMIs on this box? Those will not be detected by the latency
> tracers directly. But we do have other tools to detect them.

What are they, and how can James try them?

Alan Stern

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