linux-kernel - Re: How how latent should non-preemptive scheduling be?

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Message-ID: <alpine.DEB.1.10.0809220804090.14496@gandalf.stny.rr.com>
Date:	Mon, 22 Sep 2008 08:07:05 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Sitsofe Wheeler <sitsofe@...oo.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: How how latent should non-preemptive scheduling be?


On Mon, 22 Sep 2008, Ingo Molnar wrote:
> * Sitsofe Wheeler <sitsofe@...oo.com> wrote:
> >>
> >> sounds like potential SMM triggered latencies.
> >
> > I have just gone away and read about the SMM ( 
> > http://blogs.msdn.com/carmencr/archive/2005/08/31/458609.aspx ). If 
> > you're right there is pretty much nothing that can be done about the 
> > problem : (
> 
> well, since they went away after you enabled CONFIG_PREEMPT=y, they are 
> definitely in-kernel latencies, not any external SMM latencies.
> 
> I.e. they are inherently fixable. Could you enable:
> 
>   CONFIG_DYNAMIC_FTRACE=y
>   CONFIG_FTRACE_MCOUNT_RECORD=y
> 
> that should make the traces a lot more verbose - every kernel function 
> executed in the latency path will be logged. That way we'll be able to 
> say which one takes that long.
> 
> note, you might have to increase /debug/tracing/trace_entries to get a 
> long enough trace to capture the relevant portion of the latency.

Also note, to modify trace_entries, you must be in the none (nop?) tracer, 
otherwise the size will not be effected.

If you find the trace is also too big, you can echo a list of functions 
into:

  /debug/tracing/set_ftrace_notrace

to not trace those functions. using '>' will remove any existing function 
in that file, but using '>>' will append functions to the file.

For a list of functions that you can add, see:

  /debug/tracing/available_filter_functions

-- Steve

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