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

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Message-Id: <1221834006.5537.3.camel@lappy.programming.kicks-ass.net>
Date:	Fri, 19 Sep 2008 16:20:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: How how latent should non-preemptive scheduling be?

On Fri, 2008-09-19 at 12:54 +0100, Sitsofe Wheeler wrote:
> Ingo Molnar wrote:
> > here's two quick howtos:
> > 
> >   http://redhat.com/~mingo/sched-devel.git/readme-tracer.txt
> >   http://redhat.com/~mingo/sched-devel.git/howto-trace-latencies.txt
> 
> These two files appear to be identical. Is this intentional?
> 
> Anyway after following your instructions a putting together a small 
> script to dice the output, I collated the 10 switches which took the 
> longest:
> 
> # Top ten longest switches
> # Rel TS Process   Abs TS
> 0.122161 hald-3423 1867.821170 ***
> 0.039438 <idle>-0  1867.379054
> 0.036318 hald-3423 1867.669009
> 0.031362 <idle>-0  1868.002762
> 0.030000 hald-3423 1867.699009
> 0.028933 <idle>-0  1867.529238
> 0.028539 <idle>-0  1867.228861
> 0.028196 <idle>-0  1867.128731
> 0.027763 <idle>-0  1868.101449
> 0.027513 <idle>-0  1867.028606
> 
> # tracer: sched_switch from around longest switch
> #
> #           TASK-PID   CPU#    TIMESTAMP  FUNCTION
> #              | |      |          |         |
>            <idle>-0     [00]  1867.608017:      0:140:R   +     3:115:S
>            <idle>-0     [00]  1867.608038:      0:140:R   +  3423:120:D
>            <idle>-0     [00]  1867.608045:      0:140:R ==>     3:115:R
>       ksoftirqd/0-3     [00]  1867.608048:      3:115:S ==>  3423:120:R
>              hald-3423  [00]  1867.629350:   3423:120:R   +  6096:120:S
>              hald-3423  [00]  1867.632691:   3423:120:R   +  3827:120:S
>              hald-3423  [00]  1867.669009:   3423:120:R   +  3998:120:S
>              hald-3423  [00]  1867.699009:   3423:120:R   +  6097:120:S
>           ***hald-3423  [00]  1867.821170:   3423:120:R ==>  6096:120:R
>         rhythmbox-6096  [00]  1867.821219:   6096:120:S ==>  6097:120:R
>         rhythmbox-6097  [00]  1867.821262:   6097:120:R   +  3826:120:S
>         rhythmbox-6097  [00]  1867.821289:   6097:120:S ==>  3826:120:R
>        pulseaudio-3826  [00]  1867.821332:   3826:120:R   +  6097:120:S
>        pulseaudio-3826  [00]  1867.821374:   3826:120:S ==>  6097:120:R
>         rhythmbox-6097  [00]  1867.821380:   6097:120:S ==>  3998:120:R
>         rhythmbox-3998  [00]  1867.821709:   3998:120:S ==>  3827:120:R
>        pulseaudio-3827  [00]  1867.824041:   3827:120:R   +  3826:120:S
> 

Its actually function tracer output I'm interested in.. that shows what
all its doing to make it take 120+ms.

I thought we had a wakeup latency tracer exacty like we have preempt and
irq off latency tracers, Steve, where'd that go?

> > 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 : (

Yeah, SMM/SMI is nasty stuff :-(

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