linux-kernel - Re: [PATCH] atmel_tc clocksource/clockevent code

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Date:	Wed, 5 Mar 2008 23:52:16 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Remy Bohmer <linux@...mer.net>
cc:	David Brownell <david-b@...bell.net>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	akpm@...ux-foundation.org, LKML <linux-kernel@...r.kernel.org>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Nicolas Ferre <nicolas.ferre@....atmel.com>,
	Andrew Victor <linux@...im.org.za>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH] atmel_tc clocksource/clockevent code

On Wed, 5 Mar 2008, Remy Bohmer wrote:
> Hello Thomas,
> 
> >  > Attached I have put a screendump of my ETM debugger. It shows a
> >  > complete flow of kernel function-calls of what happens on a timer
> >  > interrupt. In this example the complete sequence takes about 154 us.
> >  > Notice that the ETM is non-intrusive, and that the times are real and
> >  > accurate in this trace. (you can even see the effects of CPU-caches,
> >  > sometimes the same code just runs faster)
> >
> > Is there any chance to convert this to a text table? Following that
> >  png is pretty hard.
> 
> I will see what I can do, but this was the easiest way (it was just a
> print-screen;-) )
> But, will text-dump make it more clear? It could also contain the time
> each assembler instruction will take behind the routines...

text files can be parsed by scripts :)

> But, I will look into that tomorrow. (approx. 12 hours from now)

thnx
 
> >  > So, hires timestamps -> really really welcome.
> >  > hires timers -> there should be a (configurable) minimal resolution
> >  > that fits the hardware to not overload the CPU.
> >
> > clockevents let you set a minimum delta already. This can be set at
> >  runtime before registering the device.
> 
> But, I want to expose a bigger risk here.
> Apparently it is possible that a non privileged user can overload the
> system easily, by starting a high frequency periodic timer. The system
> will be that busy handling that timer that the system stops
> responding, thus it will result in some kind of Denial-of-Service
> situation, even on X86.

Wrong. The rearm condition is that the task is rescheduled. On any
platform.

Thanks,

	tglx
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