linux-kernel - Re: One-shot high-resolution POSIX timer periodically late

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Message-ID: <20070124200947.GA925@elte.hu>
Date:	Wed, 24 Jan 2007 21:09:47 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	John <me@...vacy.net>, linux-kernel@...r.kernel.org,
	tglx@...esys.com, akpm@...l.org, shill@...e.fr
Subject: Re: One-shot high-resolution POSIX timer periodically late


* john stultz <johnstul@...ibm.com> wrote:

> On Wed, 2007-01-24 at 10:41 +0100, John wrote:
> > I'm using the POSIX timers API. My platform is x86 running Linux
> > 2.6.18.6 patched with the high-resolution timer subsystem.
> > 
> > http://www.tglx.de/hrtimers.html

> > My process is the only SCHED_FIFO process on the system. There are 
> > no user-space processes with a higher priority. AFAICT, only a 
> > kernel thread could keep the CPU away from my app.
> > 
> > Is there a periodic kernel thread that runs every 2 seconds, cannot 
> > be preempted, and runs for over 50 µs??
> 
> This sounds like a BIOS SMI issue. Can you reproduce this behavior on 
> different hardware?

note that only the -hrt patchset is used - not the full -rt patchset - 
so 50 usecs delays (and more) are quite possible and common.

My question would be: does the same problem occur with the full -rt 
patchset and PREEMPT_RT? (see http://rt.wiki.kernel.org for details)

	Ingo
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