System management interrupt

[Matt Domsch]

On Tue, Nov 14, 2006 at 01:56:30PM +0100, Emmanuel Pacaud wrote:
> Hi,
> 
> I'm working on a realtime setup based on rtai/linux running on a dell
> precision workstation 690 dual xeon.
> 
> When measuring execution time of a real time interrupt handler (which
> means all interrupts are disabled), it looks like sometimes execution of
> interrupt handler code is interrupted for 10 ??s.
> 
> That measurement is done with a code that looks like:
> 
> 	t_start = rdtsc ();
> 	udelay(5);
> 	t_stop = rdtsc ();
> 
> where (t_stop - t_start) distribution has a peak centered at 5 ??s, and a
> smaller peak at 15 ??s.
> 
> Since all interrupts are disabled, our assumption is this issue come
> from a SMI. 

SMIs take much longer than 10uS, and don't occur during normal system
operation.  They're used for logging of events and a few other
oddities but they don't explain your finding.  (And yes, this is the
official answer from folks who would know.)

Thanks,
Matt

-- 
Matt Domsch
Software Architect
Dell Linux Solutions linux.dell.com & www.dell.com/linux
Linux on Dell mailing lists @ http://lists.us.dell.com