From mboxrd@z Thu Jan  1 00:00:00 1970
From: Philippe Gerum <rpm@xenomai.org>
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Date: Fri, 01 May 2009 11:40:20 +0200
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Subject: Re: [Xenomai-help] rtdm_event_timedwait returns -EINTR
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To: Thomas Lockhart <Thomas.Lockhart@domain.hid>
Cc: Petr Cervenka <grugh@domain.hid>, xenomai-help <xenomai@xenomai.org>, "Lockhart, Thomas G" <thomas.g.lockhart@domain.hid>

On Thu, 2009-04-30 at 15:40 -0700, Thomas Lockhart wrote:
> > Xenomai always tells the kernel that blocking (Xenomai) syscalls
> > _should_ be restarted when interrupted by a Linux signal (i.e.
> > -ERESTARTSYS is passed down to the kernel by the Xenomai core in that
> > case), and the kernel will actually restart that (Xenomai) syscall if no
> > handler was installed for such signal, or if SA_RESTART is set in the
> > sigaction() flags for the signal.
> 
> Thanks for the clarification. I was hoping my reply would expose my lack 
> of understanding :)
> 
> So I'm getting what I think are great results on my system with my 
> Xenomai-enabled software (a few usec jitter, ~10usec latency or offset 
> without anything special done when building or calibrating Xenomai. The 
> system is a fairly modern dual-processor Xeon desktop machine and 
> Xenomai is 2.4.7; kernel is 2.6.26 (needed for a third party device 
> driver which breaks with 2.6.27).
> 

Mmm, actually, I would bet that this latency is caused by an inter-CPU
TLB flush request; this is a bug fixed in recent releases of the I-pipe
patch, both for 2.6.28.9 and 2.6.29.1. There is no backport for 2.6.26,
but you should be able to apply this commit manually:

http://git.denx.de/?p=ipipe-2.6.git;a=commit;h=69f7e0c005eca37a04104dd85f30095791e8c2f7

> When running at a very low rate (10Hz) every 30 seconds or so there is a 
> latency spike of around 50usec. Not bad (and acceptable for my system 
> even at kHz rates), but it certainly stands out from the usual case.
> 
> Could this occasional extra latency be due to this signal interrupt and 
> transparent restart? Or should I dig around elsewhere?
> 
> TIA
> 
>                            - Tom
-- 
Philippe.