Re: Using patch-2.6.33.7.2-rt30 increases latency and CPU usage?

[Grant Edwards <grant.b.edwards@gmail.com>]

On Thu, May 10, 2012 at 11:46:26AM +0200, Remy Bohmer wrote:
> 2012/5/10 Grant Edwards <grant.b.edwards@gmail.com>:
>
>> Unfortunately, the requirements are a bit fuzzy -- I've got an ISR
>> deadline of about 20us that I'm trying to meet [I wouldn't mind a
>> little chat with the person who designed _that_ requirement into the
>> hardware].  What I don't know is how hard that deadline is.  With the
>> RT patch (and without IRQF_NODELAY), I miss the deadline most of the
>> time (I'd guess about 80% of the time).
> 
> Given my experience with these cores and the RT patch: This 20usec is
> too close.

That's pretty much what I've decided.

> There are some places in the code that have interrupt disable section
> of about 30usec (also process ctx-switch cache flush have high
> impact)
>
> So, I expect you will not get this very robust.
>
> If the 20usec requirement is hard, you can also look at using an FIQ.

Does the normal AT91 kernel not use FIQs at all?  If so, I might be
able to dedicate the FIQ to this function and have it happen without
the Linux kernel knowing about it (or affecting it).  Any
communication between it and a normal user-task would have to be
handled carefully...

-- 
Grant
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