Re: Deterministic behavior for TTY serial

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, May 03, 2012 at 05:28:47PM +0200, Ivo Sieben wrote:
> Hi,
> 
> >
> > The PREEMPT_RT uses mutexes for "normal" spin locks that do not
> > disable interrupts...
> > I'll try to use raw spinlocks in this code section and for the tty flip buffer
> > See if that can solve my problem.
> >
> > If you have other ideas... let me know!
> >
> > Regards,
> > Ivo
> 
> I've changed some small things to the tty layer (see my other 3 RFC
> patches I've send).
> Performance increased with my loopback stress test:
> - Old situation: average read call last for 50us, with peaks up to 230 us
> - New situation: average read call still 50us, peak up to 60 us
> - Write was stable in both situations: average of 90 us, peak up to 100 us
> 
> Only the very first read & write took extra time (128 us for read, 143
> for write)
> I'm still investigating that...
> 
> Feedback is very appreciated.

Why are raw spinlocks "faster" here?  I like the end-result of what you
have accomplished, but I had some questions on your patches, care to
answer them?

thanks,

greg k-h