Re: [Adeos-main] [pull request] x86: critical fix for native_safe_halt

[Philippe Gerum <rpm@xenomai.org>]

On Wed, 2009-12-16 at 15:39 +0100, Jan Kiszka wrote:
> Philippe Gerum wrote:
> > On Wed, 2009-12-16 at 15:19 +0100, Philippe Gerum wrote:
> >> On Wed, 2009-12-16 at 12:53 +0100, Jan Kiszka wrote:
> >>> Philippe Gerum wrote:
> >>>> On Sun, 2009-12-13 at 19:19 +0100, Jan Kiszka wrote:
> >>>>> Philippe Gerum wrote:
> >>>>>> On Sun, 2009-12-13 at 18:48 +0100, Jan Kiszka wrote:
> >>>>>>> Philippe Gerum wrote:
> >>>>>>>> On Sat, 2009-12-12 at 22:37 +0100, Jan Kiszka wrote:
> >>>>>>>>> Recent moving of ipipe_suspend_domain finally exposed a deeper flaw in
> >>>>>>>>> cpu_idle on x86: We failed to check the pipeline log before issuing the
> >>>>>>>>> real hlt. This caused IRQ latencies or even drops for Linux,
> >>>>>>>>> specifically on SMP. Credits go to plain QEMU whose slow SMP mode caused
> >>>>>>>>> ipipe_critical_enter to deadlock frequently enough.
> >>>>>>>>>
> >>>>>>>>> The first patch of this series fixes this (see below), the second one
> >>>>>>>>> simply removes the two useless ipipe_suspend_domain calls.
> >>>>>>>>>
> >>>>>>>> What your patch does as well, is killing the ability to run low priority
> >>>>>>>> domains below the root level.
> >>>>>>> Yes, I'm killing the dream.
> >>>>>>>
> >>>>>>> I heavily doubt that the functions I removed in the second patch ever
> >>>>>>> contributed something good to this. It's always the job of the lowest
> >>>>>>> domain to issue hardware halt, not of some arbitrary mid-prio domain.
> >>>> Actually, no it's not. You may use a low-priority domain to run idle
> >>>> level jobs outside of the linux infrastructure for that purpose (e.g.
> >>>> RCU). A high priority domain may want to post events for a low priority
> >>>> domain to act upon when a mid priority domain is about to enter the CPU
> >>>> idle state.
> >>> Even if all the related bugs were fixed: When you pass down control to
> >>> the lower domain on cpu_idle via ipipe_suspend_domain, you won't get a
> >>> Linux reschedule (without CONFIG_PREEMPT) until the low-prio domain
> >>> finally returns from its event handler - likely not what "low-prio"
> >>> suggests.
> >> low prio suggests nothing else than "runs whenever nothing else has to
> >> be done higher in the pipeline".
> > 
> > Nothing, meaning nothing interrupt-wise. For the rest, it's a deal
> > between all OSes there.
> > 
> 
> Right, and that deal was, well, fairly incomplete in the previous code.
> That's my whole point.

Nope. The missing code would have been provided by the low priority
domain, without requiring anything to be changed on the Linux side, if
that use was only about running (pre-)idle context code.

Which does not removes anything to the usefulness of actually fixing CPU
halt code this said, but that is unrelated. 

> 
> Jan
> 
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-- 
Philippe.