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. > >>>> Moreover, what would be the practical use for such model in the context >>>> of Linux? >>> That is _not_ the point. The point is, when submitting a patch, please >>> make sure to raise all the concerns it might introduce wrt to changing >>> the base features. I'm not opposed to make the feature set evolve, but I >>> don't want this to happen "by mistake". >> Just pushed >> >> "x86: Drop redundant ipipe_suspend_domain from cpu_idle >> >> Allowing domains below root always required more than these calls (Linux >> would have to give up idle management). And syncing the root domain now >> takes place in __ipipe_halt_root. So remove these suspension calls." >> >> as commit message for the second patch. Is that what you are looking for? >> > > Not exactly, because your comment states that what was removed was > intrinsically useless, it was not, and has been used, even if only in a > couple of occasions, mainly to enable a debugging hack. This is why I > choked on removing a feature silently. But at the same time, we are in a > simplification trend of the I-pipe toward X3, so I agree on the final > goal you are pursuing with that patch. > > In short, let's move on, I'll merge that as well, now that everything is > on the table, and there is no objection anyway. I couldn't agree more. :) Jan
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