From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4B28CA44.2080304@domain.hid> References: <4B240D0B.50603@domain.hid> <1260723757.2216.2.camel@domain.hid> <4B252909.9090304@domain.hid> <1260727502.2216.19.camel@domain.hid> <4B253028.10607@domain.hid> <1260962769.2216.203.camel@domain.hid> <4B28CA44.2080304@domain.hid> Content-Type: text/plain; charset="UTF-8" Date: Wed, 16 Dec 2009 15:19:35 +0100 Message-ID: <1260973175.2216.371.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Adeos-main] [pull request] x86: critical fix for native_safe_halt List-Id: General discussion about Adeos List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: adeos-main 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". I just don't get why you seem to bind the Linux rescheduling logic with what a low prio domain would do; by definition, adeos-wise, both are totally non-related. > > > > >>>> 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 > -- Philippe.