On 09/27/2012 04:16 AM, Suresh Siddha wrote: > On Wed, 2012-09-26 at 23:00 +0530, Srivatsa S. Bhat wrote: >> On 09/26/2012 10:36 PM, Suresh Siddha wrote: >>> On Wed, 2012-09-26 at 21:33 +0530, Srivatsa S. Bhat wrote: >>>> I have some fundamental questions here: >>>> 1. Why was the CPU never removed from the affinity masks in the original >>>> code? I find it hard to believe that it was just an oversight, because the >>>> whole point of fixup_irqs() is to affine the interrupts to other CPUs, >>>> IIUC. >>>> So, is that really a bug or is the existing code correct for some reason >>>> which I don't know of? >>> >>> I am not aware of the history but my guess is that the affinity mask >>> which is coming from the user-space wants to be preserved. And >>> fixup_irqs() is fixing the underlying interrupt routing when the cpu >>> goes down >> >> and the code that corresponds to that is: >> irq_force_complete_move(irq); is it? > > No. irq_set_affinity() >
Um? That takes the updated/changed affinity and sets data->affinity to that value no? You mentioned that probably the intention of the original code was to preserve the user-set affinity mask, but still change the underlying interrupt routing. Sorry, but I still didn't quite understand what is that part of the code that achieves that. It appears that ->irq_set_affinity() is doing both - change the (user-set) affinity as well as the underlying irq routing. And it does it only when all CPUs in the original affinity mask go offline, not on every offline; which looks like a real bug to me. >>> with a hope that things will be corrected when the cpu comes >>> back online. But as Liu noted, we are not correcting the underlying >>> routing when the cpu comes back online. I think we should fix that >>> rather than modifying the user-specified affinity. >>> I am not able to distinguish the 2 things in the existing code, as I mentioned above :( >> >> Hmm, I didn't entirely get your suggestion. Are you saying that we should >> change >> data->affinity (by calling ->irq_set_affinity()) during offline but maintain >> a >> copy of the original affinity mask somewhere, so that we can try to match it >> when possible (ie., when CPU comes back online)? > > Don't change the data->affinity in the fixup_irqs() You mean, IOW, don't call ->irq_set_affinity() during CPU offline at all? (Would that even be correct?) > and shortly after a > cpu is online, call irq_chip's irq_set_affinity() for those irq's who > affinity included this cpu (now that the cpu is back online, > irq_set_affinity() will setup the HW routing tables correctly). > > This presumes that across the suspend/resume, cpu offline/online > operations, we don't want to break the irq affinity setup by the > user-level entity like irqbalance etc... > The only way I can think of to preserve the user-set affinity but still alter the underlying routing appropriately when needed, is by having an additional mask (per-irq) that holds the user-set affinity. >>> That happens only if the irq chip doesn't have the irq_set_affinity() setup. >> >> That is my other point of concern : setting irq affinity can fail even if >> we have ->irq_set_affinity(). (If __ioapic_set_affinity() fails, for >> example). >> Why don't we complain in that case? I think we should... and if its serious >> enough, abort the hotplug operation or atleast indicate that offline failed.. > > yes if there is a failure then we are in trouble, as the cpu is already > disappeared from the online-masks etc. For platforms with > interrupt-remapping, interrupts can be migrated from the process context > and as such this all can be done much before. > > And for legacy platforms we have done quite a few changes in the recent > past like using eoi_ioapic_irq() for level triggered interrupts etc, > that makes it as safe as it can be. Perhaps we can move most of the > fixup_irqs() code much ahead and the lost section of the current > fixup_irqs() (which check IRR bits and use the retrigger function to > trigger the interrupt on another cpu) can still be done late just like > now. > Hmm.. ok.. Thanks for the explanation! Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
