On Mon, Apr 15, 2024 at 1:56 PM Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote: > > On Mon, 15 Apr 2024 13:37:08 +0200 > "Rafael J. Wysocki" <rafael@xxxxxxxxxx> wrote: > > > On Mon, Apr 15, 2024 at 10:46 AM Jonathan Cameron > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > On Sat, 13 Apr 2024 01:23:48 +0200 > > > Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > > > > > > Russell! > > > > > > > > On Fri, Apr 12 2024 at 22:52, Russell King (Oracle) wrote: > > > > > On Fri, Apr 12, 2024 at 10:54:32PM +0200, Thomas Gleixner wrote: > > > > >> > As for the cpu locking, I couldn't find anything in arch_register_cpu() > > > > >> > that depends on the cpu_maps_update stuff nor needs the cpus_write_lock > > > > >> > being taken - so I've no idea why the "make_present" case takes these > > > > >> > locks. > > > > >> > > > > >> Anything which updates a CPU mask, e.g. cpu_present_mask, after early > > > > >> boot must hold the appropriate write locks. Otherwise it would be > > > > >> possible to online a CPU which just got marked present, but the > > > > >> registration has not completed yet. > > > > > > > > > > Yes. As far as I've been able to determine, arch_register_cpu() > > > > > doesn't manipulate any of the CPU masks. All it seems to be doing > > > > > is initialising the struct cpu, registering the embedded struct > > > > > device, and setting up the sysfs links to its NUMA node. > > > > > > > > > > There is nothing obvious in there which manipulates any CPU masks, and > > > > > this is rather my fundamental point when I said "I couldn't find > > > > > anything in arch_register_cpu() that depends on ...". > > > > > > > > > > If there is something, then comments in the code would be a useful aid > > > > > because it's highly non-obvious where such a manipulation is located, > > > > > and hence why the locks are necessary. > > > > > > > > acpi_processor_hotadd_init() > > > > ... > > > > acpi_map_cpu(pr->handle, pr->phys_id, pr->acpi_id, &pr->id); > > > > > > > > That ends up in fiddling with cpu_present_mask. > > > > > > > > I grant you that arch_register_cpu() is not, but it might rely on the > > > > external locking too. I could not be bothered to figure that out. > > > > > > > > >> Define "real hotplug" :) > > > > >> > > > > >> Real physical hotplug does not really exist. That's at least true for > > > > >> x86, where the physical hotplug support was chased for a while, but > > > > >> never ended up in production. > > > > >> > > > > >> Though virtualization happily jumped on it to hot add/remove CPUs > > > > >> to/from a guest. > > > > >> > > > > >> There are limitations to this and we learned it the hard way on X86. At > > > > >> the end we came up with the following restrictions: > > > > >> > > > > >> 1) All possible CPUs have to be advertised at boot time via firmware > > > > >> (ACPI/DT/whatever) independent of them being present at boot time > > > > >> or not. > > > > >> > > > > >> That guarantees proper sizing and ensures that associations > > > > >> between hardware entities and software representations and the > > > > >> resulting topology are stable for the lifetime of a system. > > > > >> > > > > >> It is really required to know the full topology of the system at > > > > >> boot time especially with hybrid CPUs where some of the cores > > > > >> have hyperthreading and the others do not. > > > > >> > > > > >> > > > > >> 2) Hot add can only mark an already registered (possible) CPU > > > > >> present. Adding non-registered CPUs after boot is not possible. > > > > >> > > > > >> The CPU must have been registered in #1 already to ensure that > > > > >> the system topology does not suddenly change in an incompatible > > > > >> way at run-time. > > > > >> > > > > >> The same restriction would apply to real physical hotplug. I don't think > > > > >> that's any different for ARM64 or any other architecture. > > > > > > > > > > This makes me wonder whether the Arm64 has been barking up the wrong > > > > > tree then, and whether the whole "present" vs "enabled" thing comes > > > > > from a misunderstanding as far as a CPU goes. > > > > > > > > > > However, there is a big difference between the two. On x86, a processor > > > > > is just a processor. On Arm64, a "processor" is a slice of the system > > > > > (includes the interrupt controller, PMUs etc) and we must enumerate > > > > > those even when the processor itself is not enabled. This is the whole > > > > > reason there's a difference between "present" and "enabled" and why > > > > > there's a difference between x86 cpu hotplug and arm64 cpu hotplug. > > > > > The processor never actually goes away in arm64, it's just prevented > > > > > from being used. > > > > > > > > It's the same on X86 at least in the physical world. > > > > > > There were public calls on this via the Linaro Open Discussions group, > > > so I can talk a little about how we ended up here. Note that (in my > > > opinion) there is zero chance of this changing - it took us well over > > > a year to get to this conclusion. So if we ever want ARM vCPU HP > > > we need to work within these constraints. > > > > > > The ARM architecture folk (the ones defining the ARM ARM, relevant ACPI > > > specs etc, not the kernel maintainers) are determined that they want > > > to retain the option to do real physical CPU hotplug in the future > > > with all the necessary work around dynamic interrupt controller > > > initialization, debug and many other messy corners. > > > > That's OK, but the difference is not in the ACPi CPU enumeration/removal code. > > > > > Thus anything defined had to be structured in a way that was 'different' > > > from that. > > > > Apparently, that's where things got confused. > > > > > I don't mind the proposed flattening of the 2 paths if the ARM kernel > > > maintainers are fine with it but it will remove the distinctions and > > > we will need to be very careful with the CPU masks - we can't handle > > > them the same as x86 does. > > > > At the ACPI code level, there is no distinction. > > > > A CPU that was not available before has just become available. The > > platform firmware has notified the kernel about it and now > > acpi_processor_add() runs. Why would it need to use different code > > paths depending on what _STA bits were clear before? > > I think we will continue to disagree on this. To my mind and from the > ACPI specification, they are two different state transitions with different > required actions. Well, please be specific: What exactly do you mean here and which parts of the spec are you talking about? > Those state transitions are an ACPI level thing not > an arch level one. However, I want a solution that moves things forwards > so I'll give pushing that entirely into the arch code a try. Thanks! Though I think that there is a disconnect between us that needs to be clarified first. > > > > Yes, there is some arch stuff to be called and that arch stuff should > > figure out what to do to make things actually work. > > > > > I'll get on with doing that, but do need input from Will / Catalin / James. > > > There are some quirks that need calling out as it's not quite a simple > > > as it appears from a high level. > > > > > > Another part of that long discussion established that there is userspace > > > (Android IIRC) in which the CPU present mask must include all CPUs > > > at boot. To change that would be userspace ABI breakage so we can't > > > do that. Hence the dance around adding yet another mask to allow the > > > OS to understand which CPUs are 'present' but not possible to online. > > > > > > Flattening the two paths removes any distinction between calls that > > > are for real hotplug and those that are for this online capable path. > > > > Which calls exactly do you mean? > > At the moment he distinction does not exist (because x86 only supports > fake physical CPU HP and arm64 only vCPU HP / online capable), but if > the architecture is defined for arm64 physical hotplug in the future > we would need to do interrupt controller bring up + a lot of other stuff. > > It may be possible to do that in the arch code - will be hard to verify > that until that arch is defined Today all I need to do is ensure that > any attempt to do present bit setting for ARM64 returns an error. > That looks to be straight forward. OK > > > > > > As a side note, the indicating bit for these flows is defined in ACPI > > > for x86 from ACPI 6.3 as a flag in Processor Local APIC > > > (the ARM64 definition is a cut and paste of that text). So someone > > > is interested in this distinction on x86. I can't say who but if > > > you have a mantis account you can easily follow the history and it > > > might be instructive to not everyone considering the current x86 > > > flow the right way to do it. > > > > So a physically absent processor is different from a physically > > present processor that has not been disabled. No doubt about this. > > > > That said, I'm still unsure why these two cases require two different > > code paths in acpi_processor_add(). > > It might be possible to push the checking down into arch_register_cpu() > and have that for now reject any attempt to do physical CPU HP on arm64. > It is that gate that is vital to getting this accepted by ARM. > > I'm still very much stuck on the hotadd_init flag however, so any suggestions > on that would be very welcome! I need to do some investigation which will take some time I suppose.
