On Wed, 2021-12-15 at 11:10 +0000, Mark Rutland wrote: > On Tue, Dec 14, 2021 at 08:32:29PM +0000, David Woodhouse wrote: > > On Tue, 2021-12-14 at 14:24 +0000, Mark Rutland wrote: > > > On Tue, Dec 14, 2021 at 12:32:46PM +0000, David Woodhouse wrote: > > > > From: David Woodhouse < > > > > dwmw@xxxxxxxxxxxx > > > > > > > > > > > > If the platform registers these states, bring all CPUs to each registered > > > > state in turn, before the final bringup to CPUHP_BRINGUP_CPU. This allows > > > > the architecture to parallelise the slow asynchronous tasks like sending > > > > INIT/SIPI and waiting for the AP to come to life. > > > > > > > > There is a subtlety here: even with an empty CPUHP_BP_PARALLEL_DYN step, > > > > this means that *all* CPUs are brought through the prepare states and to > > > > CPUHP_BP_PREPARE_DYN before any of them are taken to CPUHP_BRINGUP_CPU > > > > and then are allowed to run for themselves to CPUHP_ONLINE. > > > > > > > > So any combination of prepare/start calls which depend on A-B ordering > > > > for each CPU in turn, such as the X2APIC code which used to allocate a > > > > cluster mask 'just in case' and store it in a global variable in the > > > > prep stage, then potentially consume that preallocated structure from > > > > the AP and set the global pointer to NULL to be reallocated in > > > > CPUHP_X2APIC_PREPARE for the next CPU... would explode horribly. > > > > > > > > We believe that X2APIC was the only such case, for x86. But this is why > > > > it remains an architecture opt-in. For now. > > > > > > It might be worth elaborating with a non-x86 example, e.g. > > > > > > > We believe that X2APIC was the only such case, for x86. Other architectures > > > > have similar requirements with global variables used during bringup (e.g. > > > > `secondary_data` on arm/arm64), so architectures must opt-in for now. > > > > > > ... so that we have a specific example of how unconditionally enabling this for > > > all architectures would definitely break things today. > > > > I do not have such an example, and I do not know that it would > > definitely break things to turn it on for all architectures today. > > > > The x2apic one is an example of why it *might* break random > > architectures and thus why it needs to be an architecture opt-in. > > Ah; I had thought we did the `secondary_data` setup in a PREPARE step, and > hence it was a comparable example, but I was mistaken. Sorry for the noise! > Right, that's entirely within your __cpu_up(). You can stare at Thomas's patch for inspiration on how to cope with that one. In arch/arm64/kernel/smp.c you have a comment saying * as from 2.5, kernels no longer have an init_tasks structure * so we need some other way of telling a new secondary core * where to place its SVC stack In x86, the idle task pointer is in the per_cpu data. The real mode bringup now starts with the CPU's APICID (which it can get from CPUID), looks that up in the cpuid_to_apicid[] array to find the CPU#, then finds its own per_cpu data, and gets everything else it needs (including the initial stack) from there. > > > FWIW, that's something I would like to cleanup for arm64 for general > > > robustness, and if that would make it possible for us to have parallel bringup > > > in future that would be a nice bonus. > > > > Yes. But although I lay the groundwork here, the arch can't *actually* > > do parallel bringup without some arch-specific work, so auditing the > > pre-bringup states is the easy part. :) > > Sure; that was trying to be a combination of: > > * This looks nice, I'd like to use this (eventually) on arm64. > > * I'm aware of some arm64-specific groundwork we need to do before arm64 can > use this. > > So I think we're agreed. :) I'd love to have at least one more architecture come along for the ride as I do the next step. After this series, the largest chunk of time seems to be spent waiting for each AP as they transition to CPUHP_AP_ONLINE_IDLE and then all the way to CPUHP_ONLINE. So I'm going to look at making bringup_nonboot_cpus() prod *all* the APs to move to CPUHP_AP_ONLINE_IDLE without waiting for them to get there. Then do another pass waiting for that and prodding them to move to CPUHP_ONLINE. And then do a final pass of waiting for them to have got *there*. > > + int n = setup_max_cpus - num_online_cpus(); > > + > > + /* ∀ parallel pre-bringup state, bring N CPUs to it */ > > I see you have a fancy maths keyboard. ;) Nah, standard UK layout keyboard. I just happen to remember U+2200 as it's *right* at the beginning of the mathematical symbols block and is fairly easy to type ;) > It might be worth using a few more words here for clarity, e.g. > > /* > * Bring all nonboot CPUs through each pre-bringup state in turn > */ But it isn't *all* nonboot CPUs; it really is only up to N of them.
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