From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Saturday, February 17, 2024 11:17 PM > > > > > > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig > > > index 0024210..bc3f496 100644 > > > --- a/drivers/hv/Kconfig > > > +++ b/drivers/hv/Kconfig > > > @@ -9,6 +9,7 @@ config HYPERV > > > select PARAVIRT > > > select X86_HV_CALLBACK_VECTOR if X86 > > > select OF_EARLY_FLATTREE if OF > > > + select CPUMASK_OFFSTACK > > > help > > > Select this option to run Linux as a Hyper-V client operating > > > system. > > > -- > > > 1.8.3.1 > > > > > > > I'm not sure that enabling CPUMASK_OFFSTACK for Hyper-V > > guests is the right thing to do, as there's additional runtime > > cost when CPUMASK_OFFSTACK is enabled. I agree that for > > the most general case, you want NR_CPUS to be 2048, which > > requires CPUMASK_OFFSTACK. But it would be legitimate to > > build a kernel with NR_CPUS set to something like 64 or 256 > > for a more limited Hyper-V guest use case, and to not want to > > incur the cost of CPUMASK_OFFSTACK. > > > > You could consider doing something like this: > > > > select CPUMASK_OFFSTACK if NR_CPUS > 512 > > Thanks for your review. > > This was my first thought as well, but for x86, NR_CPUS itself depends > on CPUMASK_OFFSTACK and this creates some kind of circular dependency > and doesn't work effectively. > > Here are few key points to note: > > 1. In ARM64 as well for enabling CPUMASK_OFFSTACK we need to enable > DEBUG_PER_CPU_MAPS and that will have additional overhead. > This dependency is for all the archs. There was an earlier attempt > to decouple it: https://lore.kernel.org/lkml/20220412231508.32629-1-libo.chen@xxxxxxxxxx/ > > 2. However, for ARM64, NR_CPUS doesn't have dependency on CPUMASK_OFFSTACK. > In ARM64 NR_CPUS is quite independent from any policy, we can choose any > value for NR_CPUS freely, things are simple. This problem specificaly > to be solved for x86. > > 3. If we have to select more then 512 CPUs on x86, CPUMASK_OFFSTACK > needto be enabled, so this additional runtime cost is unavoidable > for NR_CPUS > 512. There is no way today to enable CPUMASK_OFFSTACK > apart from enabling MAXSMP or DEBUG_PER_CPU_MAPS. Both of these > options we don't want to use. > > I agree that we possibly don't want to enable this option for HyperV VMs > where NR_CPUS < 512. I have two thoughts here: > > 1. Enable it only for VTL platforms, as current requirement for minimal kernel > is only for VTL platforms only. > > 2. Fix this for all of x86. I couldn't find any reson why CPUMASK_OFFSTACK > dependency is there on x86 for having more than 512 CPUs. What is special > in x86 to have this restriction ? If there is no reason we should relax > the restriction of CPUMASK_OFFSTACK for NR_CPUs similar to ARM and other > archs. > You've done some deeper research than I did. :-( What a mess. ARM64 seems to have it right. On x86, the dependency between NR_CPUS and CPUMASK_OFFSTACK seems to flow the wrong direction. I would think you would select NR_CPUS first, and then if the number is large, select CPUMASK_OFFSTACK. And the display of CPUMASK_OFFSTACK in config tools should not be dependent on DEBUG_PER_CPU_MAPS. It should be easy to independently select CPUMASK_OFFSTACK (modulo architectures that don't support it). In the Libo Chen thread, I don't understand the reluctance to make CPUMASK_OFFSTACK independent of DEBUG_PER_CPU_MAPS. I don't have any great suggestions for the path forward. :-( Maybe revive the Libo Chen thread, with a better justification for removing the dependency between CPUMASK_OFFSTACK and DEBUG_PER_CPU_MAPS? Or at least clarify why the dependency should be kept? Michael
