Re: [PATCH v2] RISC-V: Increase range and default value of NR_CPUS

[Anup Patel]

On Wed, Apr 6, 2022 at 3:25 PM Heinrich Schuchardt
<heinrich.schuchardt@xxxxxxxxxxxxx> wrote:
>
> On 3/31/22 21:42, Palmer Dabbelt wrote:
> > On Sat, 19 Mar 2022 05:12:06 PDT (-0700), apatel@xxxxxxxxxxxxxxxx wrote:
> >> Currently, the range and default value of NR_CPUS is too restrictive
> >> for high-end RISC-V systems with large number of HARTs. The latest
> >> QEMU virt machine supports upto 512 CPUs so the current NR_CPUS is
> >> restrictive for QEMU as well. Other major architectures (such as
> >> ARM64, x86_64, MIPS, etc) have a much higher range and default
> >> value of NR_CPUS.
> >>
> >> This patch increases NR_CPUS range to 2-512 and default value to
> >> XLEN (i.e. 32 for RV32 and 64 for RV64).
> >>
> >> Signed-off-by: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> >> ---
> >> Changes since v1:
> >>  - Updated NR_CPUS range to 2-512 which reflects maximum number of
> >>    CPUs supported by QEMU virt machine.
> >> ---
> >>  arch/riscv/Kconfig | 7 ++++---
> >>  1 file changed, 4 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> >> index 5adcbd9b5e88..423ac17f598c 100644
> >> --- a/arch/riscv/Kconfig
> >> +++ b/arch/riscv/Kconfig
> >> @@ -274,10 +274,11 @@ config SMP
> >>        If you don't know what to do here, say N.
> >>
> >>  config NR_CPUS
> >> -    int "Maximum number of CPUs (2-32)"
> >> -    range 2 32
> >> +    int "Maximum number of CPUs (2-512)"
> >> +    range 2 512
>
> For SBI_V01=y there seems to be a hard constraint to XLEN bits.
> See __sbi_v01_cpumask_to_hartmask() in rch/riscv/kernel/sbi.c.
>
> So shouldn't this be something like:
>
> range 2 512 !SBI_V01
> range 2 32 SBI_V01 && 32BIT
> range 2 64 SBI_V01 && 64BIT

This is just making it unnecessarily complicated for supporting
SBI v0.1

How about removing SBI v0.1 support and the spin-wait CPU
operations from arch/riscv ?

>
> >>      depends on SMP
> >> -    default "8"
> >> +    default "32" if 32BIT
> >> +    default "64" if 64BIT
> >>
> >>  config HOTPLUG_CPU
> >>      bool "Support for hot-pluggable CPUs"
> >
> > I'm getting all sorts of boot issues with more than 32 CPUs, even on the
> > latest QEMU master.  I'm not opposed to increasing the CPU count in
> > theory, but if we're going to have a setting that goes up to a huge
> > number it needs to at least boot.  I've got 64 host threads, so it
> > shouldn't just be a scheduling thing.
>
> Currently high performing hardware for RISC-V is missing. So it makes
> sense to build software via QEMU on x86_64 or arm64 with as many
> hardware threads as available (128 is not uncommon).
>
> OpenSBI currently is limited to 128 threads:
> include/sbi/sbi_hartmask.h:22:
> #define SBI_HARTMASK_MAX_BITS 128
> This is just an arbitrary value we can be modified.

Yes, this limit will be gradually increased with some improvements
to optimize runtime memory used by OpenSBI.

>
> U-Boot v2022.04 qemu-riscv64_smode_defconfig has a value of
> CONFIG_SYS_MALLOC_F_LEN that is to low. This leads to a boot failure for
> more than 16 harts. A patch to correct this is pending:
> [PATCH v2 1/1] riscv: alloc space exhausted
> https://lore.kernel.org/u-boot/CAN5B=eKt=tFLZ2z3aNHJqsnJzpdA0oikcrC2i1_=ZDD=f+M0jA@xxxxxxxxxxxxxx/T/#t
>
> With QEMU 7.0 and the U-Boot fix booting into a 5.17 defconfig kernel
> with 64 virtual cores worked fine for me.

Thanks for trying this patch.

Regards,
Anup

>
> Best regards
>
> Heinrich
>
> >
> > If there was some hardware that actually boots on these I'd be happy to
> > take it, but given that it's just QEMU I'd prefer to sort out the bugs
> > first.  It's probably just latent bugs somewhere, but allowing users to
> > turn on configs we know don't work just seems like the wrong way to go.
> >
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> >
>