Re: [RFC PATCH 1/5] RISC-V: Add logical CPU indexing for RISC-V

[Anup Patel <anup@brainfault.org>]

On Thu, Aug 16, 2018 at 5:26 AM, Atish Patra <atish.patra@wdc.com> wrote:
> Currently, both linux cpu id and hardware cpu id are same.
> This is not recommended as it will lead to discontinuous cpu
> indexing in Linux. Moreover, kdump kernel will run from CPU0
> which would be absent if we follow existing scheme.
>
> Implement a logical mapping between Linux cpu id and hardware
> cpuid to decouple these two. Always mark the boot processor as
> cpu0 and all other cpus get the logical cpu id based on their
> booting order.
>
> Signed-off-by: Atish Patra <atish.patra@wdc.com>
> ---
>  arch/riscv/include/asm/smp.h | 17 ++++++++++++++++-
>  arch/riscv/kernel/setup.c    |  2 ++
>  arch/riscv/kernel/smp.c      | 19 +++++++++++++++++++
>  3 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/smp.h b/arch/riscv/include/asm/smp.h
> index 36016845..0763337b 100644
> --- a/arch/riscv/include/asm/smp.h
> +++ b/arch/riscv/include/asm/smp.h
> @@ -22,6 +22,12 @@
>  #include <linux/cpumask.h>
>  #include <linux/irqreturn.h>
>
> +/*
> + * Mapping between linux logical cpu index and hartid.
> + */
> +extern u64 __cpu_logical_map[NR_CPUS];
> +#define cpu_logical_map(cpu)    __cpu_logical_map[cpu]
> +
>  #ifdef CONFIG_SMP
>
>  /* SMP initialization hook for setup_arch */
> @@ -33,6 +39,8 @@ void arch_send_call_function_ipi_mask(struct cpumask *mask);
>  /* Hook for the generic smp_call_function_single() routine. */
>  void arch_send_call_function_single_ipi(int cpu);
>
> +int riscv_hartid_to_cpuid(int hartid);
> +void cpuid_to_hartid_mask(const struct cpumask *in, struct cpumask *out);
>  /*
>   * This is particularly ugly: it appears we can't actually get the definition
>   * of task_struct here, but we need access to the CPU this task is running on.
> @@ -41,6 +49,13 @@ void arch_send_call_function_single_ipi(int cpu);
>   */
>  #define raw_smp_processor_id() (*((int*)((char*)get_current() + TASK_TI_CPU)))
>
> -#endif /* CONFIG_SMP */
> +#else
> +
> +static inline int riscv_hartid_to_cpuid(int hartid) { return 0 ; }
> +static inline void cpuid_to_hartid_mask(const struct cpumask *in,
> +                                      struct cpumask *out) {
> +       cpumask_set_cpu(cpu_logical_map(0), out);
> +}
>
> +#endif /* CONFIG_SMP */
>  #endif /* _ASM_RISCV_SMP_H */
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index db20dc63..e21ed481 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -82,6 +82,8 @@ EXPORT_SYMBOL(empty_zero_page);
>  /* The lucky hart to first increment this variable will boot the other cores */
>  atomic_t hart_lottery;
>
> +u64 __cpu_logical_map[NR_CPUS];

If hardware IDs are always machine word size then its better to use
"unsigned long" in-place of u64.

The __cpu_logical_map[] should be zero initially because zero is a
valid hardware ID. Better set all entries to -1 by assigning { -1 } to the
array.

Also, I feel  __cpu_logical_map[] should be part of smp.c instead of
setup.c. Any particular reason for having it in setup.c?

> +
>  #ifdef CONFIG_BLK_DEV_INITRD
>  static void __init setup_initrd(void)
>  {
> diff --git a/arch/riscv/kernel/smp.c b/arch/riscv/kernel/smp.c
> index 906fe21e..d55379ee 100644
> --- a/arch/riscv/kernel/smp.c
> +++ b/arch/riscv/kernel/smp.c
> @@ -38,7 +38,26 @@ enum ipi_message_type {
>         IPI_MAX
>  };
>
> +int riscv_hartid_to_cpuid(int hartid)
> +{
> +       int i = -1;
> +
> +       for (i = 0; i < NR_CPUS; i++)
> +               if (cpu_logical_map(i) == hartid)
> +                       return i;
> +
> +       pr_err("Couldn't find cpu id for hartid [%d]\n", hartid);
> +       BUG();
> +       return i;
> +}
>
> +void cpuid_to_hartid_mask(const struct cpumask *in, struct cpumask *out)

May be rename cpuid_to_hartid_mask() to riscv_cpuid_to_hartid_mask()
for consistency.

> +{
> +       int cpu;
> +
> +       for_each_cpu(cpu, in)
> +               cpumask_set_cpu(cpu_logical_map(cpu), out);
> +}
>  /* Unsupported */
>  int setup_profiling_timer(unsigned int multiplier)
>  {
> --
> 2.7.4
>