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

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Date:   Wed, 15 Aug 2018 22:17:58 -0700
From:   Atish Patra <atish.patra@....com>
To:     Anup Patel <anup@...infault.org>
Cc:     "palmer@...ive.com" <palmer@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Damien Le Moal <Damien.LeMoal@....com>
Subject: Re: [RFC PATCH 1/5] RISC-V: Add logical CPU indexing for RISC-V

On 8/15/18 9:06 PM, Anup Patel wrote:
> On Thu, Aug 16, 2018 at 5:26 AM, Atish Patra <atish.patra@....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@....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.
> 

Good point. Thanks.

> 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.
> 

ok. I will introduce INVALID_HART_ID (=-1) and initialize with that.

> 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?
> 

currently smp.c is only compiled in if CONFIG_SMP. That's why I kept it 
in setup.c


Regards,
Atish
>> +
>>   #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
>>
>