Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.

[Christoph Hellwig]

On Wed, Aug 21, 2019 at 05:46:42PM -0700, Atish Patra wrote:
> In RISC-V, tlb flush happens via SBI which is expensive. If the local
> cpu is the only cpu in cpumask, there is no need to invoke a SBI call.
> 
> Just do a local flush and return.
> 
> Signed-off-by: Atish Patra <atish.pa...@wdc.com>
> ---
>  arch/riscv/mm/tlbflush.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index df93b26f1b9d..36430ee3bed9 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -2,6 +2,7 @@
>  
>  #include <linux/mm.h>
>  #include <linux/smp.h>
> +#include <linux/sched.h>
>  #include <asm/sbi.h>
>  
>  void flush_tlb_all(void)
> @@ -13,9 +14,23 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask, 
> unsigned long start,
>               unsigned long size)
>  {
>       struct cpumask hmask;
> +     unsigned int cpuid = get_cpu();
>  
> +     if (!cmask) {
> +             riscv_cpuid_to_hartid_mask(cpu_online_mask, &hmask);
> +             goto issue_sfence;
> +     }
> +
> +     if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
> +             local_flush_tlb_all();
> +             goto done;
> +     }

I think a single core on a SMP kernel is a valid enough use case given
how litte distros still have UP kernels.  So Maybe this shiuld rather be:

        if (!cmask)
                cmask = cpu_online_mask;

        if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
                local_flush_tlb_all();
        } else {
                riscv_cpuid_to_hartid_mask(cmask, &hmask);
                sbi_remote_sfence_vma(hmask.bits, start, size);
        }