Re: [PATCH] RISC-V: Issue a local tlb flush if possible.

[Atish Patra <Atish.Patra@wdc.com>]

On 8/9/19, 8:30 PM, "Anup Patel" <anup@brainfault.org> wrote:

    On Sat, Aug 10, 2019 at 7:13 AM Atish Patra <atish.patra@wdc.com> wrote:
    >
    > In RISC-V, tlb flush happens via SBI which is expensive.
    > If the target cpumask contains a local hartid, some cost
    > can be saved by issuing a local tlb flush as we do that
    > in OpenSBI anyways.
    >
    > Signed-off-by: Atish Patra <atish.patra@wdc.com>
    > ---
    >  arch/riscv/include/asm/tlbflush.h | 33 +++++++++++++++++++++++++++----
    >  1 file changed, 29 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
    > index 687dd19735a7..b32ba4fa5888 100644
    > --- a/arch/riscv/include/asm/tlbflush.h
    > +++ b/arch/riscv/include/asm/tlbflush.h
    > @@ -8,6 +8,7 @@
    >  #define _ASM_RISCV_TLBFLUSH_H
    >
    >  #include <linux/mm_types.h>
    > +#include <linux/sched.h>
    >  #include <asm/smp.h>
    >
    >  /*
    > @@ -46,14 +47,38 @@ static inline void remote_sfence_vma(struct cpumask *cmask, unsigned long start,
    >                                      unsigned long size)
    >  {
    >         struct cpumask hmask;
    > +       struct cpumask tmask;
    > +       int cpuid = smp_processor_id();
    >
    >         cpumask_clear(&hmask);
    > -       riscv_cpuid_to_hartid_mask(cmask, &hmask);
    > -       sbi_remote_sfence_vma(hmask.bits, start, size);
    > +       cpumask_clear(&tmask);
    > +
    > +       if (cmask)
    > +               cpumask_copy(&tmask, cmask);
    > +       else
    > +               cpumask_copy(&tmask, cpu_online_mask);
    
    This can be further simplified.
    
    We can totally avoid tmask, cpumask_copy(), and cpumask_clear()
    by directly updating hmask.
    
Ahh yes. Thanks for pointing out.

    In addition to this patch, we should also handle the case of
    empty hart mask in OpenSBI.
    
Yes. I have few other fixes as well (around fifo race condition and local flushing in OpenSBI).
I will send them soon.

Regards,
Atish
    > +
    > +       if (cpumask_test_cpu(cpuid, &tmask)) {
    > +               /* Save trap cost by issuing a local tlb flush here */
    > +               if ((start == 0 && size == -1) || (size > PAGE_SIZE))
    > +                       local_flush_tlb_all();
    > +               else if (size == PAGE_SIZE)
    > +                       local_flush_tlb_page(start);
    > +               cpumask_clear_cpu(cpuid, &tmask);
    > +       } else if (cpumask_empty(&tmask)) {
    > +               /* cpumask is empty. So just do a local flush */
    > +               local_flush_tlb_all();
    > +               return;
    > +       }
    > +
    > +       if (!cpumask_empty(&tmask)) {
    > +               riscv_cpuid_to_hartid_mask(&tmask, &hmask);
    > +               sbi_remote_sfence_vma(hmask.bits, start, size);
    > +       }
    >  }
    >
    > -#define flush_tlb_all() sbi_remote_sfence_vma(NULL, 0, -1)
    > -#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, 0)
    > +#define flush_tlb_all() remote_sfence_vma(NULL, 0, -1)
    > +#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, (addr) + PAGE_SIZE)
    >  #define flush_tlb_range(vma, start, end) \
    >         remote_sfence_vma(mm_cpumask((vma)->vm_mm), start, (end) - (start))
    >  #define flush_tlb_mm(mm) \
    > --
    > 2.21.0
    >
    >
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    Regards,
    Anup