linux-kernel - Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

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Date:   Tue, 20 Aug 2019 20:29:47 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "anup@...infault.org" <anup@...infault.org>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "schwab@...ux-m68k.org" <schwab@...ux-m68k.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "hch@...radead.org" <hch@...radead.org>,
        "allison@...utok.net" <allison@...utok.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "palmer@...ive.com" <palmer@...ive.com>
Subject: Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

On Tue, 2019-08-20 at 14:21 +0530, Anup Patel wrote:
> On Tue, Aug 20, 2019 at 6:17 AM Atish Patra <atish.patra@....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. There is also no need of SBI call if
> > cpumask is empty.
> > 
> > Do a local flush first if current cpu is present in cpumask.
> > Invoke SBI call only if target cpumask contains any cpus
> > other than local cpu.
> > 
> > Signed-off-by: Atish Patra <atish.patra@....com>
> > ---
> >  arch/riscv/include/asm/tlbflush.h | 37 ++++++++++++++++++++++++++-
> > ----
> >  1 file changed, 31 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/tlbflush.h
> > b/arch/riscv/include/asm/tlbflush.h
> > index b5e64dc19b9e..3f9cd17b5402 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>
> > 
> >  /*
> > @@ -42,20 +43,44 @@ static inline void flush_tlb_range(struct
> > vm_area_struct *vma,
> > 
> >  #include <asm/sbi.h>
> > 
> > -static inline void remote_sfence_vma(struct cpumask *cmask,
> > unsigned long start,
> > -                                    unsigned long size)
> > +static void __riscv_flush_tlb(struct cpumask *cmask, unsigned long
> > start,
> > +                             unsigned long size)
> >  {
> >         struct cpumask hmask;
> > +       unsigned int hartid;
> > +       unsigned int cpuid;
> > 
> >         cpumask_clear(&hmask);
> > +
> > +       if (!cmask) {
> > +               riscv_cpuid_to_hartid_mask(cpu_online_mask,
> > &hmask);
> > +               goto issue_sfence;
> > +       }
> > +
> > +       cpuid = get_cpu();
> > +       if (cpumask_test_cpu(cpuid, cmask)) {
> > +               /* 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);
> > +       }
> > +       if (cpumask_any_but(cmask, cpuid) >= nr_cpu_ids)
> > +               goto done;
> > +
> >         riscv_cpuid_to_hartid_mask(cmask, &hmask);
> > +       hartid = cpuid_to_hartid_map(cpuid);
> > +       cpumask_clear_cpu(hartid, &hmask);
> > +
> > +issue_sfence:
> >         sbi_remote_sfence_vma(hmask.bits, start, size);
> > +done:
> > +       put_cpu();
> >  }
> > 
> > -#define flush_tlb_all() sbi_remote_sfence_vma(NULL, 0, -1)
> > -
> > +#define flush_tlb_all() __riscv_flush_tlb(NULL, 0, -1)
> >  #define flush_tlb_range(vma, start, end) \
> > -       remote_sfence_vma(mm_cpumask((vma)->vm_mm), start, (end) -
> > (start))
> > +       __riscv_flush_tlb(mm_cpumask((vma)->vm_mm), start, (end) -
> > (start))
> > 
> >  static inline void flush_tlb_page(struct vm_area_struct *vma,
> >                                   unsigned long addr) {
> > @@ -63,7 +88,7 @@ static inline void flush_tlb_page(struct
> > vm_area_struct *vma,
> >  }
> > 
> >  #define flush_tlb_mm(mm)                               \
> > -       remote_sfence_vma(mm_cpumask(mm), 0, -1)
> > +       __riscv_flush_tlb(mm_cpumask(mm), 0, -1)
> > 
> >  #endif /* CONFIG_SMP */
> > 
> > --
> > 2.21.0
> > 
> 
> I think we should move __riscv_flush_tlb() to mm/tlbflush.c because
> it's quite
> big now.
> 
> In future, we will also have __riscv_flush_tlb_asid() which will
> flush TLB based
> on ASID.
> 

Sounds good to me. Christoph has already mm/tlbflush.c in his mmu
series. I will rebase on top of it.

> Regards,
> Anup

-- 
Regards,
Atish