Re: [External] [PATCH v5 08/13] riscv: Avoid TLB flush loops when affected by SiFive CIP-1200

[yunhui cui <cuiyunhui@bytedance.com>]

H Samuel,

On Tue, Mar 12, 2024 at 8:36 AM Samuel Holland
<samuel.holland@sifive.com> wrote:
>
> Hi Yunhui,
>
> On 2024-02-29 8:48 PM, yunhui cui wrote:
> > Hi Samuel,
> >
> > On Fri, Mar 1, 2024 at 7:22 AM Samuel Holland <samuel.holland@sifive.com> wrote:
> >>
> >> Since implementations affected by SiFive errata CIP-1200 always use the
> >> global variant of the sfence.vma instruction, they only need to execute
> >> the instruction once. The range-based loop only hurts performance.
> >>
> >> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> >> ---
> >>
> >> (no changes since v4)
> >>
> >> Changes in v4:
> >>  - Only set tlb_flush_all_threshold when CONFIG_MMU=y.
> >>
> >> Changes in v3:
> >>  - New patch for v3
> >>
> >>  arch/riscv/errata/sifive/errata.c | 5 +++++
> >>  arch/riscv/include/asm/tlbflush.h | 2 ++
> >>  arch/riscv/mm/tlbflush.c          | 2 +-
> >>  3 files changed, 8 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/riscv/errata/sifive/errata.c b/arch/riscv/errata/sifive/errata.c
> >> index 3d9a32d791f7..716cfedad3a2 100644
> >> --- a/arch/riscv/errata/sifive/errata.c
> >> +++ b/arch/riscv/errata/sifive/errata.c
> >> @@ -42,6 +42,11 @@ static bool errata_cip_1200_check_func(unsigned long  arch_id, unsigned long imp
> >>                 return false;
> >>         if ((impid & 0xffffff) > 0x200630 || impid == 0x1200626)
> >>                 return false;
> >> +
> >> +#ifdef CONFIG_MMU
> >> +       tlb_flush_all_threshold = 0;
> >> +#endif
> >> +
> >>         return true;
> >>  }
> >>
> >> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> >> index 463b615d7728..8e329721375b 100644
> >> --- a/arch/riscv/include/asm/tlbflush.h
> >> +++ b/arch/riscv/include/asm/tlbflush.h
> >> @@ -66,6 +66,8 @@ void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
> >>                                unsigned long uaddr);
> >>  void arch_flush_tlb_batched_pending(struct mm_struct *mm);
> >>  void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
> >> +
> >> +extern unsigned long tlb_flush_all_threshold;
> >>  #else /* CONFIG_MMU */
> >>  #define local_flush_tlb_all()                  do { } while (0)
> >>  #endif /* CONFIG_MMU */
> >> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> >> index 365e0a0e4725..22870f213188 100644
> >> --- a/arch/riscv/mm/tlbflush.c
> >> +++ b/arch/riscv/mm/tlbflush.c
> >> @@ -11,7 +11,7 @@
> >>   * Flush entire TLB if number of entries to be flushed is greater
> >>   * than the threshold below.
> >>   */
> >> -static unsigned long tlb_flush_all_threshold __read_mostly = 64;
> >> +unsigned long tlb_flush_all_threshold __read_mostly = 64;
> >>
> >>  static void local_flush_tlb_range_threshold_asid(unsigned long start,
> >>                                                  unsigned long size,
> >> --
> >> 2.43.1
> >>
> >
> > If local_flush_tlb_all_asid() is used every time, more PTWs will be
> > generated. Will such modifications definitely improve the overall
> > performance?
>
> This change in this commit specifically applies to older SiFive SoCs with a bug
> making single-page sfence.vma instructions unsafe to use. In this case, a single
> call to local_flush_tlb_all_asid() is optimal, yes.
Would it be more clear to add this content to the git commit
description appropriately?

>
> > Hi Alex, Samuel,
> > The relationship between flush_xx_range_asid() and nr_ptes is
> > basically linear growth (y=kx +b), while flush_all_asid() has nothing
> > to do with nr_ptes (y=c).
> > Some TLBs may do some optimization. The operation of flush all itself
> > requires very few cycles, but there is a certain delay between
> > consecutive flush all.
> > The intersection of the two straight lines is the optimal solution of
> > tlb_flush_all_threshold. In actual situations, continuous
> > flush_all_asid will not occur. One problem caused by flush_all_asid()
> > is that multiple flush entries require PTW, which causes greater
> > latency.
> > Therefore, the value of tlb_flush_all_threshold needs to be considered
> > or quantified. Maybe doing local_flush_tlb_page_asid() based on the
> > actual nr_ptes_in_range would give better overall performance.
> > What do you think?
>
> Yes, this was something Alex brought up when adding this threshold, that it
> should be tuned for various scenarios. That still needs to be done. This patch
> just covers one specific case where we know the optimal answer due to an erratum.
>
> Regards,
> Samuel
>

Thanks,
Yunhui