Re: [PATCH v2 1/7] powerpc/64s/radix: do not flush TLB on spurious fault

On Mon, 21 May 2018 11:36:12 +0530
"Aneesh Kumar K.V" [off-list ref] wrote:

Nicholas Piggin [off-list ref] writes:

quoted

In the case of a spurious fault (which can happen due to a race with
another thread that changes the page table), the default Linux mm code
calls flush_tlb_page for that address. This is not required because
the pte will be re-fetched. Hash does not wire this up to a hardware
TLB flush for this reason. This patch avoids the flush for radix.

From Power ISA v3.0B, p.1090:

    Setting a Reference or Change Bit or Upgrading Access Authority
    (PTE Subject to Atomic Hardware Updates)

    If the only change being made to a valid PTE that is subject to
    atomic hardware updates is to set the Refer- ence or Change bit to
    1 or to add access authorities, a simpler sequence suffices
    because the translation hardware will refetch the PTE if an access
    is attempted for which the only problems were reference and/or
    change bits needing to be set or insufficient access authority.

The nest MMU on POWER9 does not re-fetch the PTE after such an access
attempt before faulting, so address spaces with a coprocessor
attached will continue to flush in these cases.

This reduces tlbies for a kernel compile workload from 0.95M to 0.90M.

fork --fork --exec benchmark improved 0.5% (12300->12400).
 

Reviewed-by: Aneesh Kumar K.V <redacted>

Do you want to use flush_tlb_fix_spurious_fault in
ptep_set_access_flags() also?. That would bring it closer to generic version?

I'm not sure it adds much, it does bring it closer to generic version
and they do happen to do the same thing, but it's not really fixing a
spurious fault in ptep_set_access_flags(). I think adding that just
means you would have to follow another indirection to work out what it
does.

Thanks,
Nick