Re: [PATCH v3 15/15] arm64/mm: Implement clear_ptes() to optimize exit()

Alistair Popple
2023-12-08T01:45:56Z

Ryan Roberts <ryan.roberts@arm.com> writes:

> With the core-mm changes in place to batch-clear ptes during
> zap_pte_range(), we can take advantage of this in arm64 to greatly
> reduce the number of tlbis we have to issue, and recover the lost exit
> performance incured when adding support for transparent contiguous ptes.
>
> If we are clearing a whole contpte range, we can elide first unfolding
> that range and save the tlbis. We just clear the whole range.
>
> The following shows the results of running a kernel compilation workload
> and measuring the cost of arm64_sys_exit_group() (which at ~1.5% is a
> very small part of the overall workload).
>
> Benchmarks were run on Ampere Altra in 2 configs; single numa node and 2
> numa nodes (tlbis are more expensive in 2 node config).
>
> - baseline: v6.7-rc1 + anonfolio-v7
> - no-opt: contpte series without any attempt to optimize exit()
> - simple-ptep_get_clear_full: simple optimization to exploit full=1.
> ptep_get_clear_full() does not fully conform to its intended semantic
> - robust-ptep_get_clear_full: similar to previous but
> ptep_get_clear_full() fully conforms to its intended semantic
> - clear_ptes: optimization implemented by this patch
>
> | config | numa=1 | numa=2 |
> |----------------------------|--------|--------|
> | baseline | 0% | 0% |
> | no-opt | 190% | 768% |
> | simple-ptep_get_clear_full | 8% | 29% |
> | robust-ptep_get_clear_full | 21% | 19% |
> | clear_ptes | 13% | 9% |
>
> In all cases, the cost of arm64_sys_exit_group() increases; this is
> anticipated because there is more work to do to tear down the page
> tables. But clear_ptes() gives the smallest increase overall.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> arch/arm64/include/asm/pgtable.h | 32 ++++++++++++++++++++++++
> arch/arm64/mm/contpte.c | 42 ++++++++++++++++++++++++++++++++
> 2 files changed, 74 insertions(+)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 9bd2f57a9e11..ff6b3cc9e819 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1145,6 +1145,8 @@ extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> pte_t *ptep, pte_t pte, unsigned int nr);
> +extern pte_t contpte_clear_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, unsigned int nr);
> extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> unsigned long addr, pte_t *ptep);
> extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> @@ -1270,6 +1272,36 @@ static inline void pte_clear(struct mm_struct *mm,
> __pte_clear(mm, addr, ptep);
> }
>
> +#define clear_ptes clear_ptes
> +static inline pte_t clear_ptes(struct mm_struct *mm,
> + unsigned long addr, pte_t *ptep, int full,
> + unsigned int nr)
> +{
> + pte_t pte;
> +
> + if (!contpte_is_enabled(mm)) {

I think it would be better to call the generic definition of
clear_ptes() here. Obviously that won't exist if clear_ptes is defined
here, but you could alcall it __clear_ptes() and #define clear_ptes
__clear_ptes when the arch specific helper isn't defined.

> + unsigned int i;
> + pte_t tail;
> +
> + pte = __ptep_get_and_clear(mm, addr, ptep);
> + for (i = 1; i < nr; i++) {
> + addr += PAGE_SIZE;
> + ptep++;
> + tail = __ptep_get_and_clear(mm, addr, ptep);
> + if (pte_dirty(tail))
> + pte = pte_mkdirty(pte);
> + if (pte_young(tail))
> + pte = pte_mkyoung(pte);
> + }
> + } else if (nr == 1) {
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + pte = __ptep_get_and_clear(mm, addr, ptep);
> + } else
> + pte = contpte_clear_ptes(mm, addr, ptep, nr);
> +
> + return pte;
> +}
> +
> #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
> static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> unsigned long addr, pte_t *ptep)
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index 2a57df16bf58..34b43bde3fcd 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -257,6 +257,48 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> }
> EXPORT_SYMBOL(contpte_set_ptes);
>
> +pte_t contpte_clear_ptes(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
> + unsigned int nr)
> +{
> + /*
> + * If we cover a partial contpte block at the beginning or end of the
> + * batch, unfold if currently folded. This makes it safe to clear some
> + * of the entries while keeping others. contpte blocks in the middle of
> + * the range, which are fully covered don't need to be unfolded because
> + * we will clear the full block.
> + */
> +
> + unsigned int i;
> + pte_t pte;
> + pte_t tail;
> +
> + if (ptep != contpte_align_down(ptep) || nr < CONT_PTES)
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +
> + if (ptep + nr != contpte_align_down(ptep + nr))
> + contpte_try_unfold(mm, addr + PAGE_SIZE * (nr - 1),
> + ptep + nr - 1,
> + __ptep_get(ptep + nr - 1));
> +
> + pte = __ptep_get_and_clear(mm, addr, ptep);
> +
> + for (i = 1; i < nr; i++) {
> + addr += PAGE_SIZE;
> + ptep++;
> +
> + tail = __ptep_get_and_clear(mm, addr, ptep);
> +
> + if (pte_dirty(tail))
> + pte = pte_mkdirty(pte);
> +
> + if (pte_young(tail))
> + pte = pte_mkyoung(pte);
> + }
> +
> + return pte;
> +}
> +EXPORT_SYMBOL(contpte_clear_ptes);
> +
> int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> unsigned long addr, pte_t *ptep)
> {