Re: [PATCH v6 6/9] mm: thp: Add "recommend" option for anon_orders

[David Hildenbrand]

On 29.09.23 13:44, Ryan Roberts wrote:
> In addition to passing a bitfield of folio orders to enable for THP,
> allow the string "recommend" to be written, which has the effect of
> causing the system to enable the orders preferred by the architecture
> and by the mm. The user can see what these orders are by subsequently
> reading back the file.
>
> Note that these recommended orders are expected to be static for a given
> boot of the system, and so the keyword "auto" was deliberately not used,
> as I want to reserve it for a possible future use where the "best" order
> is chosen more dynamically at runtime.
>
> Recommended orders are determined as follows:
>    - PMD_ORDER: The traditional THP size
>    - arch_wants_pte_order() if implemented by the arch
>    - PAGE_ALLOC_COSTLY_ORDER: The largest order kept on per-cpu free list
>
> arch_wants_pte_order() can be overridden by the architecture if desired.
> Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
> set of ptes map physically contigious, naturally aligned memory, so this
> mechanism allows the architecture to optimize as required.
>
> Here we add the default implementation of arch_wants_pte_order(), used
> when the architecture does not define it, which returns -1, implying
> that the HW has no preference.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>   Documentation/admin-guide/mm/transhuge.rst |  4 ++++
>   include/linux/pgtable.h                    | 13 +++++++++++++
>   mm/huge_memory.c                           | 14 +++++++++++---
>   3 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 732c3b2f4ba8..d6363d4efa3a 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -187,6 +187,10 @@ pages (=16K if the page size is 4K). The example above enables order-9
>   By enabling multiple orders, allocation of each order will be
>   attempted, highest to lowest, until a successful allocation is made.
>   If the PMD-order is unset, then no PMD-sized THPs will be allocated.
> +It is also possible to enable the recommended set of orders, which
> +will be optimized for the architecture and mm::
> +
> +	echo recommend >/sys/kernel/mm/transparent_hugepage/anon_orders
>   
>   The kernel will ignore any orders that it does not support so read the
>   file back to determine which orders are enabled::
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index af7639c3b0a3..0e110ce57cc3 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -393,6 +393,19 @@ static inline void arch_check_zapped_pmd(struct vm_area_struct *vma,
>   }
>   #endif
>   
> +#ifndef arch_wants_pte_order
> +/*
> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
> + * PMD_ORDER) and must not be order-1 since THP requires large folios to be at
> + * least order-2. Negative value implies that the HW has no preference and mm
> + * will choose it's own default order.
> + */
> +static inline int arch_wants_pte_order(void)
> +{
> +	return -1;
> +}
> +#endif
> +
>   #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>   static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>   				       unsigned long address,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bcecce769017..e2e2d3906a21 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -464,10 +464,18 @@ static ssize_t anon_orders_store(struct kobject *kobj,
>   	int err;
>   	int ret = count;
>   	unsigned int orders;
> +	int arch;
>   
> -	err = kstrtouint(buf, 0, &orders);
> -	if (err)
> -		ret = -EINVAL;
> +	if (sysfs_streq(buf, "recommend")) {
> +		arch = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
> +		orders = BIT(arch);
> +		orders |= BIT(PAGE_ALLOC_COSTLY_ORDER);
> +		orders |= BIT(PMD_ORDER);
> +	} else {
> +		err = kstrtouint(buf, 0, &orders);
> +		if (err)
> +			ret = -EINVAL;
> +	}
>   
>   	if (ret > 0) {
>   		orders &= THP_ORDERS_ALL_ANON;

:/ don't really like that. Regarding my proposal, one could have 
something like that in an "auto" setting for the "enabled" value, or a 
"recommended" setting [not sure].

--
Cheers,

David / dhildenb