Re: [PATCH] numa: Use LAST_CPUPID_SHIFT to calculate LAST_CPUPID_MASK

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:

> LAST_CPUPID_MASK is calculated using LAST_CPUPID_WIDTH.  However
> LAST_CPUPID_WIDTH itself can be 0. (when LAST_CPUPID_NOT_IN_PAGE_FLAGS
> is set). In such a case LAST_CPUPID_MASK turns out to be 0.
>
> But with recent commit 1ae71d0319: (mm: numa: bugfix for
> LAST_CPUPID_NOT_IN_PAGE_FLAGS) if LAST_CPUPID_MASK is 0,
> page_cpupid_xchg_last() and page_cpupid_reset_last() causes
> page->_last_cpupid to be set to 0.
>
> This causes performance regression. Its almost as if numa_balancing is
> off.
>
> Fix LAST_CPUPID_MASK by using LAST_CPUPID_SHIFT instead of
> LAST_CPUPID_WIDTH.
>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c1b7414..b9765bf 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -684,7 +684,7 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>  #define ZONES_MASK		((1UL << ZONES_WIDTH) - 1)
>  #define NODES_MASK		((1UL << NODES_WIDTH) - 1)
>  #define SECTIONS_MASK		((1UL << SECTIONS_WIDTH) - 1)
> -#define LAST_CPUPID_MASK	((1UL << LAST_CPUPID_WIDTH) - 1)
> +#define LAST_CPUPID_MASK	((1UL << LAST_CPUPID_SHIFT) - 1)
>  #define ZONEID_MASK		((1UL << ZONEID_SHIFT) - 1)
>
>  static inline enum zone_type page_zonenum(const struct page *page)
>
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