Re: [RFC PATCH V1 1/1] sched/numa: Enhance vma scanning logic

[David Hildenbrand <david@redhat.com>]

On 16.01.23 03:25, Raghavendra K T wrote:
>   During the Numa scanning make sure only relevant vmas of the
> tasks are scanned.
> 
> Logic:
> 1) For the first two time allow unconditional scanning of vmas
> 2) Store recent 4 unique tasks (last 8bits of PIDs) accessed the vma.
>    False negetives in case of collison should be fine here.
> 3) If more than 4 pids exist assume task indeed accessed vma to
>   to avoid false negetives
> 
> Co-developed-by: Bharata B Rao <bharata@amd.com>
> (initial patch to store pid information)
> 
> Suggested-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Bharata B Rao <bharata@amd.com>
> Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
> ---
>   include/linux/mm_types.h |  2 ++
>   kernel/sched/fair.c      | 32 ++++++++++++++++++++++++++++++++
>   mm/memory.c              | 21 +++++++++++++++++++++
>   3 files changed, 55 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 500e536796ca..07feae37b8e6 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -506,6 +506,8 @@ struct vm_area_struct {
>   	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
>   #endif
>   	struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> +	unsigned int accessing_pids;
> +	int next_pid_slot;
>   } __randomize_layout;

What immediately jumps at me is the unconditional grow of a VMA by 8 
bytes. A process with 64k mappings consumes 512 KiB more of memory, 
possibly completely unnecessarily.

This at least needs to be fenced by CONFIG_NUMA_BALANCING.

-- 
Thanks,

David / dhildenb