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

[Raghavendra K T <raghavendra.kt@amd.com>]

On 1/17/2023 4:44 PM, David Hildenbrand wrote:
> 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 for the review David. Good point..  I do agree. I see I will have
to fence further in memory.c only since fair.c is already taken care.