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

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Date:   Tue, 17 Jan 2023 12:14:29 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Raghavendra K T <raghavendra.kt@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Peter Xu <peterx@...hat.com>, xu xin <cgel.zte@...il.com>,
        Yu Zhao <yuzhao@...gle.com>, Colin Cross <ccross@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Hugh Dickins <hughd@...gle.com>,
        Bharata B Rao <bharata@....com>,
        Disha Talreja <dishaa.talreja@....com>
Subject: Re: [RFC PATCH V1 1/1] sched/numa: Enhance vma scanning logic

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@....com>
> (initial patch to store pid information)
> 
> Suggested-by: Mel Gorman <mgorman@...hsingularity.net>
> Signed-off-by: Bharata B Rao <bharata@....com>
> Signed-off-by: Raghavendra K T <raghavendra.kt@....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