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Message-ID: <4ac11b6a-284c-c763-f3c6-45a53c272f47@redhat.com>
Date:   Tue, 17 Jan 2023 12:14:29 +0100
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Subject: Re: [RFC PATCH V1 1/1] sched/numa: Enhance vma scanning logic
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To:     Raghavendra K T <raghavendra.kt@amd.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Liam R . Howlett" <Liam.Howlett@Oracle.com>,
        Peter Xu <peterx@redhat.com>, xu xin <cgel.zte@gmail.com>,
        Yu Zhao <yuzhao@google.com>, Colin Cross <ccross@google.com>,
        Arnd Bergmann <arnd@arndb.de>, Hugh Dickins <hughd@google.com>,
        Bharata B Rao <bharata@amd.com>,
        Disha Talreja <dishaa.talreja@amd.com>
References: <cover.1673610485.git.raghavendra.kt@amd.com>
 <67bf778d592c39d02444825c416c2ed11d2ef4b2.1673610485.git.raghavendra.kt@amd.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <67bf778d592c39d02444825c416c2ed11d2ef4b2.1673610485.git.raghavendra.kt@amd.com>
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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