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Date:   Wed, 3 May 2023 23:12:33 +0530
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Subject: Re: [RFC PATCH V1 1/2] sched/numa: Introduce per vma scan counter
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To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>, rppt@kernel.org,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Bharata B Rao <bharata@amd.com>
References: <cover.1683033105.git.raghavendra.kt@amd.com>
 <abd037023141f25f79c6bbbb801c8405e4c449a1.1683033105.git.raghavendra.kt@amd.com>
From:   Raghavendra K T <raghavendra.kt@amd.com>
In-Reply-To: <abd037023141f25f79c6bbbb801c8405e4c449a1.1683033105.git.raghavendra.kt@amd.com>
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On 5/3/2023 7:35 AM, Raghavendra K T wrote:
> With the recent numa scan enhancements, only the tasks which had
> previously accessed vma are allowed to scan.
> 
> While this has improved significant system time overhead, there are
> corner cases, which genuinely needs some relaxation for e.g., concern
> raised by PeterZ where unfairness amongst the theread belonging to
> disjoint set of VMSs can potentially amplify the side effects of vma
> regions belonging to some of the tasks being left unscanned.
> 
> To address this, allow scanning for first few times with a per vma
> counter.
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
> ---

Some clarification:
base was linux-next-20230411 (because I have some issue with
linux-next-20230425 onwards and linux master branch, which I am diging.


>   include/linux/mm_types.h |  1 +
>   kernel/sched/fair.c      | 30 +++++++++++++++++++++++++++---
>   2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3fc9e680f174..f66e6b4e0620 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -479,6 +479,7 @@ struct vma_numab_state {
>   	unsigned long next_scan;
>   	unsigned long next_pid_reset;
>   	unsigned long access_pids[2];
> +	unsigned int scan_counter;
>   };
>   
>   /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a29ca11bead2..3c50dc3893eb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2928,19 +2928,38 @@ static void reset_ptenuma_scan(struct task_struct *p)
>   	p->mm->numa_scan_offset = 0;
>   }
>   
> +/* Scan 1GB or 4 * scan_size */
> +#define VMA_DISJOINT_SET_ACCESS_THRESH		4U
> +
>   static bool vma_is_accessed(struct vm_area_struct *vma)
>   {
>   	unsigned long pids;
> +	unsigned int windows;

Missed windows = 0 while splitting the patch
will be corrected in next posting.

/me Remembered after kernel test robot noticed
[...]