linux-kernel - Re: [RFC PATCH V1 1/2] sched/numa: Introduce per vma scan counter

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Message-ID: <8213bb71-9924-4f98-eb85-1a3d6401c0f6@amd.com>
Date:   Wed, 3 May 2023 23:12:33 +0530
From:   Raghavendra K T <raghavendra.kt@....com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, rppt@...nel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bharata B Rao <bharata@....com>
Subject: Re: [RFC PATCH V1 1/2] sched/numa: Introduce per vma scan counter

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@....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
[...]