linux-kernel - Re: [RFC PATCH V3 1/1] sched/numa: Fix disjoint set vma scan regression

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Date:   Wed, 5 Jul 2023 11:18:37 +0530
From:   Raghavendra K T <raghavendra.kt@....com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel test robot <oliver.sang@...el.com>
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>,
        Aithal Srikanth <sraithal@....com>
Subject: Re: [RFC PATCH V3 1/1] sched/numa: Fix disjoint set vma scan
 regression

On 5/31/2023 9:55 AM, Raghavendra K T wrote:
>   With the numa scan enhancements [1], only the threads which had previously
> accessed vma are allowed to scan.
> 
> While this had improved significant system time overhead, there were corner
> cases, which genuinely need some relaxation. For e.g.,
> 
> 1) Concern raised by PeterZ, where if there are N partition sets of vmas
> belonging to tasks, then unfairness in allowing these threads to scan could
> potentially amplify the side effect of some of the vmas being left
> unscanned.
> 
> 2) Below reports of LKP numa01 benchmark regression.
> 
> Currently this is handled by allowing first two scanning unconditional
> as indicated by mm->numa_scan_seq. This is imprecise since for some
> benchmark vma scanning might itself start at numa_scan_seq > 2.
> 
> Solution:
> Allow unconditional scanning of vmas of tasks depending on vma size. This
> is achieved by maintaining a per vma scan counter, where
> 
> f(allowed_to_scan) = f(scan_counter <  vma_size / scan_size)
> 
> Result:
> numa01_THREAD_ALLOC result on 6.4.0-rc2 (that has numascan enhancement)
>                  	base-numascan	base		base+fix
> real    		1m1.507s	1m23.259s	1m2.632s
> user    		213m51.336s	251m46.363s	220m35.528s
> sys     		3m3.397s	0m12.492s	2m41.393s
> 
> numa_hit 		5615517		4560123		4963875
> numa_local 		5615505		4560024		4963700
> numa_other 		12		99		175
> numa_pte_updates 	1822797		493		1559111
> numa_hint_faults 	1307113		523		1469031
> numa_hint_faults_local 	612617		488		884829
> numa_pages_migrated 	694370		35		584202
> 
> Summary: Regression in base is recovered by allowing scanning as required.
> 
> [1] https://lore.kernel.org/lkml/cover.1677672277.git.raghavendra.kt@amd.com/T/#t
> 
> Fixes: fc137c0ddab2 ("sched/numa: enhance vma scanning logic")
> regression.
> Reported-by: Aithal Srikanth <sraithal@....com>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/lkml/db995c11-08ba-9abf-812f-01407f70a5d4@amd.com/T/
> Signed-off-by: Raghavendra K T <raghavendra.kt@....com>

Hello kernel test robot,

Gentle ping to check if the patch has helped your regression report.

Thanks
- Raghu