linux-kernel - Re: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: batch HVO work when demoting

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Message-Id: <C5E8EB48-1B4F-4C40-A580-8C99C128ACCD@linux.dev>
Date: Mon, 12 Aug 2024 17:40:02 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Linux Memory Management List <linux-mm@...ck.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: batch HVO work when
 demoting



> On Aug 11, 2024, at 12:17, Yu Zhao <yuzhao@...gle.com> wrote:
> 
> Batch the HVO work, including de-HVO of the source and HVO of the
> destination hugeTLB folios, to speed up demotion.
> 
> After commit bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with
> speculative PFN walkers"), each request of HVO or de-HVO, batched or
> not, invokes synchronize_rcu() once. For example, when not batched,
> demoting one 1GB hugeTLB folio to 512 2MB hugeTLB folios invokes
> synchronize_rcu() 513 times (1 de-HVO plus 512 HVO requests), whereas
> when batched, only twice (1 de-HVO plus 1 HVO request). And
> performance between the two cases are significantly different, e.g.,
>  echo 2048kB >/sys/kernel/mm/hugepages/hugepages-1048576kB/demote_size
>  time echo 100 >/sys/kernel/mm/hugepages/hugepages-1048576kB/demote
> 
> Before this patch:
>  real     8m58.158s
>  user     0m0.009s
>  sys      0m5.900s
> 
> After this patch:
>  real     0m0.900s
>  user     0m0.000s
>  sys      0m0.851s
> 
> Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
> Signed-off-by: Yu Zhao <yuzhao@...gle.com>

Reviewed-by: Muchun Song <muchun.song@...ux.dev>

Thanks.