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

[Muchun Song <muchun.song@linux.dev>]

> On Aug 11, 2024, at 12:17, Yu Zhao <yuzhao@google.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@google.com>

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

Thanks.