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

[Yu Zhao <yuzhao@google.com>]

On Mon, Aug 12, 2024 at 3:40 AM Muchun Song <muchun.song@linux.dev> wrote:
>
>
>
> > 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!

I forgot to mention (in the commit message) that:

This patch changes the behavior of the `demote` interface
when de-HVO fails. Before, the interface aborts immediately upon
failure; now, it tries to finish an entire batch, meaning it can make
extra progress if the rest of the batch contains folios that do not
need to de-HVO.

Will post v2 to fix this.