Re: [PATCH] mm: huge_memory: batch tlb flush when splitting a pte-mapped THP

[Baolin Wang]

On 11/1/2023 2:13 PM, Alistair Popple wrote:
> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>
> > I can observe an obvious tlb flush hotpot when splitting a pte-mapped THP on
>
>   A tlb flush hotpot does sound delicious, but I think you meant hotspot :-)

Ah, yes. Hope Andrew can help to fix it :)

> > my ARM64 server, and the distribution of this hotspot is as follows:
> >
> >     - 16.85% split_huge_page_to_list
> >        + 7.80% down_write
> >        - 7.49% try_to_migrate
> >           - 7.48% rmap_walk_anon
> >                7.23% ptep_clear_flush
> >        + 1.52% __split_huge_page
> >
> > The reason is that the split_huge_page_to_list() will build migration entries
> > for each subpage of a pte-mapped Anon THP by try_to_migrate(), or unmap for
> > file THP, and it will clear and tlb flush for each subpage's pte. Moreover,
> > the split_huge_page_to_list() will set TTU_SPLIT_HUGE_PMD flag to ensure
> > the THP is already a pte-mapped THP before splitting it to some normal pages.
>
> The only other user of TTU_SPLIT_HUGE_PMD is vmscan which also sets
> TTU_BATCH_FLUSH so we could make the former imply the latter but that
> seem dangerous given the requirement to call try_to_unmap_flush() so
> best not to.
>
> Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>

Thanks for reviewing, and also thanks to Ying and Yang.