Re: [PATCH v6 0/4] mm: introduce THP deferred setting

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, May 15, 2025 at 11:41 AM Nico Pache <npache@redhat.com> wrote:
>
> This series is a follow-up to [1], which adds mTHP support to khugepaged.
> mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> configs to make sense. Without it global="defer" and  mTHP="inherit" case
> is "undefined" behavior.
>
> We've seen cases were customers switching from RHEL7 to RHEL8 see a
> significant increase in the memory footprint for the same workloads.
>
> Through our investigations we found that a large contributing factor to
> the increase in RSS was an increase in THP usage.
>
> For workloads like MySQL, or when using allocators like jemalloc, it is
> often recommended to set /transparent_hugepages/enabled=never. This is
> in part due to performance degradations and increased memory waste.
>
> This series introduces enabled=defer, this setting acts as a middle
> ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> page fault handler will act normally, making a hugepage if possible. If
> the allocation is not MADV_HUGEPAGE, then the page fault handler will
> default to the base size allocation. The caveat is that khugepaged can
> still operate on pages that are not MADV_HUGEPAGE.
>
> This allows for three things... one, applications specifically designed to
> use hugepages will get them, and two, applications that don't use
> hugepages can still benefit from them without aggressively inserting
> THPs at every possible chance. This curbs the memory waste, and defers
> the use of hugepages to khugepaged. Khugepaged can then scan the memory
> for eligible collapsing. Lastly there is the added benefit for those who
> want THPs but experience higher latency PFs. Now you can get base page
> performance at the PF handler and Hugepage performance for those mappings
> after they collapse.
>
> Admins may want to lower max_ptes_none, if not, khugepaged may
> aggressively collapse single allocations into hugepages.
>
> TESTING:
> - Built for x86_64, aarch64, ppc64le, and s390x
> - selftests mm
> - In [1] I provided a script [2] that has multiple access patterns
> - lots of general use.
> - redis testing. This test was my original case for the defer mode. What I
>    was able to prove was that THP=always leads to increased max_latency
>    cases; hence why it is recommended to disable THPs for redis servers.
>    However with 'defer' we dont have the max_latency spikes and can still
>    get the system to utilize THPs. I further tested this with the mTHP
>    defer setting and found that redis (and probably other jmalloc users)
>    can utilize THPs via defer (+mTHP defer) without a large latency
>    penalty and some potential gains. I uploaded some mmtest results
>    here[3] which compares:
>        stock+thp=never
>        stock+(m)thp=always
>        khugepaged-mthp + defer (max_ptes_none=64)
>
>   The results show that (m)THPs can cause some throughput regression in
>   some cases, but also has gains in other cases. The mTHP+defer results
>   have more gains and less losses over the (m)THP=always case.
>
> V6 Changes:
> - nits
> - rebased dependent series and added review tags
>
> V5 Changes:
> - rebased dependent series
> - added reviewed-by tag on 2/4
>
> V4 Changes:
> - Minor Documentation fixes
> - rebased the dependent series [1] onto mm-unstable
>     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
>
> V3 Changes:
> - Combined the documentation commits into one, and moved a section to the
>   khugepaged mthp patchset
>
> V2 Changes:
> - base changes on mTHP khugepaged support
> - Fix selftests parsing issue
> - add mTHP defer option
> - add mTHP defer Documentation
>
> [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> [2] - https://gitlab.com/npache/khugepaged_mthp_test
> [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>
> Nico Pache (4):
>   mm: defer THP insertion to khugepaged
>   mm: document (m)THP defer usage
>   khugepaged: add defer option to mTHP options
>   selftests: mm: add defer to thp setting parser
>
>  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
>  include/linux/huge_mm.h                    | 18 +++++-
>  mm/huge_memory.c                           | 69 +++++++++++++++++++---
>  mm/khugepaged.c                            |  8 +--
>  tools/testing/selftests/mm/thp_settings.c  |  1 +
>  tools/testing/selftests/mm/thp_settings.h  |  1 +
>  6 files changed, 106 insertions(+), 22 deletions(-)
>
> --
> 2.49.0
>
>

Hello Nico,

Upon reviewing the series, it occurred to me that BPF could solve this
more cleanly. Adding a 'tva_flags' parameter to the BPF hook would
handle this case and future scenarios without requiring new modes. The
BPF mode could then serve as a unified solution.

-- 
Regards
Yafang