Re: [RFC PATCH v2 0/5] mm, bpf: BPF based THP adjustment

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

On Mon, May 26, 2025 at 10:32 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 24 May 2025, at 23:01, Yafang Shao wrote:
>
> > On Tue, May 20, 2025 at 2:05 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >>
> >> Background
> >> ----------
> >>
> >> At my current employer, PDD, we have consistently configured THP to "never"
> >> on our production servers due to past incidents caused by its behavior:
> >>
> >> - Increased memory consumption
> >>   THP significantly raises overall memory usage.
> >>
> >> - Latency spikes
> >>   Random latency spikes occur due to more frequent memory compaction
> >>   activity triggered by THP.
> >>
> >> These issues have made sysadmins hesitant to switch to "madvise" or
> >> "always" modes.
> >>
> >> New Motivation
> >> --------------
> >>
> >> We have now identified that certain AI workloads achieve substantial
> >> performance gains with THP enabled. However, we’ve also verified that some
> >> workloads see little to no benefit—or are even negatively impacted—by THP.
> >>
> >> In our Kubernetes environment, we deploy mixed workloads on a single server
> >> to maximize resource utilization. Our goal is to selectively enable THP for
> >> services that benefit from it while keeping it disabled for others. This
> >> approach allows us to incrementally enable THP for additional services and
> >> assess how to make it more viable in production.
> >>
> >> Proposed Solution
> >> -----------------
> >>
> >> For this use case, Johannes suggested introducing a dedicated mode [0]. In
> >> this new mode, we could implement BPF-based THP adjustment for fine-grained
> >> control over tasks or cgroups. If no BPF program is attached, THP remains
> >> in "never" mode. This solution elegantly meets our needs while avoiding the
> >> complexity of managing BPF alongside other THP modes.
> >>
> >> A selftest example demonstrates how to enable THP for the current task
> >> while keeping it disabled for others.
> >>
> >> Alternative Proposals
> >> ---------------------
> >>
> >> - Gutierrez’s cgroup-based approach [1]
> >>   - Proposed adding a new cgroup file to control THP policy.
> >>   - However, as Johannes noted, cgroups are designed for hierarchical
> >>     resource allocation, not arbitrary policy settings [2].
> >>
> >> - Usama’s per-task THP proposal based on prctl() [3]:
> >>   - Enabling THP per task via prctl().
> >>   - As David pointed out, neither madvise() nor prctl() works in "never"
> >>     mode [4], making this solution insufficient for our needs.
> >>
> >> Conclusion
> >> ----------
> >>
> >> Introducing a new "bpf" mode for BPF-based per-task THP adjustments is the
> >> most effective solution for our requirements. This approach represents a
> >> small but meaningful step toward making THP truly usable—and manageable—in
> >> production environments.
> >>
> >> This is currently a PoC implementation. Feedback of any kind is welcome.
> >>
> >> Link: https://lore.kernel.org/linux-mm/20250509164654.GA608090@cmpxchg.org/ [0]
> >> Link: https://lore.kernel.org/linux-mm/20241030083311.965933-1-gutierrez.asier@huawei-partners.com/ [1]
> >> Link: https://lore.kernel.org/linux-mm/20250430175954.GD2020@cmpxchg.org/ [2]
> >> Link: https://lore.kernel.org/linux-mm/20250519223307.3601786-1-usamaarif642@gmail.com/ [3]
> >> Link: https://lore.kernel.org/linux-mm/41e60fa0-2943-4b3f-ba92-9f02838c881b@redhat.com/ [4]
> >>
> >> RFC v1->v2:
> >> The main changes are as follows,
> >> - Use struct_ops instead of fmod_ret (Alexei)
> >> - Introduce a new THP mode (Johannes)
> >> - Introduce new helpers for BPF hook (Zi)
> >> - Refine the commit log
> >>
> >> RFC v1: https://lwn.net/Articles/1019290/
> >>
> >> Yafang Shao (5):
> >>   mm: thp: Add a new mode "bpf"
> >>   mm: thp: Add hook for BPF based THP adjustment
> >>   mm: thp: add struct ops for BPF based THP adjustment
> >>   bpf: Add get_current_comm to bpf_base_func_proto
> >>   selftests/bpf: Add selftest for THP adjustment
> >>
> >>  include/linux/huge_mm.h                       |  15 +-
> >>  kernel/bpf/cgroup.c                           |   2 -
> >>  kernel/bpf/helpers.c                          |   2 +
> >>  mm/Makefile                                   |   3 +
> >>  mm/bpf_thp.c                                  | 120 ++++++++++++
> >>  mm/huge_memory.c                              |  65 ++++++-
> >>  mm/khugepaged.c                               |   3 +
> >>  tools/testing/selftests/bpf/config            |   1 +
> >>  .../selftests/bpf/prog_tests/thp_adjust.c     | 175 ++++++++++++++++++
> >>  .../selftests/bpf/progs/test_thp_adjust.c     |  39 ++++
> >>  10 files changed, 414 insertions(+), 11 deletions(-)
> >>  create mode 100644 mm/bpf_thp.c
> >>  create mode 100644 tools/testing/selftests/bpf/prog_tests/thp_adjust.c
> >>  create mode 100644 tools/testing/selftests/bpf/progs/test_thp_adjust.c
> >>
> >> --
> >> 2.43.5
> >>
> >
> > Hi all,
> >
> > Let’s summarize the current state of the discussion and identify how
> > to move forward.
> >
> > - Global-Only Control is Not Viable
> > We all seem to agree that a global-only control for THP is unwise. In
> > practice, some workloads benefit from THP while others do not, so a
> > one-size-fits-all approach doesn’t work.
> >
> > - Should We Use "Always" or "Madvise"?
> > I suspect no one would choose 'always' in its current state. ;)
> > Both Lorenzo and David propose relying on the madvise mode. However,
> > since madvise is an unprivileged userspace mechanism, any user can
> > freely adjust their THP policy. This makes fine-grained control
> > impossible without breaking userspace compatibility—an undesirable
> > tradeoff.
> > Given these limitations, the community should consider introducing a
> > new "admin" mode for privileged THP policy management.
> >
>
> I agree with the above two points.
>
> > - Can the Kernel Automatically Manage THP Without User Input?
> > In practice, users define their own success metrics—such as latency
> > (RT), queries per second (QPS), or throughput—to evaluate a feature’s
> > usefulness. If a feature fails to improve these metrics, it provides
> > no practical value.
> > Currently, the kernel lacks visibility into user-defined metrics,
> > making fully automated optimization impossible (at least without user
> > input). More importantly, automatic management offers no benefit if it
> > doesn’t align with user needs.
>
> Yes, kernel is basically guessing what userspace wants with some hints
> like MADV_HUGEPAGE/MADV_NOHUGEPAGE. But kernel has the global view
> of memory fragmentation, which userspace cannot get easily.

Correct, memory fragmentation is another critical factor in
determining whether to allocate THP.

> I wonder
> if it is possible that userspace tuning might benefit one set of
> applications but hurt others or overall performance. Right now,
> THP tuning is 0 or 1, either an application wants THPs or not.
> We might need a way of ranking THP requests from userspace to
> let kernel prioritize them (I am not sure if we can add another
> user input parameter, like THP_nice, to get this done, since
> apparently everyone will set THP_nice to -100 to get themselves
> at the top of the list).

Interesting idea. Perhaps we could make this configurable only by sysadmins.

-- 
Regards
Yafang