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

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

On Tue, May 27, 2025 at 8:19 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 27.05.25 11:43, Yafang Shao wrote:
> > On Tue, May 27, 2025 at 5:27 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 27.05.25 10:40, Yafang Shao wrote:
> >>> On Tue, May 27, 2025 at 4:30 PM David Hildenbrand <david@redhat.com> wrote:
> >>>>
> >>>>>> I don't think we want to add such a mechanism (new mode) where the
> >>>>>> primary configuration mechanism is through bpf.
> >>>>>>
> >>>>>> Maybe bpf could be used as an alternative, but we should look into a
> >>>>>> reasonable alternative first, like the discussed mctrl()/.../ raised in
> >>>>>> the process_madvise() series.
> >>>>>>
> >>>>>> No "bpf" mode in disguise, please :)
> >>>>>
> >>>>> This goal can be readily achieved using a BPF program. In any case, it
> >>>>> is a feasible solution.
> >>>>
> >>>> No BPF-only solution.
> >>>>
> >>>>>
> >>>>>>
> >>>>>>> We could define
> >>>>>>> the API as follows:
> >>>>>>>
> >>>>>>> struct bpf_thp_ops {
> >>>>>>>            /**
> >>>>>>>             * @task_thp_mode: Get the THP mode for a specific task
> >>>>>>>             *
> >>>>>>>             * Return:
> >>>>>>>             * - TASK_THP_ALWAYS: "always" mode
> >>>>>>>             * - TASK_THP_MADVISE: "madvise" mode
> >>>>>>>             * - TASK_THP_NEVER: "never" mode
> >>>>>>>             * Future modes can also be added.
> >>>>>>>             */
> >>>>>>>            int (*task_thp_mode)(struct task_struct *p);
> >>>>>>> };
> >>>>>>>
> >>>>>>> For observability, we could add a "THP mode" field to
> >>>>>>> /proc/[pid]/status. For example:
> >>>>>>>
> >>>>>>> $ grep "THP mode" /proc/123/status
> >>>>>>> always
> >>>>>>> $ grep "THP mode" /proc/456/status
> >>>>>>> madvise
> >>>>>>> $ grep "THP mode" /proc/789/status
> >>>>>>> never
> >>>>>>>
> >>>>>>> The THP mode for each task would be determined by the attached BPF
> >>>>>>> program based on the task's attributes. We would place the BPF hook in
> >>>>>>> appropriate kernel functions. Note that this setting wouldn't be
> >>>>>>> inherited during fork/exec - the BPF program would make the decision
> >>>>>>> dynamically for each task.
> >>>>>>
> >>>>>> What would be the mode (default) when the bpf program would not be active?
> >>>>>>
> >>>>>>> This approach also enables runtime adjustments to THP modes based on
> >>>>>>> system-wide conditions, such as memory fragmentation or other
> >>>>>>> performance overheads. The BPF program could adapt policies
> >>>>>>> dynamically, optimizing THP behavior in response to changing
> >>>>>>> workloads.
> >>>>>>
> >>>>>> I am not sure that is the proper way to handle these scenarios: I never
> >>>>>> heard that people would be adjusting the system-wide policy dynamically
> >>>>>> in that way either.
> >>>>>>
> >>>>>> Whatever we do, we have to make sure that what we add won't
> >>>>>> over-complicate things in the future. Having tooling dynamically adjust
> >>>>>> the THP policy of processes that coarsely sounds ... very wrong long-term.
> >>>>>
> >>>>> This is just an example demonstrating how BPF can be used to adjust
> >>>>> its flexibility. Notably, all these policies can be implemented
> >>>>> without modifying the kernel.
> >>>>
> >>>> See below on "policy".
> >>>>
> >>>>>
> >>>>>>
> >>>>>>     > > As Liam pointed out in another thread, naming is challenging here -
> >>>>>>> "process" might not be the most accurate term for this context.
> >>>>>>
> >>>>>> No, it's not even a per-process thing. It is per MM, and a MM might be
> >>>>>> used by multiple processes ...
> >>>>>
> >>>>> I consistently use 'thread' for the latter case.
> >>>>
> >>>> You can use CLONE_VM without CLONE_THREAD ...
> >>>
> >>> If I understand correctly, this can only occur for shared THP but not
> >>> anonymous THP. For instance, if either process allocates an anonymous
> >>> THP, it would trigger the creation of a new MM. Please correct me if
> >>> I'm mistaken.
> >>
> >> What clone(CLONE_VM) will do is essentially create a new process, that
> >> shares the MM with the original process. Similar to a thread, just that
> >> the new process will show up in /proc/ as ... a new process, not as a
> >> thread under /prod/$pid/tasks of the original process.
> >>
> >> Both processes will operate on the shared MM struct as if they were
> >> ordinary threads. No Copy-on-Write involved.
> >>
> >> One example use case I've been involved in is async teardown in QEMU [1].
> >>
> >> [1] https://kvm-forum.qemu.org/2022/ibm_async_destroy.pdf
> >
> > I understand what you mean, but what I'm really confused about is how
> > this relates to allocating anonymous THP.  If either one allocates
> > anon THP, it will definitely create a new MM, right ?
>
> No. They work on the same address space - same MM. Either can allocate a
> new anon THP and the other one would be able to modify it. No fork/CoW.
>
> I only bring it up because it's two "processes" sharing the same MM. And
> the THP mode in your proposal would actually be per-MM and not per process.
>
> It's confusing ... :)

Thanks for the explanation.

>
> >
> >>
> >>>
> >>>>
> >>>> Additionally, this
> >>>>> can be implemented per-MM without kernel code modifications.
> >>>>> With a well-designed API, users can even implement custom THP
> >>>>> policies—all without altering kernel code.
> >>>>
> >>>> You can switch between modes, that' all you can do. I wouldn't really
> >>>> call that "custom policy" as it is extremely limited.
> >>>>
> >>>> And that's exactly my point: it's basic switching between modes ... a
> >>>> reasonable policy in the future will make placement decisions and not
> >>>> just state "always/never/madvise".
> >>>
> >>> Could you please elaborate further on 'make placement decisions'? As
> >>> previously mentioned, we (including the broader community) really need
> >>> the user input to determine whether THP allocation is appropriate in a
> >>> given case.
> >>
> >> The glorious future were we make smarter decisions where to actually
> >> place THPs even in the "always" mode.
> >>
> >> E.g., just because we enable "always" for a process does not mean that
> >> we really want a THP everywhere; quite the opposite.
> >
> > So 'always' simply means "the system doesn't guarantee THP allocation
> > will succeed" ?
>
> I mean, with THPs, there are no guarantees, ever :(
>
> > If that's the case, we should revisit RFC v1 [0],
> > where we proposed rejecting THP allocations in certain scenarios for
> > specific tasks.
>
> Hooking into actual page allocation during page faults (e.g., THP size,
> khugepaged collapse decisions) is IMHO a much better application of ebpf
> than setting a THP mode per process (or MM ... ) using epbf.
>
> So yes, you could drive the system in "always" mode and decide to not
> allocate THPs during page faults / khugepaged for specific processes.
>
> IMHO that also does not contradict the VM_HUGEPAGE / VM_NOHUGEPAGE
> default setting proposal: VM_HUGEPAGE could feed into the epbf program
> as yet another parameter to make a decision.

That seems like a viable solution. Thank you for your help.

-- 
Regards
Yafang