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

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

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.

>
> 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.

-- 
Regards
Yafang