Re: [DISCUSSION] proposed mctl() API

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> On Thu, May 29, 2025 at 03:43:26PM +0100, Lorenzo Stoakes wrote:
> > After discussions in various threads (Usama's series adding a new prctl()
> > in [0], and a proposal to adapt process_madvise() to do the same -
> > conception in [1] and RFC in [2]), it seems fairly clear that it would make
> > sense to explore a dedicated API to explicitly allow for actions which
> > affect the virtual address space as a whole.
> >
> > Also, Barry is implementing a feature (currently under RFC) which could
> > additionally make use of this API (see [3]).
>
> I think the reason that you're having trouble coming up with a good
> place to put these ideas is because they are all bad ideas.  Do none of
> them.  Problem solved.
>
> People should put more effort into allocating THPs automatically and
> monitoring where they're helping performance and where they're hurting
> performance, instead of coming up with these baroque reasons to blame
> the sysadmin for not having tweaked some magic knob.
>
> Barry's problem is that we're all nervous about possibly regressing
> performance on some unknown workloads.  Just try Barry's proposal, see
> if anyone actually compains or if we're just afraid of our own shadows.
>

So from my perspective - I very much agree with the concept of doing nothing
here, ideally :)

But I feel we need to look at this problem from both the short term and the
long term - in the long run I absolutely agree with what you say.

In the short term this proposal is broadly 'what is the least worst means
of establishing policy like this?'.

I think there is broad agreement that prctl() is sub-optimal, so an
mm-specific API makes sense.

So it comes down to a question of - how badly do we need to be able to do
this _now_?