Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag refs in page flags

[Suren Baghdasaryan]

On Tue, Oct 15, 2024 at 8:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 15.10.24 16:59, Suren Baghdasaryan wrote:
> > On Tue, Oct 15, 2024 at 12:32 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 15.10.24 01:53, John Hubbard wrote:
> >>> On 10/14/24 4:48 PM, Yosry Ahmed wrote:
> >>>> On Mon, Oct 14, 2024 at 1:37 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >>>>>
> >>>>> Add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to store allocation tag
> >>>>> references directly in the page flags. This eliminates memory
> >>>>> overhead caused by page_ext and results in better performance
> >>>>> for page allocations.
> >>>>> If the number of available page flag bits is insufficient to
> >>>>> address all kernel allocations, profiling falls back to using
> >>>>> page extensions with an appropriate warning to disable this
> >>>>> config.
> >>>>> If dynamically loaded modules add enough tags that they can't
> >>>>> be addressed anymore with available page flag bits, memory
> >>>>> profiling gets disabled and a warning is issued.
> >>>>
> >>>> Just curious, why do we need a config option? If there are enough bits
> >>>> in page flags, why not use them automatically or fallback to page_ext
> >>>> otherwise?
> >>>
> >>> Or better yet, *always* fall back to page_ext, thus leaving the
> >>> scarce and valuable page flags available for other features?
> >>>
> >>> Sorry Suren, to keep coming back to this suggestion, I know
> >>> I'm driving you crazy here! But I just keep thinking it through
> >>> and failing to see why this feature deserves to consume so
> >>> many page flags.
> >>
> >> My 2 cents: there is nothing wrong about consuming unused page flags in
> >> a configuration. No need to let them stay unused in a configuration :)
> >>
> >> The real issue starts once another feature wants to make use of some of
> >> them ... in such configuration there would be less available for
> >> allocation tags and the performance of allocations tags might
> >> consequently get worse again.
> >
> > Thanks for the input and indeed this is the case. If this happens, we
> > will get a warning telling us that page flags could not be used and
> > page_ext will be used instead. I think that's the best I can do given
> > that page flag bits is a limited resource.
>
> Right, I think what John is concerned about (and me as well) is that
> once a new feature really needs a page flag, there will be objection
> like "no you can't, we need them for allocation tags otherwise that
> feature will be degraded".

I do understand your concern but IMHO the possibility of degrading a
feature should not be a reason to always operate at degraded capacity
(which is what we have today). If one is really concerned about
possible future regression they can set
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS=n and keep what we have today. That's
why I'm strongly advocating that we do need
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS so that the user has control over how
this scarce resource is used.

>
> So a "The Lord has given, and the Lord has taken away!" mentality might
> be required when consuming that many scarce resources, meaning, as long
> as they are actually unused, use them, but it should not block other
> features that really need them.

I agree and I think that's what I implemented here. If there are
enough page flag bits we use them, otherwise we automatically fall
back to page_ext.

>
> Does that make sense?

Absolutely and thank you all for the feedback.

>
> --
> Cheers,
>
> David / dhildenb
>