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

[David Hildenbrand]

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".

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.

Does that make sense?

--
Cheers,

David / dhildenb