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

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Message-ID: <CAJuCfpGkuaCh+PxKbzMbu-81oeEdzcfjFThoRk+-Cezf0oJWZg@mail.gmail.com>
Date: Tue, 15 Oct 2024 07:59:08 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: John Hubbard <jhubbard@...dia.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	akpm@...ux-foundation.org, kent.overstreet@...ux.dev, corbet@....net, 
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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

On Tue, Oct 15, 2024 at 12:32 AM David Hildenbrand <david@...hat.com> 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@...gle.com> 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.

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