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
