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: <6a2a84f5-8474-432f-b97e-18552a9d993c@redhat.com>
Date: Tue, 15 Oct 2024 09:32:01 +0200
From: David Hildenbrand <david@...hat.com>
To: John Hubbard <jhubbard@...dia.com>, Yosry Ahmed <yosryahmed@...gle.com>,
 Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

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.

-- 
Cheers,

David / dhildenb