linux-kernel - Re: [PATCH v4 15/36] lib: introduce support for page allocation tagging

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Message-ID: <z3uitmi57ccg2iifn5nb3pav6skh4zjfvemhuxqdlmwdij3242@wx2lbakzwrxc>
Date: Tue, 27 Feb 2024 04:45:54 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, 
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Subject: Re: [PATCH v4 15/36] lib: introduce support for page allocation
 tagging

On Tue, Feb 27, 2024 at 10:30:53AM +0100, Vlastimil Babka wrote:
> 
> 
> On 2/26/24 18:11, Suren Baghdasaryan wrote:
> > On Mon, Feb 26, 2024 at 9:07 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> On 2/21/24 20:40, Suren Baghdasaryan wrote:
> >>> Introduce helper functions to easily instrument page allocators by
> >>> storing a pointer to the allocation tag associated with the code that
> >>> allocated the page in a page_ext field.
> >>>
> >>> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >>> Co-developed-by: Kent Overstreet <kent.overstreet@...ux.dev>
> >>> Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> >>
> >> The static key usage seems fine now. Even if the page_ext overhead is still
> >> always paid when compiled in, you mention in the cover letter there's a plan
> >> for boot-time toggle later, so
> > 
> > Yes, I already have a simple patch for that to be included in the next
> > revision: https://github.com/torvalds/linux/commit/7ca367e80232345f471b77b3ea71cf82faf50954
> 
> This opt-out logic would require a distro kernel with allocation
> profiling compiled-in to ship together with something that modifies
> kernel command line to disable it by default, so it's not very
> practical. Could the CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT be
> turned into having 3 possible choices, where one of them would
> initialize mem_profiling_enabled to false?
> 
> Or, taking a step back, is it going to be a common usecase to pay the
> memory overhead unconditionally, but only enable the profiling later
> during runtime? Also what happens if someone would enable and disable it
> multiple times during one boot? Would the statistics get all skewed
> because some frees would be not accounted while it's disabled?

I already wrote the code for fast lookup from codetag index -> codetag -
i.e. pointer compression - so this is all going away shortly.

It just won't be in the initial pull request because of other
dependencies (it requires my eytzinger code, which I was already lifting
from fs/bcachefs/ for 6.9), but it can still probably make 6.9 in a
second smaller pull.