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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 27, 2024 at 1:30 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
>
>
> On 2/26/24 18:11, Suren Baghdasaryan wrote:
> > On Mon, Feb 26, 2024 at 9:07 AM Vlastimil Babka <vbabka@suse.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@google.com>
> >>> Co-developed-by: Kent Overstreet <kent.overstreet@linux.dev>
> >>> Signed-off-by: Kent Overstreet <kent.overstreet@linux.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?

I was thinking about a similar approach of having the early boot
parameter to be a tri-state with "0 | 1 | Never". The default option
would be "Never" if CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n
and "1" if CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y. Would that
solve the problem for distributions?

>
> 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?

I think that would be the option one would use in the early
deployments, to be able to enable the feature on specific devices
without a reboot. Pasha brought up also an option when we disable the
feature initially (via early boot option) but can enable it and reboot
the system that will come up with enabled option.

As Kent mentioned, he has been working on a pointer compression
mechanism to cut the overhead of each codtag reference from one
pointer (8 bytes) to 2 bytes index. I'm yet to check the performance
but if that works and we can fit this index into page flags, that
would completely eliminate dependency on page_ext and this memory
overhead will be gone. This mechanism is not mature enough and I don't
want to include these optimizations into the initial patchset, that's
why it's not included in this patchset.

> 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?

Yes and this was discussed during last LSFMM when the runtime control
was brought up for the first time. That loss of accounting while the
feature is disabled seems to be expected and acceptable. One could
snapshot the state before re-enabling the feature and then compare
later results with the initial snapshot to figure out the allocation
growth.

>
> >>
> >> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> >
> > Thanks!
> >
> >>
> >>
>
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