Message-ID: <CAJuCfpFBov_2F9Kx5Csio=hOe8kY1yXjmg_z8dXU=ZUQ_-wmaQ@mail.gmail.com>
Date: Wed, 17 Sep 2025 16:04:48 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org, kent.overstreet@...ux.dev, 
	hannes@...xchg.org, rientjes@...gle.com, roman.gushchin@...ux.dev, 
	harry.yoo@...cle.com, shakeel.butt@...ux.dev, 00107082@....com, 
	pyyjason@...il.com, pasha.tatashin@...een.com, souravpanda@...gle.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] alloc_tag: mark inaccurate allocation counters in
 /proc/allocinfo output

On Wed, Sep 17, 2025 at 2:10 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 16/09/2025 23:27, Suren Baghdasaryan wrote:
> > On Tue, Sep 16, 2025 at 10:26 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >>
> >> On Tue, Sep 16, 2025 at 9:52 PM Usama Arif <usamaarif642@...il.com> wrote:
> >>>
> >>>
> >>>
> >>> On 16/09/2025 22:46, Suren Baghdasaryan wrote:
> >>>> On Tue, Sep 16, 2025 at 2:11 PM Usama Arif <usamaarif642@...il.com> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 16/09/2025 16:51, Suren Baghdasaryan wrote:
> >>>>>> On Tue, Sep 16, 2025 at 5:57 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>>>>>>
> >>>>>>> On 9/16/25 01:02, Suren Baghdasaryan wrote:
> >>>>>>>> While rare, memory allocation profiling can contain inaccurate counters
> >>>>>>>> if slab object extension vector allocation fails. That allocation might
> >>>>>>>> succeed later but prior to that, slab allocations that would have used
> >>>>>>>> that object extension vector will not be accounted for. To indicate
> >>>>>>>> incorrect counters, "accurate:no" marker is appended to the call site
> >>>>>>>> line in the /proc/allocinfo output.
> >>>>>>>> Bump up /proc/allocinfo version to reflect the change in the file format
> >>>>>>>> and update documentation.
> >>>>>>>>
> >>>>>>>> Example output with invalid counters:
> >>>>>>>> allocinfo - version: 2.0
> >>>>>>>>            0        0 arch/x86/kernel/kdebugfs.c:105 func:create_setup_data_nodes
> >>>>>>>>            0        0 arch/x86/kernel/alternative.c:2090 func:alternatives_smp_module_add
> >>>>>>>>            0        0 arch/x86/kernel/alternative.c:127 func:__its_alloc accurate:no
> >>>>>>>>            0        0 arch/x86/kernel/fpu/regset.c:160 func:xstateregs_set
> >>>>>>>>            0        0 arch/x86/kernel/fpu/xstate.c:1590 func:fpstate_realloc
> >>>>>>>>            0        0 arch/x86/kernel/cpu/aperfmperf.c:379 func:arch_enable_hybrid_capacity_scale
> >>>>>>>>            0        0 arch/x86/kernel/cpu/amd_cache_disable.c:258 func:init_amd_l3_attrs
> >>>>>>>>        49152       48 arch/x86/kernel/cpu/mce/core.c:2709 func:mce_device_create accurate:no
> >>>>>>>>        32768        1 arch/x86/kernel/cpu/mce/genpool.c:132 func:mce_gen_pool_create
> >>>>>>>>            0        0 arch/x86/kernel/cpu/mce/amd.c:1341 func:mce_threshold_create_device
> >>>>>>>>
> >>>>>>>> Suggested-by: Johannes Weiner <hannes@...xchg.org>
> >>>>>>>> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >>>>>>>> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> >>>>>>>> Acked-by: Usama Arif <usamaarif642@...il.com>
> >>>>>>>> Acked-by: Johannes Weiner <hannes@...xchg.org>
> >>>>>>>
> >>>>>>> With this format you could instead print the accumulated size of allocations
> >>>>>>> that could not allocate their objext (for the given tag). It should be then
> >>>>>>> an upper bound of the actual error, because obviously we cannot recognize
> >>>>>>> moments where these allocations are freed - so we don't know for which tag
> >>>>>>> to decrement. Maybe it could be more useful output than the yes/no
> >>>>>>> information, although of course require more storage in struct codetag, so I
> >>>>>>> don't know if it's worth it.
> >>>>>>
> >>>>>> Yeah, I'm reluctant to add more fields to the codetag and increase the
> >>>>>> overhead until we have a usecases. If that happens and with the new
> >>>>>> format we can add something like error_size:<value> to indicate the
> >>>>>> amount of the error.
> >>>>>>
> >>>>>>>
> >>>>>>> Maybe a global counter of sum size for all these missed objexts could be
> >>>>>>> also maintained, and that wouldn't be an upper bound but an actual current
> >>>>>>> error, that is if we can precisely determine that when freeing an object, we
> >>>>>>> don't have a tag to decrement because objext allocation had failed on it and
> >>>>>>> thus that allocation had incremented this global error counter and it's
> >>>>>>> correct to decrement it.
> >>>>>>
> >>>>>> That's a good idea and should be doable without too much overhead. Thanks!
> >>>>>> For the UAPI... I think for this case IOCTL would work and the use
> >>>>>> scenario would be that the user sees the "accurate:no" mark and issues
> >>>>>> ioctl command to retrieve this global counter value.
> >>>>>> Usama, since you initiated this feature request, do you think such a
> >>>>>> counter would be useful?
> >>>>>>
> >>>>>
> >>>>>
> >>>>> hmm, I really dont like suggesting changing /proc/allocinfo as it will break parsers,
> >>>>> but it might be better to put it there?
> >>>>> If the value is in the file, I imagine people will be more prone to looking at it?
> >>>>> I am not completely sure if everyone will do an ioctl to try and find this out?
> >>>>> Especially if you just have infra that is just automatically collecting info from
> >>>>> this file.
> >>>>
> >>>> The current file reports per-codetag data and not global counters. We
> >>>> could report it somewhere in the header but the first question to
> >>>> answer is: would this be really useful (not in a way of  "nice to
> >>>> have" but for a concrete usecase)? If not then I would suggest keeping
> >>>> things simple until there is a need for it.
> >>>>
> >>>
> >>> I think its a nice to have. I can't think of a concrete usecase at present.
> >>>
> >>> I guess a potential usecase is if you are trying to use memory allocation
> >>> profiling to debug OOMs and the missed objects size is very large. I guess we
> >>> wont know until this happens, but I would hope this number is usually small.
> >>
> >> Hmm. Missing a large allocation and not knowing about it can be a problem...
> >> I'll start sketching a patch to see if tracking such a global counter
> >> has any drawbacks and in the meantime I'm open to suggestions on how
> >> to expose it to the userspace.
> >>
> >> About concerns on the IOCTL interface, would it be more usable if we
> >> get the alloctop [1] or a similar tool which can be used to easily
> >> issue such commands into kernel/tools?
> >>
> >> [1] https://android-review.git.corp.google.com/c/platform/system/memory/libmeminfo/+/3431860
> >
> > Ugh, sorry. Externally accesible link would be
> > https://android-review.googlesource.com/c/platform/system/memory/libmeminfo/+/3431860
> >
>
> Yeah this would be nice to have. We do have something very similar in our infra, to basically
> sort by size and store only top x entries.
>
> When doing manually, I just do sort -g /proc/allocinfo|tail -n 30|numfmt --to=iec which is copied from
> the kernel doc.

Got it. I guess if we get an upstream tool like that which is kept
in-sync with kernel's UAPI and new features, that would make the
maintenance easier for everyone.

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