Re: [PATCH v2 1/1] alloc_tag: mark inaccurate allocation counters in /proc/allocinfo output

[Usama Arif <usamaarif642@gmail.com>]

On 16/09/2025 22:46, Suren Baghdasaryan wrote:
> On Tue, Sep 16, 2025 at 2:11 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 16/09/2025 16:51, Suren Baghdasaryan wrote:
>>> On Tue, Sep 16, 2025 at 5:57 AM Vlastimil Babka <vbabka@suse.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@cmpxchg.org>
>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>>>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>>>>> Acked-by: Usama Arif <usamaarif642@gmail.com>
>>>>> Acked-by: Johannes Weiner <hannes@cmpxchg.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.