Date:   Wed, 3 May 2023 09:25:29 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH 00/40] Memory allocation profiling

On Mon 01-05-23 09:54:10, Suren Baghdasaryan wrote:
> Memory allocation profiling infrastructure provides a low overhead
> mechanism to make all kernel allocations in the system visible. It can be
> used to monitor memory usage, track memory hotspots, detect memory leaks,
> identify memory regressions.
> 
> To keep the overhead to the minimum, we record only allocation sizes for
> every allocation in the codebase. With that information, if users are
> interested in more detailed context for a specific allocation, they can
> enable in-depth context tracking, which includes capturing the pid, tgid,
> task name, allocation size, timestamp and call stack for every allocation
> at the specified code location.
[...]
> Implementation utilizes a more generic concept of code tagging, introduced
> as part of this patchset. Code tag is a structure identifying a specific
> location in the source code which is generated at compile time and can be
> embedded in an application-specific structure. A number of applications
> for code tagging have been presented in the original RFC [1].
> Code tagging uses the old trick of "define a special elf section for
> objects of a given type so that we can iterate over them at runtime" and
> creates a proper library for it. 
> 
> To profile memory allocations, we instrument page, slab and percpu
> allocators to record total memory allocated in the associated code tag at
> every allocation in the codebase. Every time an allocation is performed by
> an instrumented allocator, the code tag at that location increments its
> counter by allocation size. Every time the memory is freed the counter is
> decremented. To decrement the counter upon freeing, allocated object needs
> a reference to its code tag. Page allocators use page_ext to record this
> reference while slab allocators use memcg_data (renamed into more generic
> slabobj_ext) of the slab page.
[...]
> [1] https://lore.kernel.org/all/20220830214919.53220-1-surenb@google.com/
[...]
>  70 files changed, 2765 insertions(+), 554 deletions(-)

Sorry for cutting the cover considerably but I believe I have quoted the
most important/interesting parts here. The approach is not fundamentally
different from the previous version [1] and there was a significant
discussion around this approach. The cover letter doesn't summarize nor
deal with concerns expressed previous AFAICS. So let me bring those up
back. At least those I find the most important:
- This is a big change and it adds a significant maintenance burden
  because each allocation entry point needs to be handled specifically.
  The cost will grow with the intended coverage especially there when
  allocation is hidden in a library code.
- It has been brought up that this is duplicating functionality already
  available via existing tracing infrastructure. You should make it very
  clear why that is not suitable for the job
- We already have page_owner infrastructure that provides allocation
  tracking data. Why it cannot be used/extended?

Thanks!
-- 
Michal Hocko
SUSE Labs

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