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Date: Thu, 7 Mar 2024 13:17:48 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Randy Dunlap <rdunlap@infradead.org>
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Subject: Re: [PATCH v5 37/37] memprofiling: Documentation
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On Wed, Mar 06, 2024 at 07:18:57PM -0800, Randy Dunlap wrote:
> Hi,
> This includes some editing suggestions and some doc build fixes.
> 
> 
> On 3/6/24 10:24, Suren Baghdasaryan wrote:
> > From: Kent Overstreet <kent.overstreet@linux.dev>
> > 
> > Provide documentation for memory allocation profiling.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  Documentation/mm/allocation-profiling.rst | 91 +++++++++++++++++++++++
> >  1 file changed, 91 insertions(+)
> >  create mode 100644 Documentation/mm/allocation-profiling.rst
> > 
> > diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst
> > new file mode 100644
> > index 000000000000..8a862c7d3aab
> > --- /dev/null
> > +++ b/Documentation/mm/allocation-profiling.rst
> > @@ -0,0 +1,91 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===========================
> > +MEMORY ALLOCATION PROFILING
> > +===========================
> > +
> > +Low overhead (suitable for production) accounting of all memory allocations,
> > +tracked by file and line number.
> > +
> > +Usage:
> > +kconfig options:
> > + - CONFIG_MEM_ALLOC_PROFILING
> > + - CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
> > + - CONFIG_MEM_ALLOC_PROFILING_DEBUG
> > +   adds warnings for allocations that weren't accounted because of a
> > +   missing annotation
> > +
> > +Boot parameter:
> > +  sysctl.vm.mem_profiling=0|1|never
> > +
> > +  When set to "never", memory allocation profiling overheads is minimized and it
> 
>                                                       overhead is
> 
> > +  cannot be enabled at runtime (sysctl becomes read-only).
> > +  When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y, default value is "1".
> > +  When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n, default value is "never".
> > +
> > +sysctl:
> > +  /proc/sys/vm/mem_profiling
> > +
> > +Runtime info:
> > +  /proc/allocinfo
> > +
> > +Example output:
> > +  root@moria-kvm:~# sort -g /proc/allocinfo|tail|numfmt --to=iec
> > +        2.8M    22648 fs/kernfs/dir.c:615 func:__kernfs_new_node
> > +        3.8M      953 mm/memory.c:4214 func:alloc_anon_folio
> > +        4.0M     1010 drivers/staging/ctagmod/ctagmod.c:20 [ctagmod] func:ctagmod_start
> > +        4.1M        4 net/netfilter/nf_conntrack_core.c:2567 func:nf_ct_alloc_hashtable
> > +        6.0M     1532 mm/filemap.c:1919 func:__filemap_get_folio
> > +        8.8M     2785 kernel/fork.c:307 func:alloc_thread_stack_node
> > +         13M      234 block/blk-mq.c:3421 func:blk_mq_alloc_rqs
> > +         14M     3520 mm/mm_init.c:2530 func:alloc_large_system_hash
> > +         15M     3656 mm/readahead.c:247 func:page_cache_ra_unbounded
> > +         55M     4887 mm/slub.c:2259 func:alloc_slab_page
> > +        122M    31168 mm/page_ext.c:270 func:alloc_page_ext
> > +===================
> > +Theory of operation
> > +===================
> > +
> > +Memory allocation profiling builds off of code tagging, which is a library for
> > +declaring static structs (that typcially describe a file and line number in
> 
>                                   typically
> 
> > +some way, hence code tagging) and then finding and operating on them at runtime
> 
>                                                                         at runtime,
> 
> > +- i.e. iterating over them to print them in debugfs/procfs.
> 
>   i.e., iterating

i.e. latin id est, that is: grammatically my version is fine

> 
> > +
> > +To add accounting for an allocation call, we replace it with a macro
> > +invocation, alloc_hooks(), that
> > + - declares a code tag
> > + - stashes a pointer to it in task_struct
> > + - calls the real allocation function
> > + - and finally, restores the task_struct alloc tag pointer to its previous value.
> > +
> > +This allows for alloc_hooks() calls to be nested, with the most recent one
> > +taking effect. This is important for allocations internal to the mm/ code that
> > +do not properly belong to the outer allocation context and should be counted
> > +separately: for example, slab object extension vectors, or when the slab
> > +allocates pages from the page allocator.
> > +
> > +Thus, proper usage requires determining which function in an allocation call
> > +stack should be tagged. There are many helper functions that essentially wrap
> > +e.g. kmalloc() and do a little more work, then are called in multiple places;
> > +we'll generally want the accounting to happen in the callers of these helpers,
> > +not in the helpers themselves.
> > +
> > +To fix up a given helper, for example foo(), do the following:
> > + - switch its allocation call to the _noprof() version, e.g. kmalloc_noprof()
> > + - rename it to foo_noprof()
> > + - define a macro version of foo() like so:
> > +   #define foo(...) alloc_hooks(foo_noprof(__VA_ARGS__))
> > +
> > +It's also possible to stash a pointer to an alloc tag in your own data structures.
> > +
> > +Do this when you're implementing a generic data structure that does allocations
> > +"on behalf of" some other code - for example, the rhashtable code. This way,
> > +instead of seeing a large line in /proc/allocinfo for rhashtable.c, we can
> > +break it out by rhashtable type.
> > +
> > +To do so:
> > + - Hook your data structure's init function, like any other allocation function
> 
> maybe end the line above with a '.' like the following line.
> 
> > + - Within your init function, use the convenience macro alloc_tag_record() to
> > +   record alloc tag in your data structure.
> > + - Then, use the following form for your allocations:
> > +   alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...))
> 
> 
> Finally, there are a number of documentation build warnings in this patch.
> I'm no ReST expert, but the attached patch fixes them for me.
> 
> -- 
> #Randy