Message-ID: <CAJuCfpHXAhGZb1aOPyHOPiTWSwQJi570THqJQcjrVPf=4Dt3xQ@mail.gmail.com>
Date: Tue, 16 Sep 2025 08:51:51 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, kent.overstreet@...ux.dev, hannes@...xchg.org, 
	usamaarif642@...il.com, 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 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?

>
> > ---
> > Changes since v1[1]:
> > - Changed the marker from asterisk to accurate:no pair, per Andrew Morton
> > - Documented /proc/allocinfo v2 format
> > - Update the changelog
> > - Added Acked-by from v2 since the functionality is the same,
> > per Shakeel Butt, Usama Arif and Johannes Weiner
> >
> > [1] https://lore.kernel.org/all/20250909234942.1104356-1-surenb@google.com/
> >
> >  Documentation/filesystems/proc.rst |  4 ++++
> >  include/linux/alloc_tag.h          | 12 ++++++++++++
> >  include/linux/codetag.h            |  5 ++++-
> >  lib/alloc_tag.c                    |  4 +++-
> >  mm/slub.c                          |  2 ++
> >  5 files changed, 25 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > index 915a3e44bc12..1776a06571c2 100644
> > --- a/Documentation/filesystems/proc.rst
> > +++ b/Documentation/filesystems/proc.rst
> > @@ -1009,6 +1009,10 @@ number, module (if originates from a loadable module) and the function calling
> >  the allocation. The number of bytes allocated and number of calls at each
> >  location are reported. The first line indicates the version of the file, the
> >  second line is the header listing fields in the file.
> > +If file version is 2.0 or higher then each line may contain additional
> > +<key>:<value> pairs representing extra information about the call site.
> > +For example if the counters are not accurate, the line will be appended with
> > +"accurate:no" pair.
> >
> >  Example output.
> >
> > diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> > index 9ef2633e2c08..d40ac39bfbe8 100644
> > --- a/include/linux/alloc_tag.h
> > +++ b/include/linux/alloc_tag.h
> > @@ -221,6 +221,16 @@ static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes)
> >       ref->ct = NULL;
> >  }
> >
> > +static inline void alloc_tag_set_inaccurate(struct alloc_tag *tag)
> > +{
> > +     tag->ct.flags |= CODETAG_FLAG_INACCURATE;
> > +}
> > +
> > +static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
> > +{
> > +     return !!(tag->ct.flags & CODETAG_FLAG_INACCURATE);
> > +}
> > +
> >  #define alloc_tag_record(p)  ((p) = current->alloc_tag)
> >
> >  #else /* CONFIG_MEM_ALLOC_PROFILING */
> > @@ -230,6 +240,8 @@ static inline bool mem_alloc_profiling_enabled(void) { return false; }
> >  static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag *tag,
> >                                size_t bytes) {}
> >  static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {}
> > +static inline void alloc_tag_set_inaccurate(struct alloc_tag *tag) {}
> > +static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag) { return false; }
> >  #define alloc_tag_record(p)  do {} while (0)
> >
> >  #endif /* CONFIG_MEM_ALLOC_PROFILING */
> > diff --git a/include/linux/codetag.h b/include/linux/codetag.h
> > index 457ed8fd3214..8ea2a5f7c98a 100644
> > --- a/include/linux/codetag.h
> > +++ b/include/linux/codetag.h
> > @@ -16,13 +16,16 @@ struct module;
> >  #define CODETAG_SECTION_START_PREFIX "__start_"
> >  #define CODETAG_SECTION_STOP_PREFIX  "__stop_"
> >
> > +/* codetag flags */
> > +#define CODETAG_FLAG_INACCURATE      (1 << 0)
> > +
> >  /*
> >   * An instance of this structure is created in a special ELF section at every
> >   * code location being tagged.  At runtime, the special section is treated as
> >   * an array of these.
> >   */
> >  struct codetag {
> > -     unsigned int flags; /* used in later patches */
> > +     unsigned int flags;
> >       unsigned int lineno;
> >       const char *modname;
> >       const char *function;
> > diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> > index 79891528e7b6..12ff80bbbd22 100644
> > --- a/lib/alloc_tag.c
> > +++ b/lib/alloc_tag.c
> > @@ -80,7 +80,7 @@ static void allocinfo_stop(struct seq_file *m, void *arg)
> >  static void print_allocinfo_header(struct seq_buf *buf)
> >  {
> >       /* Output format version, so we can change it. */
> > -     seq_buf_printf(buf, "allocinfo - version: 1.0\n");
> > +     seq_buf_printf(buf, "allocinfo - version: 2.0\n");
> >       seq_buf_printf(buf, "#     <size>  <calls> <tag info>\n");
> >  }
> >
> > @@ -92,6 +92,8 @@ static void alloc_tag_to_text(struct seq_buf *out, struct codetag *ct)
> >
> >       seq_buf_printf(out, "%12lli %8llu ", bytes, counter.calls);
> >       codetag_to_text(out, ct);
> > +     if (unlikely(alloc_tag_is_inaccurate(tag)))
> > +             seq_buf_printf(out, " accurate:no");
> >       seq_buf_putc(out, ' ');
> >       seq_buf_putc(out, '\n');
> >  }
> > diff --git a/mm/slub.c b/mm/slub.c
> > index af343ca570b5..9c04f29ee8de 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2143,6 +2143,8 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
> >        */
> >       if (likely(obj_exts))
> >               alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
> > +     else
> > +             alloc_tag_set_inaccurate(current->alloc_tag);
> >  }
> >
> >  static inline void
>

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