linux-kernel - Re: [PATCH v3 20/35] lib: add codetag reference into slabobj

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Date: Fri, 16 Feb 2024 09:04:06 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
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Subject: Re: [PATCH v3 20/35] lib: add codetag reference into slabobj_ext

On Fri, Feb 16, 2024 at 7:36 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/12/24 22:39, Suren Baghdasaryan wrote:
> > To store code tag for every slab object, a codetag reference is embedded
> > into slabobj_ext when CONFIG_MEM_ALLOC_PROFILING=y.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > Co-developed-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > ---
> >  include/linux/memcontrol.h | 5 +++++
> >  lib/Kconfig.debug          | 1 +
> >  mm/slab.h                  | 4 ++++
> >  3 files changed, 10 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index f3584e98b640..2b010316016c 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1653,7 +1653,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> >   * if MEMCG_DATA_OBJEXTS is set.
> >   */
> >  struct slabobj_ext {
> > +#ifdef CONFIG_MEMCG_KMEM
> >       struct obj_cgroup *objcg;
> > +#endif
> > +#ifdef CONFIG_MEM_ALLOC_PROFILING
> > +     union codetag_ref ref;
> > +#endif
> >  } __aligned(8);
>
> So this means that compiling with CONFIG_MEM_ALLOC_PROFILING will increase
> the memory overhead of arrays allocated for CONFIG_MEMCG_KMEM, even if
> allocation profiling itself is not enabled in runtime? Similar concern to
> the unconditional page_ext usage, that this would hinder enabling in a
> general distro kernel.
>
> The unused field overhead would be smaller than currently page_ext, but
> getting rid of it when alloc profiling is not enabled would be more work
> than introducing an early boot param for the page_ext case. Could be however
> solved similarly to how page_ext is populated dynamically at runtime.
> Hopefully it wouldn't add noticeable cpu overhead.

Yes, slabobj_ext overhead is much smaller than page_ext one but still
considerable and it would be harder to eliminate. Boot-time resizing
of the extension object might be doable but that again would be quite
complex and better be done as a separate patchset. This is lower on my
TODO list than page_ext ones since the overhead is order of magnitude
smaller.

>
> >  static inline void __inc_lruvec_kmem_state(void *p, enum node_stat_item idx)
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 7bbdb0ddb011..9ecfcdb54417 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -979,6 +979,7 @@ config MEM_ALLOC_PROFILING
> >       depends on !DEBUG_FORCE_WEAK_PER_CPU
> >       select CODE_TAGGING
> >       select PAGE_EXTENSION
> > +     select SLAB_OBJ_EXT
> >       help
> >         Track allocation source code and record total allocation size
> >         initiated at that code location. The mechanism can be used to track
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 77cf7474fe46..224a4b2305fb 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -569,6 +569,10 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >
> >  static inline bool need_slab_obj_ext(void)
> >  {
> > +#ifdef CONFIG_MEM_ALLOC_PROFILING
> > +     if (mem_alloc_profiling_enabled())
> > +             return true;
> > +#endif
> >       /*
> >        * CONFIG_MEMCG_KMEM creates vector of obj_cgroup objects conditionally
> >        * inside memcg_slab_post_alloc_hook. No other users for now.
>