Re: [PATCH v3 20/35] lib: add codetag reference into slabobj_ext

Suren Baghdasaryan
2024-02-16T17:04:21Z

On Fri, Feb 16, 2024 at 7:36 AM Vlastimil Babka <vbabka@suse.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@google.com>
> > Co-developed-by: Kent Overstreet <kent.overstreet@linux.dev>
> > Signed-off-by: Kent Overstreet <kent.overstreet@linux.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.
>