Re: [PATCH RFC 3/4] mm, slab: add static key for should_failslab()

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, May 31, 2024 at 9:44 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Fri, May 31, 2024 at 2:33 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> >
> > Since commit 4f6923fbb352 ("mm: make should_failslab always available for
> > fault injection") should_failslab() is unconditionally a noinline
> > function. This adds visible overhead to the slab allocation hotpath,
> > even if the function is empty. With CONFIG_FAILSLAB=y there's additional
> > overhead when the functionality is not enabled by a boot parameter or
> > debugfs.
> >
> > The overhead can be eliminated with a static key around the callsite.
> > Fault injection and error injection frameworks can now be told that the
> > this function has a static key associated, and are able to enable and
> > disable it accordingly.
> >
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > ---
> >  mm/failslab.c |  2 +-
> >  mm/slab.h     |  3 +++
> >  mm/slub.c     | 10 +++++++---
> >  3 files changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/failslab.c b/mm/failslab.c
> > index ffc420c0e767..878fd08e5dac 100644
> > --- a/mm/failslab.c
> > +++ b/mm/failslab.c
> > @@ -9,7 +9,7 @@ static struct {
> >         bool ignore_gfp_reclaim;
> >         bool cache_filter;
> >  } failslab = {
> > -       .attr = FAULT_ATTR_INITIALIZER,
> > +       .attr = FAULT_ATTR_INITIALIZER_KEY(&should_failslab_active.key),
> >         .ignore_gfp_reclaim = true,
> >         .cache_filter = false,
> >  };
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 5f8f47c5bee0..792e19cb37b8 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -11,6 +11,7 @@
> >  #include <linux/memcontrol.h>
> >  #include <linux/kfence.h>
> >  #include <linux/kasan.h>
> > +#include <linux/jump_label.h>
> >
> >  /*
> >   * Internal slab definitions
> > @@ -160,6 +161,8 @@ static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(freelist_aba_t)
> >   */
> >  #define slab_page(s) folio_page(slab_folio(s), 0)
> >
> > +DECLARE_STATIC_KEY_FALSE(should_failslab_active);
> > +
> >  /*
> >   * If network-based swap is enabled, sl*b must keep track of whether pages
> >   * were allocated from pfmemalloc reserves.
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 0809760cf789..3bb579760a37 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3874,13 +3874,15 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
> >                         0, sizeof(void *));
> >  }
> >
> > +DEFINE_STATIC_KEY_FALSE(should_failslab_active);
> > +
> >  noinline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
> >  {
> >         if (__should_failslab(s, gfpflags))
> >                 return -ENOMEM;
> >         return 0;
> >  }
> > -ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
> > +ALLOW_ERROR_INJECTION_KEY(should_failslab, ERRNO, &should_failslab_active);
> >
> >  static __fastpath_inline
> >  struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags)
> > @@ -3889,8 +3891,10 @@ struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags)
> >
> >         might_alloc(flags);
> >
> > -       if (unlikely(should_failslab(s, flags)))
> > -               return NULL;
> > +       if (static_branch_unlikely(&should_failslab_active)) {
> > +               if (should_failslab(s, flags))
> > +                       return NULL;
> > +       }
>
> makes sense.
> Acked-by: Alexei Starovoitov <ast@kernel.org>
>
> Do you have any microbenchmark numbers before/after this optimization?

There are numbers in the cover letter for the entire series:
https://lore.kernel.org/lkml/20240531-fault-injection-statickeys-v1-0-a513fd0a9614@suse.cz/