Re: futex performance regression from "futex: Allow automatic allocation of process wide futex hash"

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 27, 2025 at 03:48:12PM -0700, Tim Chen wrote:
> On Tue, 2025-06-24 at 21:01 +0200, Peter Zijlstra wrote:
> > 
> > How about something like this (very lightly tested)...
> > 
> > the TL;DR is that it turns all those refcounts into per-cpu ops when
> > there is no hash replacement pending (eg. the normal case), and only
> > folds the lot into an atomic when we really care about it.
> > 
> > There's some sharp corners still.. but it boots and survives the
> > (slightly modified) selftest.
> > 
> > ---
> >  include/linux/futex.h                              |  12 +-
> >  include/linux/mm_types.h                           |   5 +
> >  kernel/futex/core.c                                | 238 +++++++++++++++++++--
> >  .../selftests/futex/functional/futex_priv_hash.c   |  11 +-
> >  4 files changed, 239 insertions(+), 27 deletions(-)
> > 
> > diff --git a/include/linux/futex.h b/include/linux/futex.h
> > index b37193653e6b..5f92c7a8ba57 100644
> > --- a/include/linux/futex.h
> > +++ b/include/linux/futex.h
> > @@ -85,17 +85,11 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
> >  #ifdef CONFIG_FUTEX_PRIVATE_HASH
> >  int futex_hash_allocate_default(void);
> >  void futex_hash_free(struct mm_struct *mm);
> > -
> > -static inline void futex_mm_init(struct mm_struct *mm)
> > -{
> > -	RCU_INIT_POINTER(mm->futex_phash, NULL);
> > -	mm->futex_phash_new = NULL;
> > -	mutex_init(&mm->futex_hash_lock);
> > -}
> > +int futex_mm_init(struct mm_struct *mm);
> >  
> >  #else /* !CONFIG_FUTEX_PRIVATE_HASH */
> >  static inline int futex_hash_allocate_default(void) { return 0; }
> > -static inline void futex_hash_free(struct mm_struct *mm) { }
> > +static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
> 
> Minor nit.
> 
> futex_hash_free() is defined to return void for CONFIG_FUTEX_PRIVATE_HASH
> config. But is now defined to return int for !CONFIG_FUTEX_PRIVATE_HASH and !CONFIG_FUTEX configs.
> But it seems like nobody is actually checking the return value.  It
> seems to make more sense to keep the return value as void here so
> the return value is consistent?

YEah, not sure what happened there. There's a TODO item to actually make
sure someone looks at the futex_mm_init() return value, I guess that's a
good moment to clean this up too :-)