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Message-ID: <20250628082327.GC1613200@noisy.programming.kicks-ass.net>
Date: Sat, 28 Jun 2025 10:23:27 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Chris Mason <clm@...a.com>, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: futex performance regression from "futex: Allow automatic
allocation of process wide futex hash"
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 :-)
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