Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Guo Ren <guoren@kernel.org>]

On Sun, Mar 28, 2021 at 2:43 AM Waiman Long <longman@redhat.com> wrote:
>
> On 3/27/21 2:06 PM, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > Some architectures don't have sub-word swap atomic instruction,
> > they only have the full word's one.
> >
> > The sub-word swap only improve the performance when:
> > NR_CPUS < 16K
> >   *  0- 7: locked byte
> >   *     8: pending
> >   *  9-15: not used
> >   * 16-17: tail index
> >   * 18-31: tail cpu (+1)
> >
> > The 9-15 bits are wasted to use xchg16 in xchg_tail.
> >
> > Please let architecture select xchg16/xchg32 to implement
> > xchg_tail.
> >
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Waiman Long <longman@redhat.com>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Anup Patel <anup@brainfault.org>
> > ---
> >   kernel/Kconfig.locks       |  3 +++
> >   kernel/locking/qspinlock.c | 44 +++++++++++++++++++++-----------------
> >   2 files changed, 27 insertions(+), 20 deletions(-)
> >
> > diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
> > index 3de8fd11873b..d02f1261f73f 100644
> > --- a/kernel/Kconfig.locks
> > +++ b/kernel/Kconfig.locks
> > @@ -239,6 +239,9 @@ config LOCK_SPIN_ON_OWNER
> >   config ARCH_USE_QUEUED_SPINLOCKS
> >       bool
> >
> > +config ARCH_USE_QUEUED_SPINLOCKS_XCHG32
> > +     bool
> > +
> >   config QUEUED_SPINLOCKS
> >       def_bool y if ARCH_USE_QUEUED_SPINLOCKS
> >       depends on SMP
> > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> > index cbff6ba53d56..54de0632c6a8 100644
> > --- a/kernel/locking/qspinlock.c
> > +++ b/kernel/locking/qspinlock.c
> > @@ -163,26 +163,6 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
> >       WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
> >   }
> >
> > -/*
> > - * xchg_tail - Put in the new queue tail code word & retrieve previous one
> > - * @lock : Pointer to queued spinlock structure
> > - * @tail : The new queue tail code word
> > - * Return: The previous queue tail code word
> > - *
> > - * xchg(lock, tail), which heads an address dependency
> > - *
> > - * p,*,* -> n,*,* ; prev = xchg(lock, node)
> > - */
> > -static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> > -{
> > -     /*
> > -      * We can use relaxed semantics since the caller ensures that the
> > -      * MCS node is properly initialized before updating the tail.
> > -      */
> > -     return (u32)xchg_relaxed(&lock->tail,
> > -                              tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
> > -}
> > -
> >   #else /* _Q_PENDING_BITS == 8 */
> >
> >   /**
> > @@ -206,6 +186,30 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
> >   {
> >       atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
> >   }
> > +#endif
> > +
> > +#if _Q_PENDING_BITS == 8 && !defined(CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32)
> > +/*
> > + * xchg_tail - Put in the new queue tail code word & retrieve previous one
> > + * @lock : Pointer to queued spinlock structure
> > + * @tail : The new queue tail code word
> > + * Return: The previous queue tail code word
> > + *
> > + * xchg(lock, tail), which heads an address dependency
> > + *
> > + * p,*,* -> n,*,* ; prev = xchg(lock, node)
> > + */
> > +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> > +{
> > +     /*
> > +      * We can use relaxed semantics since the caller ensures that the
> > +      * MCS node is properly initialized before updating the tail.
> > +      */
> > +     return (u32)xchg_relaxed(&lock->tail,
> > +                              tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
> > +}
> > +
> > +#else
> >
> >   /**
> >    * xchg_tail - Put in the new queue tail code word & retrieve previous one
>
> I don't have any problem adding a
> CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 config option to control that.
Thx

>
> One minor nit:
>
> #endif /* _Q_PENDING_BITS == 8 */
>
> You should probably remove the comment at the trailing end of the
> corresponding "#endif" as it is now wrong.
I'll fix it in next patch

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/