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

[Waiman Long <longman@redhat.com>]

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.

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.

Cheers,
Longman