Re: [PATCH v4] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

[Ingo Molnar <mingo@kernel.org>]

* Waiman Long <longman@redhat.com> wrote:

> All the locking related cmpxchg's in the following functions are
> replaced with the _acquire variants:
>  - pv_queued_spin_steal_lock()
>  - trylock_clear_pending()
> 
> This change should help performance on architectures that use LL/SC.
> 
> On a 2-core 16-thread Power8 system with pvqspinlock explicitly
> enabled, the performance of a locking microbenchmark with and without
> this patch on a 4.10-rc8 kernel with Xinhui's PPC qspinlock patch
> were as follows:
> 
>   # of thread     w/o patch    with patch      % Change
>   -----------     ---------    ----------      --------
>        4         4053.3 Mop/s  4223.7 Mop/s     +4.2%
>        8         3310.4 Mop/s  3406.0 Mop/s     +2.9%
>       12         2576.4 Mop/s  2674.6 Mop/s     +3.8%
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> 
>  v3->v4:
>   - Update the comment in pv_kick_node() to mention that the code
>     may not work in some archs.
> 
>  v2->v3:
>   - Reduce scope by relaxing cmpxchg's in fast path only.
> 
>  v1->v2:
>   - Add comments in changelog and code for the rationale of the change.
> 
>  kernel/locking/qspinlock_paravirt.h | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index e6b2f7a..93e271d 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock)
>  	struct __qspinlock *l = (void *)lock;
>  
>  	if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
> -	    (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
> +	    (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
>  		qstat_inc(qstat_pv_lock_stealing, true);
>  		return true;
>  	}
> @@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct qspinlock *lock)
>  
>  /*
>   * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
> - * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock
> - * just to be sure that it will get it.
> + * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
> + * lock just to be sure that it will get it.
>   */
>  static __always_inline int trylock_clear_pending(struct qspinlock *lock)
>  {
>  	struct __qspinlock *l = (void *)lock;
>  
>  	return !READ_ONCE(l->locked) &&
> -	       (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)
> -			== _Q_PENDING_VAL);
> +	       (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
> +				_Q_LOCKED_VAL) == _Q_PENDING_VAL);
>  }
>  #else /* _Q_PENDING_BITS == 8 */
>  static __always_inline void set_pending(struct qspinlock *lock)
> @@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
>  		 */
>  		old = val;
>  		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> -		val = atomic_cmpxchg(&lock->val, old, new);
> +		val = atomic_cmpxchg_acquire(&lock->val, old, new);
>  
>  		if (val == old)
>  			return 1;
> @@ -361,6 +361,13 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>  	 * observe its next->locked value and advance itself.
>  	 *
>  	 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
> +	 *
> +	 * The write to next->locked in arch_mcs_spin_unlock_contended()
> +	 * must be ordered before the read of pn->state in the cmpxchg()
> +	 * below for the code to work correctly. However, this is not
> +	 * guaranteed in all architectures when the cmpxchg() fails.
> +	 * Both x86 and PPC can provide that guarantee, but probably
> +	 * not in some other architectures.

ob- spelling pedantry:

  s/not guaranteed in all architectures
   /not guaranteed on all architectures

  s/not in some other architectures
   /not on some other architectures

Also, "the cmpxchg() fails" should either be "when the cmpxchg() call fails" or 
"when cmpxchg() fails".

Plus the last sentence reads a bit funny.

Something like this would work for me:

	 * The write to next->locked in arch_mcs_spin_unlock_contended()
	 * must be ordered before the read of pn->state in the cmpxchg()
	 * below for the code to work correctly. However, this is not
	 * guaranteed on all architectures when cmpxchg() fails.
	 * Both x86 and PPC can provide that guarantee, but other
	 * architectures not necessarily.

(or so)

Thanks,

	Ingo