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Date:   Mon, 20 Feb 2017 05:20:52 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On Fri, Feb 17, 2017 at 03:43:40PM -0500, Waiman Long 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@...hat.com>
> ---
> 
>  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 | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index e6b2f7a..a59dc05 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 +361,9 @@ 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()
> +	 *
> +	 * We can't used relaxed form of cmpxchg here as the loading of
> +	 * pn->state can happen before setting next->locked in some archs.
>  	 */
>  	if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)

Hi Waiman.

cmpxchg() does not guarantee the (here implied) smp_mb(), in general; c.f.,
e.g., arch/arm64/include/asm/atomic_ll_sc.h, where cmpxchg() get "compiled"
to something like:

    _loop: ldxr; eor; cbnz _exit; stlxr; cbnz _loop; dmb ish; _exit:

  Andrea


>  		return;
> -- 
> 1.8.3.1
> 

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