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Date:   Mon, 20 Feb 2017 10:58:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On 02/19/2017 11:53 PM, Boqun Feng wrote:
> On Mon, Feb 20, 2017 at 05:20:52AM +0100, Andrea Parri wrote:
>> 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:
>>
> Yes, sorry for be late for this one.
>
> So Waiman, the fact is that in this case, we want the following code
> sequence:
>
> 	CPU 0					CPU 1
> 	=================			====================
> 	{pn->state = vcpu_running, node->locked = 0}
>
> 	smp_store_smb(&pn->state, vcpu_halted):
> 	  WRITE_ONCE(pn->state, vcpu_halted);
> 	  smp_mb();
> 	r1 = READ_ONCE(node->locked);
> 						arch_mcs_spin_unlock_contented();
> 						  WRITE_ONCE(node->locked, 1)
>
> 						cmpxchg(&pn->state, vcpu_halted, vcpu_hashed);
>
> never ends up in:
>
> 	r1 == 0 && cmpxchg fail(i.e. the read part of cmpxchg reads the
> 	value vcpu_running).
>
> We can have such a guarantee if cmpxchg has a smp_mb() before its load
> part, which is true for PPC. But semantically, cmpxchg() doesn't provide
> any order guarantee if it fails, which is true on ARM64, IIUC. (Add Will
> in Cc for his insight ;-)).
>
> So a possible "fix"(in case ARM64 will use qspinlock some day), would be
> replace cmpxchg() with smp_mb() + cmpxchg_relaxed().
>

It is the pvqspinlock code, the native qspinlock will not have this
problem. So we will need to make sure that the write to node->locked
will always precede the read of pn->state whether the cmpxchg is
successful or not. We are not going to replace cmpxchg() with smp_mb() +
cmpxchg_relaxed() as that will impact x86 performance. Perhaps, we could
provide another cmpxchg variant that can provide that memory barrier
guarantee for all archs.

In the mean time, I am going to update the patch to document this
limitation so that we could do something about it when archs like ARM64
want pvqspinlock support.

Cheers,
Longman

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