Linux Virtualization - May 2014 - [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long
wrote:
> If unfair lock is supported, the lock acquisition loop at the end of
> the queue_spin_lock_slowpath() function may need to detect the fact
> the lock can be stolen. Code are added for the stolen lock detection.
> 
> A new qhead macro is also defined as a shorthand for mcs.locked.
NAK, unfair should be a pure test-and-set lock.
>  /**
>   * get_qlock - Set the lock bit and own the lock
> - * @lock: Pointer to queue spinlock structure
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 otherwise
>   *
>   * This routine should only be called when the caller is the only one
>   * entitled to acquire the lock.
>   */
> -static __always_inline void get_qlock(struct qspinlock *lock)
> +static __always_inline int get_qlock(struct qspinlock *lock)
>  {
>  	struct __qspinlock *l = (void *)lock;
>  
>  	barrier();
>  	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>  	barrier();
> +	return 1;
>  }
and here you make a horribly named function more horrible;
try_set_locked() is that its now.

On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
>> If unfair lock is supported, the lock acquisition loop at the end of
>> the queue_spin_lock_slowpath() function may need to detect the fact
>> the lock can be stolen. Code are added for the stolen lock detection.
>>
>> A new qhead macro is also defined as a shorthand for mcs.locked.
> NAK, unfair should be a pure test-and-set lock.
I have performance data showing that a simple test-and-set lock does not 
scale well. That is the primary reason of ditching the test-and-set lock 
and use a more complicated scheme which scales better. Also, it will be 
hard to make the unfair test-and-set lock code to coexist nicely with PV 
spinlock code.
>>   /**
>>    * get_qlock - Set the lock bit and own the lock
>> - * @lock: Pointer to queue spinlock structure
>> + * @lock : Pointer to queue spinlock structure
>> + * Return: 1 if lock acquired, 0 otherwise
>>    *
>>    * This routine should only be called when the caller is the only one
>>    * entitled to acquire the lock.
>>    */
>> -static __always_inline void get_qlock(struct qspinlock *lock)
>> +static __always_inline int get_qlock(struct qspinlock *lock)
>>   {
>>   	struct __qspinlock *l = (void *)lock;
>>
>>   	barrier();
>>   	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>>   	barrier();
>> +	return 1;
>>   }
> and here you make a horribly named function more horrible;
> try_set_locked() is that its now.
Will do.

-Longman

On Fri, May 09, 2014 at 09:19:32PM -0400, Waiman Long
wrote:
> On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
> >On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> >>If unfair lock is supported, the lock acquisition loop at the end
of
> >>the queue_spin_lock_slowpath() function may need to detect the fact
> >>the lock can be stolen. Code are added for the stolen lock
detection.
> >>
> >>A new qhead macro is also defined as a shorthand for mcs.locked.
> >NAK, unfair should be a pure test-and-set lock.
> 
> I have performance data showing that a simple test-and-set lock does not
> scale well. That is the primary reason of ditching the test-and-set lock
and
> use a more complicated scheme which scales better.
Nobody should give a fuck about scalability in this case anyway.

Also, as I explained/asked earlier:

  lkml.kernel.org/r/20140314083001.GN27965 at twins.programming.kicks-ass.net

Lock holder preemption is _way_ worse with any kind of queueing. You've
not explained how the simple 3 cpu example in that email gets better
performance than a test-and-set lock.
> Also, it will be hard to
> make the unfair test-and-set lock code to coexist nicely with PV spinlock
> code.
That's just complete crap as the test-and-set lock is like 3 lines of
code.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 836 bytes
Desc: not available
URL:
<http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20140510/e2368bff/attachment.sig>