Re: [PATCH [RT] 11/14] optimize the !printk fastpath throughthe lock acquisition

[Gregory Haskins]

>>> On Thu, Feb 21, 2008 at 11:36 AM, in message <200802211736.26719.ak@xxxxxxx>,
Andi Kleen <ak@xxxxxxx> wrote: 
> On Thursday 21 February 2008 16:27:22 Gregory Haskins wrote:
> 
>> @@ -660,12 +660,12 @@ rt_spin_lock_fastlock(struct rt_mutex *lock,
>>  		void fastcall (*slowfn)(struct rt_mutex *lock))
>>  {
>>  	/* Temporary HACK! */
>> -	if (!current->in_printk)
>> -		might_sleep();
>> -	else if (in_atomic() || irqs_disabled())
>> +	if (unlikely(current->in_printk) && (in_atomic() || irqs_disabled()))
> 
> I have my doubts that gcc will honor unlikelies that don't affect
> the complete condition of an if.
> 
> Also conditions guarding returns are by default predicted unlikely
> anyways AFAIK. 
> 
> The patch is likely a nop.
> 

Yeah, you are probably right.  We have found that the system is *extremely* touchy on how much overhead we have in the lock-acquisition path.  For instance, using a non-inline version of adaptive_wait() can cost 5-10% in disk-io throughput.  So we were trying to find places to shave anywhere we could.  That being said, I didn't record any difference from this patch, so you are probably exactly right.  It just seemed like "the right thing to do" so I left it in.

-Greg



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