Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression

[H. Peter Anvin]

On 12/19/2013 10:09 AM, H. Peter Anvin wrote:
> On 12/19/2013 09:07 AM, Ingo Molnar wrote:
>>
>> Likewise, having a barrier before the MONITOR looks sensible as well. 
>> Having it _after_ monitor looks weird and is probably wrong. [It might 
>> have been the effects of someone seeing the spurious wakeup problems 
>> with realizing the true source, or so.]
>>
> 
> Does anyone know the history of this barrier after the monitor?  I know
> Len is looking for a minimal patchset that can go into -stable, and it
> seems prudent to not preturb the code more than necessary, but going
> forward it would be nice to know...
> 

Hmm... it *looks* like it is intended to be part of the construct:

	smp_mb();
	if (!need_resched())
		...

I found a note in the HLT variant of the function saying:

/*
 * TS_POLLING-cleared state must be visible before we
 * test NEED_RESCHED:
 */

... which presumably has been copied elsewhere.

	-hpa


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