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

[Ingo Molnar <mingo@kernel.org>]

* H. Peter Anvin <hpa@zytor.com> wrote:

> 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:
>  */

Yes, that makes sense: the need_resched test is a load, and MONITOR is 
a load as well. Can the two ever cross, or does the CPU guarantee that 
because it's the same address, the loads don't cross?

Thanks,

	Ingo