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 09:36 AM, Peter Zijlstra wrote:
> > On Thu, Dec 19, 2013 at 06:25:35PM +0100, Peter Zijlstra wrote:
> >> That said, I would find it very strange indeed if a CLFLUSH doesn't also
> >> flush the store buffer.
> > 
> > OK, it explicitly states it does not do that and you indeed need 
> > an mfence before the clflush.
> 
> So, MONITOR is defined to be ordered as a load, which I think should 
> be adequate, but I really wonder if we should have mfence on both 
> sides of clflush.  This now is up to 9 bytes, and perhaps pushing it 
> a bit with how much we would be willing to patch out.
> 
> On the other hand - the CLFLUSH seems to have worked well enough by 
> itself, and this is all probabilistic anyway, so perhaps we should 
> just leave the naked CLFLUSH in and not worry about it unless 
> measurements say otherwise?

So I think the window of breakage was rather large here, and since it 
seems to trigger on rare types of hardware I think we'd be better off 
by erring on the side of robustness this time around ...

This is the 'go to idle' path, which isn't as time-critical as the 
'get out of idle' code path.

Thanks,

	Ingo