Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline

[Ingo Molnar <mingo@elte.hu>]

* Luck, Tony <tony.luck@intel.com> wrote:

> > The question is, how realistically does this report true CPU 
> > troubles, statistically? The on-die cache might have the 
> > highest transistor count, but it's not under nearly the same 
> > thermal stress as functional units.
> >
> > If 90% of all hard CPU failures can be predicted that way 
> > then it's probably useful. If it's only 20%, then not so 
> > much.
> 
> Intel doesn't release error rates - so I can't help with data 
> here.

Well, precise data won't be needed - but we need *something* 
indicative to justify the feature - faith alone won't be enough.

Is there any third party research on this? I remember that 
Google released hard drive failure stats a few years ago, maybe 
there's some approximate data about CPU "soft" failure rates. 
Even anecdotal data and speculation/estimation would be a start 
- it could be contradicted later on by more precise data, once 
people start using the "generic CPU hot-unplug" feature. (which 
this feature should really be named, instead of the 'BSP unplug' 
name.)

> > Also, it's still all theoretical until there's systems out 
> > there where the CPU socket is physically hotpluggable. If 
> > there's such plans in the works then sure, theory becomes 
> > reality and then it's all useful - and then we can do these 
> > patches (and more).
> 
> No - physical removal of the cpu is not a requirement for this 
> to be useful. [...]

Indeed, you are right, i stand corrected there.

Okay, i'm convinced, i guess we can do this.

> [...]
>
> Physical removal of the cpu is a problem for Linux since 
> Nehalem (when memory controller moved on-die). Take away the 
> cpu, and you lose access to the memory connected to that 
> socket - and we don't have general solutions for memory 
> removal.

It's possible technically but not the easiest of features - also 
i suspect Linus would object to the naive breaking of the 
semi-linear kernel mapping we do today ;-)

But if someone implements that in a sane way, using at least 2MB 
granular mappings [or maybe ORDER_MAX granular mappings], which 
keeps 2MB TLBs, and uses a quick hash table for __pa() and 
__va(), i would definitely take a look at how ugly it ends up 
being. Our hibernation code already gives us a generic way to 
quiescence all DMA activity on the system, so most of the 
building blocks are in place.

Thanks,

	Ingo