[PATCH 00/16] big.LITTLE low-level CPU and cluster power management

[Nicolas Pitre]

On Thu, 7 Mar 2013, Pavel Machek wrote:

> Hi!
> 
> > > > Review comments are welcome!
> > > > 
> > > > [*] General design information on the b.L switcher can be found here:
> > > >     http://lwn.net/Articles/481055/
> > > >     However the code is only accessible to Linaro members for the
> > > >     time being.
> > > 
> > > Assuming the lwn article is accurate:
> > > 
> > > Hmm. So we have hw capable of running 8 cores, and then we introduce strange
> > > switching code, because "scheduler is not ready". Sounds like a bad idea.
> > > 
> > > I'd say:
> > > 
> > > * expose all 8 cores.
> > 
> > You may do that now.  However the resulting power efficiency is far from 
> > optimal.
> 
> Really? Assuming the lwn article is accurate, it will be exactly the
> same.
> 
> I propose exposing all 8 cores, then keeping 4 of them offline in
> normal operation. This should have characteristics of your solution,
> except that cpu does not change unexpectedly under the pinned tasks.
> 
> > > * as long as scheduler is not ready, you can offline "the other" set of cores...
> > > 	/sys/.../cpu/..../online
> > 
> > You may do that now also.  But system performance is far from optimal.
> 
> Really? Do the offlining from the cpufreq code, and solution will be
> equivalent to yours, except the hacks.

No it is not equivalent.  Offlining a CPU is a fairly heavy operation in 
Linux and you don't want to do that multiple times per second for 
example.  Whereas the switcher "hack" is similar to a context switch in 
terms of software complexity.


Nicolas