[RFC PATCH v3 3/6] sched: pack small tasks

[Catalin Marinas]

On Wed, Mar 27, 2013 at 02:13:54PM +0000, Peter Zijlstra wrote:
> On Wed, 2013-03-27 at 11:18 +0000, Catalin Marinas wrote:
> > On Wed, Mar 27, 2013 at 09:00:20AM +0000, Peter Zijlstra wrote:
> > > On Wed, 2013-03-27 at 09:54 +0100, Vincent Guittot wrote:
> > > > It's not mandatory to have little cores on low numbers even if it's
> > > > advised
> > > 
> > > ARGH!
> > 
> > I haven't followed this thread closely, so just a random comment from
> > me. An argument from some is that they want to boot Linux on the big CPU
> > to be quicker. The most convenient is to have the big CPU at 0.
> 
> I suppose that's almost sensible ;-) I just despair at the amount of
> variation that's allowed.
> 
> I'm guessing that swapping cpus in the bootloader or someplace really
> early is equally hard in that we (Linux) assume we boot on cpu 0 or
> something like that?

I think it's worth trying (by changing the CPU topology in the DT). At a
quick look, I don't see anything hard-coded in the kernel boot sequence.
It uses smp_processor_id() which translates to
current_thread_info()->cpu on ARM. I'm not sure how early we need this
but it's probably after DT parsing, so we could set 'cpu' to a non-zero
value for the booting processor. There are a few tweaks in the arch/arm
code code with cpu_logical_map setup (which maps between
smp_processor_id and the actual hardware CPU id and assumes 0 is the
booting CPU).

So if the above works, the scheduler guys can mandate that little CPUs
are always first and for ARM it would be a matter of getting the right
CPU topology in the DT (independent of what hw vendors think of CPU
topology) and booting Linux on CPU 4 etc.

-- 
Catalin