Re: [PATCH v2 05/13] sched: Enable SD_BALANCE_WAKE for asymmetric capacity systems

[Morten Rasmussen <morten.rasmussen@arm.com>]

On Mon, Jul 11, 2016 at 11:37:18AM +0100, Morten Rasmussen wrote:
> On Mon, Jul 11, 2016 at 12:04:49PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 22, 2016 at 06:03:16PM +0100, Morten Rasmussen wrote:
> > > Systems with the SD_ASYM_CPUCAPACITY flag set indicate that sched_groups
> > > at this level or below do not include cpus of all capacities available
> > > (e.g. group containing little-only or big-only cpus in big.LITTLE
> > > systems). It is therefore necessary to put in more effort in finding an
> > > appropriate cpu at task wake-up by enabling balancing at wake-up
> > > (SD_BALANCE_WAKE).
> > 
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -6397,6 +6397,9 @@ sd_init(struct sched_domain_topology_level *tl, int cpu)
> > >  	 * Convert topological properties into behaviour.
> > >  	 */
> > >  
> > > +	if (sd->flags & SD_ASYM_CPUCAPACITY)
> > > +		sd->flags |= SD_BALANCE_WAKE;
> > > +
> > 
> > So I'm a bit confused on the exact requirements for this; as also per
> > the previous patch.
> > 
> > Should all sched domains get BALANCE_WAKE if one (typically the top)
> > domain has ASYM_CAP set?
> > 
> > The previous patch set it on the actual asym one and one below that, but
> > what if there's more levels below that? Imagine ARM gaining SMT or
> > somesuch. Should not then that level also get BALANCE_WAKE in order to
> > 'correctly' place light/heavy tasks?
> > 
> > IOW, are you trying to fudge the behaviour semantics by creating 'weird'
> > ASYM_CAP rules instead of having a more complex behaviour rule here?
> 
> That is one possible way of describing it :-)
> 
> The proposed semantic is to set ASYM_CAP at all levels starting from the
> bottom up until you have sched_groups containing all types of cpus
> available in the system, or reach the top level.
> 
> The fundamental reason for this weird semantics is that we somehow need
> to know at the lower levels, which may be capacity symmetric, if we need
> to consider balancing at a higher level to see the asymmetry or not.
> 
> If the flag isn't set bottom up we need some other way of knowing if the
> system is asymmetric, or we would have to go look for the flag further
> up the sched_domain hierarchy each time.
> 
> I'm not saying this is the perfect solution, I'm happy to discuss
> alternatives.

One alternative to setting ASYM_CAP bottom up would be to set it only
where the asymmetry can be observed, and instead come up with a more
complicated way of setting BALANCE_WAKE bottom up until and including
the first level having the ASYM_CAP.

I looked at it briefly an realized that I couldn't find a clean way of
implementing it as I don't think we have visibility of which flags that
will be set at higher levels in the sched_domain hierarchy when the
lower levels are initialized. IOW, we have behavioural flags settings
depend on topology flags settings at a different level.