On Fri, Feb 16, 2018 at 03:41:01PM +0000, Morten Rasmussen wrote:
> On Fri, Feb 16, 2018 at 02:47:04PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 15, 2018 at 04:20:48PM +0000, Morten Rasmussen wrote:
> > > +static void update_asym_cpucapacity(int cpu)
> > > +{
> > > + if (!static_branch_unlikely(&sched_asym_cpucapacity) &&
> > > + lowest_flag_domain(cpu, SD_ASYM_CPUCAPACITY))
> > > + static_branch_enable(&sched_asym_cpucapacity);
> > > +}
> >
> > That looks odd, why not just:
> >
> > if (lowest_flag_domain(cpu, SD_ASYM_CPUCAPACITY))
> > static_branch_enable(&sched_asym_cpucapacity);
>
> I actually had that initially and then I misread the implementation of
> static_key_enable() as if it trigger the WARN_ON_ONCE() condition if I
> enable an already enabled static key. But I see now that it should be
> safe to do.
Right, that WARN is there for when we use enable/disable on a key with a
value outside of [0,1].
> > ? possibly with:
> >
> > else
> > static_branch_disable(&sched_asym_cpucapacity);
> >
> > if you want to play funny games :-)
>
> I thought about that too. It could make certain hotplug scenarios even
> more expensive. I think we want the sched_asym_cpucapacity code to behave
> even if SD_ASYM_CPUCAPACITY isn't set anywhere, so the static key would
> be permanently from the point we detect asymmetry and leave it set. This
> would be in line with how the smt static key works.
Fair enough..
