Re: RFC [patch] sched,cgroup_sched: convince RT_GROUP_SCHED throttle to work

[Yong Zhang <yong.zhang0@gmail.com>]

On Wed, Apr 18, 2012 at 10:38:07AM +0200, Mike Galbraith wrote:
> On Wed, 2012-04-18 at 15:48 +0800, Yong Zhang wrote: 
> > On Wed, Apr 18, 2012 at 08:27:50AM +0200, Mike Galbraith wrote:
> > > On Wed, 2012-04-18 at 13:20 +0800, Yong Zhang wrote:
> > > 
> > > > > --- a/kernel/sched/core.c
> > > > > +++ b/kernel/sched/core.c
> > > > > @@ -5875,6 +5875,11 @@ cpu_attach_domain(struct sched_domain *s
> > > > >  			sd->child = NULL;
> > > > >  	}
> > > > >  
> > > > > +	if (sd)
> > > > > +		cpumask_clear_cpu(cpu, cpu_isolated_map);
> > > > > +	else
> > > > > +		cpumask_set_cpu(cpu, cpu_isolated_map);
> > > > > +
> > > > 
> > > > Do we allow this?
> > > 
> > > Yeah, isolating CPUS 2-3...
> > 
> > Hmm...magic cpuset ;-)
> 
> The _only_ cpuset if you have a low jitter requirement. 
> 
> > > [ 3011.444345] CPU0 attaching NULL sched-domain.
> > > [ 3011.448719] CPU1 attaching NULL sched-domain.
> > > [ 3011.453107] CPU2 attaching NULL sched-domain.
> > > [ 3011.457466] CPU3 attaching NULL sched-domain.
> > > [ 3011.461892] CPU0 attaching sched-domain:
> > > [ 3011.465813]  domain 0: span 0-1 level MC
> > > [ 3011.469761]   groups: 0 1
> > > [ 3011.472415] CPU1 attaching sched-domain:
> > > [ 3011.476333]  domain 0: span 0-1 level MC
> > > [ 3011.480266]   groups: 1 0
> > > [ 3011.482988] CPU2 attaching sched-domain:
> > > [ 3011.486919]  domain 0: span 2-3 level MC
> > > [ 3011.490851]   groups: 2 3
> > > [ 3011.493502] CPU3 attaching sched-domain:
> > > [ 3011.497422]  domain 0: span 2-3 level MC
> > > [ 3011.501367]   groups: 3 2
> > > [ 3011.504214] CPU2 attaching NULL sched-domain.
> > > [ 3011.508569] CPU3 attaching NULL sched-domain.
> > 
> > But another scenario comes into head:
> > What will happen when a rt_rq is throttled the very CPU is
> > attached to NULL domain?
> 
> Yup, _somebody_ will hit the throttle only once :)
> 
> That's what gets fixed by either killing the throttle entirely for
> isolated CPUs, or making the throttle work until the guy who needed
> isolation turns noise maker off.  I prefer reconnect the dots, because
> that doesn't touch the fast path.

I like it better too :)

Thanks,
Yong