Re: [RFC/PATCH] cpuset: cpuset irq affinities

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2008-03-03 at 11:36 -0600, Paul Jackson wrote:
> Peter wrote:
> > The system group can overlap with anything that does need system services.
> 
> I suppose IRQs need to overlap like this, but cpusets often can't
> overlap like this.

Due to CS_CPU_EXCLUSIVE usage?

I had hoped system would be allowed to overlap.

> If a system has the cgroup hierarchy you draw:
> 
>   /cgroup
>   /cgroup/system
>   /cgroup/system/boot
> 
>   /cgroup/big_honking_app
>   /cgroup/rt_domain
> 
> this must not force the cpuset hierarchy to be:
> 
>   /dev/cpuset
>   /dev/cpuset/system
>   /dev/cpuset/system/boot
> 
>   /dev/cpuset/big_honking_app
>   /dev/cpuset/rt_domain
> 
> I guess this means IRQs cannot be added to the cpuset subsystem
> of cgroups.  Rather they have to be added to some other cgroup
> subsystem, perhaps a new one just for IRQs.

The trouble is, cgroups are primarily about tasks, whereas IRQs are not.
So we would create a cgroup that does not manage tasks, but rather
associates irqs with sets of cpus - which are not cpusets.

See how that would be awkward?

> In perhaps the most common sort of cpuset hierarchy:
> 
>   /dev/cpuset
>   /dev/cpuset/boot
>   /dev/cpuset/batch_sched
>   /dev/cpuset/big_honking_app
>   /dev/cpuset/rt_domain
> 
> none of boot or its siblings overlap.

But as long as nobody does CS_CPU_EXCLUSIVE they may overlap, right?