Re: [PATCH] cpusets: Make cpus_allowed and mems_allowed masks hotplug invariant

[Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Thu, Oct 09, 2014 at 01:50:52PM +0530, Preeti U Murthy wrote:
> >> SMT 8 on Power8 can help/hinder workloads. Hence we dynamically switch
> >> the modes at runtime.
> > 
> > That's just a horrible piece of crap hack and you deserve any and all
> > pain you get from doing it.
> > 
> > Randomly removing/adding cpus like that is horrible and makes a mockery
> > of all the affinity interfaces we have.
> 
> We observed this on ubuntu kernel, in which systemd explicitly mounts
> cgroup controllers under a child cgroup identified by the user pid.
> Since we had not observed this additional cgroup being added under the
> hood, it came as a surprise to us that cgroup/cpuset handling in the
> kernel should indeed kick in.
> 
> At best we expect hotplug to be handled well if the users have not
> explicitly configured cpusets, hence implicitly specifying that task
> affinity is for all online cpus. This is indeed the case today, so that
> is good.
> 
> However what remains to be answered is that the V2 of cgroup design -
> the default hierarchy, tracks hotplug operations for children cgroups as
> well. Tejun, Li, will not the concerns that Peter raised above hold for
> the default hierarchy as well?

None of this addresses the piece of crap thing you did with power8. You
cannot just make CPUs go away at random.