Re: [PATCH 3/9] x86/intel_rdt: Cache Allocation documentation and cgroup usage guide

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Jul 30, 2015 at 04:03:07PM -0700, Vikas Shivappa wrote:
> 
> 
> On Thu, 30 Jul 2015, Marcelo Tosatti wrote:
> 
> >On Thu, Jul 30, 2015 at 10:47:23AM -0700, Vikas Shivappa wrote:
> >>
> >>
> >>Marcello,
> >>
> >>
> >>On Wed, 29 Jul 2015, Marcelo Tosatti wrote:
> >>>
> >>>How about this:
> >>>
> >>>desiredclos (closid  p1  p2  p3 p4)
> >>>	     1       1   0   0  0
> >>>	     2	     0	 0   0  1
> >>>	     3	     0   1   1  0
> >>
> >>#1 Currently in the rdt cgroup , the root cgroup always has all the
> >>bits set and cant be changed (because the cgroup hierarchy would by
> >>default make this to have all bits as all the children need to have
> >>a subset of the root's bitmask). So if the user creates a cgroup and
> >>not put any task in it , the tasks in the root cgroup could be still
> >>using that part of the cache. Thats the reason i say we can have
> >>really 'exclusive' masks.
> >>
> >>Or in other words - there is always a desired clos (0) which has all
> >>parts set which acts like a default pool.
> >>
> >>Also the parts can overlap.  Please apply this for all the below
> >>comments which will change the way they work.
> >
> >
> >>
> >>>
> >>>p means part.
> >>
> >>I am assuming p = (a contiguous cache capacity bit mask)
> >>
> >>>closid 1 is a exclusive cgroup.
> >>>closid 2 is a "cache hog" class.
> >>>closid 3 is "default closid".
> >>>
> >>>Desiredclos is what user has specified.
> >>>
> >>>Transition 1: desiredclos --> effectiveclos
> >>>Clean all bits of unused closid's
> >>>(that must be updated whenever a
> >>>closid1 cgroup goes from empty->nonempty
> >>>and vice-versa).
> >>>
> >>>effectiveclos (closid  p1  p2  p3 p4)
> >>>	       1       0   0   0  0
> >>>	       2       0   0   0  1
> >>>	       3       0   1   1  0
> >>
> >>>
> >>>Transition 2: effectiveclos --> expandedclos
> >>>expandedclos (closid  p1  p2  p3 p4)
> >>>	       1       0   0   0  0
> >>>	       2       0   0   0  1
> >>>	       3       1   1   1  0
> >>>Then you have different inplacecos for each
> >>>CPU (see pseudo-code below):
> >>>
> >>>On the following events.
> >>>
> >>>- task migration to new pCPU:
> >>>- task creation:
> >>>
> >>>	id = smp_processor_id();
> >>>	for (part = desiredclos.p1; ...; part++)
> >>>		/* if my cosid is set and any other
> >>>	   	   cosid is clear, for the part,
> >>>		   synchronize desiredclos --> inplacecos */
> >>>		if (part[mycosid] == 1 &&
> >>>		    part[any_othercosid] == 0)
> >>>			wrmsr(part, desiredclos);
> >>>
> >>
> >>Currently the root cgroup would have all the bits set which will act
> >>like a default cgroup where all the otherwise unused parts (assuming
> >>they are a set of contiguous cache capacity bits) will be used.
> >
> >Right, but we don't want to place tasks in there in case one cgroup
> >wants exclusive cache access.
> >
> >So whenever you want an exclusive cgroup you'd do:
> >
> >create cgroup-exclusive; reserve desired part of the cache
> >for it.
> >create cgroup-default; reserved all cache minus that of cgroup-exclusive
> >for it.
> >
> >place tasks that belong to cgroup-exclusive into it.
> >place all other tasks (including init) into cgroup-default.
> >
> >Is that right?
> 
> Yes you could do that.
> 
> You can create cgroups to have masks which are exclusive in todays
> implementation, just that you could also created more cgroups to
> overlap the masks again.. iow we dont have an exclusive flag for the
> cgroup mask.
> Is that a common use case in the server environment that you need to
> prevent other cgroups from using a certain mask ? (since the root
> user should control these allocations .. he should know?)

Yes, there are two known use-cases that have this characteristic:

1) High performance numeric application which has been optimized
to a certain fraction of the cache.

2) Low latency application in multi-application OS.

For both cases exclusive cache access is wanted.