Re: [PATCH V16 11/11] x86,cgroup/intel_rdt : Add a cgroup interface to manage Intel cache allocation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Dec 21, 2015 at 10:48:22AM -0500, Luiz Capitulino wrote:
> On Sat, 19 Dec 2015 22:57:30 -0200
> Marcelo Tosatti <mtosatti@redhat.com> wrote:
> 
> > On Sat, Dec 19, 2015 at 11:42:57AM +0100, Thomas Gleixner wrote:
> > > On Thu, 17 Dec 2015, Fenghua Yu wrote:
> > > 
> > > > From: Fenghua Yu <fenghua.yu@intel.com>
> > > > 
> > > > From: Vikas Shivappa <vikas.shivappa@linux.intel.com>
> > > > 
> > > > Add a new cgroup 'intel_rdt' to manage cache allocation. Each cgroup
> > > > directory is associated with a class of service id(closid). To map a
> > > > task with closid during scheduling, this patch removes the closid field
> > > > from task_struct and uses the already existing 'cgroups' field in
> > > > task_struct.
> > > > 
> > > > The cgroup has a file 'l3_cbm' which represents the L3 cache capacity
> > > > bitmask(CBM). The CBM is global for the whole system currently. The
> > > > capacity bitmask needs to have only contiguous bits set and number of
> > > > bits that can be set is less than the max bits that can be set. The
> > > > tasks belonging to a cgroup get to fill in the L3 cache represented by
> > > > the capacity bitmask of the cgroup. For ex: if the max bits in the CBM
> > > > is 10 and the cache size is 10MB, each bit represents 1MB of cache
> > > > capacity.
> > > > 
> > > > Root cgroup always has all the bits set in the l3_cbm. User can create
> > > > more cgroups with mkdir syscall. By default the child cgroups inherit
> > > > the capacity bitmask(CBM) from parent. User can change the CBM specified
> > > > in hex for each cgroup. Each unique bitmask is associated with a class
> > > > of service ID and an -ENOSPC is returned once we run out of
> > > > closids.
> > > 
> > > This is still the original crap. No, we are not introducing this
> > > interface now just because we can. I explained in great length why
> > > this is completely useless and what we really need.
> > > 
> > > Thanks,
> > > 
> > > 	tglx
> > 
> > Can you make a summary of the points, and enumerate them, please.
> > (what are the problems of the current interface, and why such problems
> > are fixed in the new interface?).
> 
> Marcelo, you participated on the discussions. We discussed why this
> is a bad interface a *lot* in the v15 posting. There are two writeups
> that summarize all the problems:
> 
>  - https://lkml.org/lkml/2015/11/18/637

OK cool hopefully that makes it clear to Fenghua Yu what must
be changed in the patchset.

>  - http://lkml.iu.edu/hypermail/linux/kernel/1511.0/02375.html