Re: Cache Allocation Technology Design

[Vikas Shivappa]

On Thu, 6 Nov 2014, Matt Fleming wrote:

> On Thu, 30 Oct, at 11:47:40PM, Peter Zijlstra wrote:
> > Let me reply to just this one, I'll do the rest tomorrow, need sleeps.
> >
> > On Thu, Oct 30, 2014 at 06:22:36PM -0400, Tejun Heo wrote:
> >
> > > > > This controller might not even require the distinction between
> > > > > configured and effective tho?  Can't a new child just inherit the
> > > > > parent's configuration and never allow the config to become completely
> > > > > empty?
> > > >
> > > > It can do that. But that still has a problem, there is a mapping in
> > > > hardware which restricts the number of active configurations. The total
> > > > configuration space is larger than the supported active configurations.
> > > >
> > > > So _something_ must fail. The initial proposal was mkdir failing when
> > > > there were more than the hardware supported active config cgroup
> > > > directories. The alternative was on-demand activation where we only
> > > > allocate the hardware resource when the first task gets moved into the
> > > > group -- which then clearly can fail.
> > >
> > > Hmmm... why can't it just refuse creating a different configuration
> > > when its config space is full?  Make children inherit the parent's
> > > configuration and refuse config writes which require it to create a
> > > new one if the config space is full.  Seems pretty straight-forward.
> > > What am I missing?
> >
> > We could do that I suppose, there is the one corner case that would not
> > allow, intermediate directories with a restricted config that also have
> > priv restrictions but no actual tasks. Not sure that makes sense though.
>
> Could you elaborate on this configuration?
>
> > Are there any other cases I might have missed?
>
> I don't think so.
>
> So, for the specific CAT case what you're proposing is make the failure
> case happen when writing to the cache bitmask file instead of failing
> mkdir() or echo $tid > tasks ?
>
> I think that's OK. If we've run out of CLOS ids I would expect to see
> -ENOSPC returned, whereas if we try and set an invalid bitmask we'd get
> -EINVAL.
>
> Vikas, Will?

Yes that is correct. You can always create more cgroups and the new cgroup 
just inherits the mask from the parent and uses the same CLOSid as its 
parent , so it wont fail because of lack of CLOSids.

The only case of failure as you said is when user tries to modify a cbm to 
a different one.

 >
> --
> Matt Fleming, Intel Open Source Technology Center
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