linux-kernel - Re: Cache Allocation Technology Design

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Message-ID: <20141106162713.GI3592@console-pimps.org>
Date:	Thu, 6 Nov 2014 16:27:13 +0000
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>,
	Vikas Shivappa <vikas.shivappa@...el.com>,
	"Auld, Will" <will.auld@...el.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Fleming, Matt" <matt.fleming@...el.com>
Subject: Re: Cache Allocation Technology Design

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?

-- 
Matt Fleming, Intel Open Source Technology Center
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