linux-kernel - Re: Cache Allocation Technology Design

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Message-ID: <20141030224740.GC12706@worktop.programming.kicks-ass.net>
Date:	Thu, 30 Oct 2014 23:47:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vikas Shivappa <vikas.shivappa@...el.com>,
	"Auld, Will" <will.auld@...el.com>,
	Matt Fleming <matt@...sole-pimps.org>,
	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


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.

Are there any other cases I might have missed?
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