linux-kernel - Re: [PATCH 5/9] x86/intel_rdt: Add new cgroup and Class of service management

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Date:	Tue, 4 Aug 2015 14:32:26 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	linux-kernel@...r.kernel.org, vikas.shivappa@...el.com,
	x86@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, peterz@...radead.org, matt.fleming@...el.com,
	will.auld@...el.com, glenn.p.williamson@...el.com,
	kanaka.d.juvva@...el.com
Subject: Re: [PATCH 5/9] x86/intel_rdt: Add new cgroup and Class of service
 management

Hello,

On Mon, Aug 03, 2015 at 05:32:50PM -0300, Marcelo Tosatti wrote:
> You really want to specify the cache configuration "at once": 
> having process-A exclusive access to 2MB of cache at all times,
> and process-B 4MB exclusive, means you can't have process-C use 4MB of 
> cache exclusively (consider 8MB cache machine).

This is akin to arguing for implementing cpuset without
sched_setaffinity() or any other facility to adjust affinity.  People
have been using affinity fine before cgroups.  Sure, certain things
are cumbersome but cgroups isn't a replacement for a proper API.

> > cgroups is not a superset of a programmable interface.  It has
> > distinctive disadvantages and not a substitute with hirearchy support
> > for regular systemcall-like interface.  I don't think it makes sense
> > to go full-on hierarchical cgroups when we don't have basic interface
> > which is likely to cover many use cases better.  A syscall-like
> > interface combined with a tool similar to taskset would cover a lot in
> > a more accessible way.
> 
> How are you going to specify sharing of portions of cache by two sets
> of tasks with a syscall interface?

Again, think about how people have been using CPU affinity.

> > cpuset-style allocation can be easier for things like this but that
> > should be an addition on top not the one and only interface.  How is
> > it gonna handle if multiple threads of a process want to restrict
> > cache usages to avoid stepping on each other's toes?  Delegate the
> > subdirectory and let the process itself open it and write to files to
> > configure when there isn't even a way to atomically access the
> > process's own directory or a way to synchronize against migration?
> 
> One would preconfigure that in advance - but you are right, a 
> syscall interface is more flexible in that respect.

I'm not trying to say cgroup controller would be useless but the
current approach seems somewhat backwards and over-engineered.  Can't
we just start with something simple?  e.g. a platform device driver
that allows restricting cache usage of a target thread (be that self
or ptraceable target)?

Thanks.

-- 
tejun
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