linux-kernel - Re: [PATCH RFC] ioctl based CAT interface

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Message-ID: <20151116090756.GM17308@twins.programming.kicks-ass.net>
Date:	Mon, 16 Nov 2015 10:07:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Luiz Capitulino <lcapitulino@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vikas Shivappa <vikas.shivappa@...el.com>,
	Tejun Heo <tj@...nel.org>, Yu Fenghua <fenghua.yu@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] ioctl based CAT interface

On Fri, Nov 13, 2015 at 03:33:04PM -0200, Marcelo Tosatti wrote:
> On Fri, Nov 13, 2015 at 05:51:00PM +0100, Peter Zijlstra wrote:
> > On Fri, Nov 13, 2015 at 02:39:33PM -0200, Marcelo Tosatti wrote:
> > > + * 	* one tcrid entry can be in different locations 
> > > + * 	  in different sockets.
> > 
> > NAK on that without cpuset integration.
> > 
> > I do not want freely migratable tasks having radically different
> > performance profiles depending on which CPU they land.
> 
> Ok, so, configuration:
> 
> 
> Socket-1				Socket-2
> 
> pinned thread-A with			100% L3 free
> 80% of L3 
> reserved
> 
> 
> So it is a problem if a thread running on socket-2 is scheduled to 
> socket-1 because performance is radically different, fine.
> 
> Then one way to avoid that is to not allow freely migratable tasks
> to move to Socket-1. Fine.
> 
> Then you want to use cpusets for that.
> 
> Can you fill in the blanks what is missing here?

I'm still not seeing what the problem with CAT-cgroup is.

/cgroups/cpuset/
  socket-1/cpus = $socket-1
           tasks = $thread-A

  socket-2/cpus = $socket-2
           tasks = $thread-B

/cgroups/cat/
  group-A/bitmap = 0x3F / 0xFF
  group-A/tasks = $thread-A

  group-B/bitmap = 0xFF / 0xFF
  group-B/tasks = $thread-B


That gets you thread-A on socket-1 with 6/8 of the L3 and thread-B on
socket-2 with 8/8 of the L3.
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