linux-kernel - Re: [PATCH V16 11/11] x86,cgroup/intel_rdt : Add a cgroup interface to manage Intel cache allocation

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Message-ID: <20151221104822.6d33aed0@redhat.com>
Date:	Mon, 21 Dec 2015 10:48:22 -0500
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>, Tony Luck <tony.luck@...el.com>,
	Ravi V Shankar <ravi.v.shankar@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	x86 <x86@...nel.org>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Subject: Re: [PATCH V16 11/11] x86,cgroup/intel_rdt : Add a cgroup interface
 to manage Intel cache allocation

On Sat, 19 Dec 2015 22:57:30 -0200
Marcelo Tosatti <mtosatti@...hat.com> wrote:

> On Sat, Dec 19, 2015 at 11:42:57AM +0100, Thomas Gleixner wrote:
> > On Thu, 17 Dec 2015, Fenghua Yu wrote:
> > 
> > > From: Fenghua Yu <fenghua.yu@...el.com>
> > > 
> > > From: Vikas Shivappa <vikas.shivappa@...ux.intel.com>
> > > 
> > > Add a new cgroup 'intel_rdt' to manage cache allocation. Each cgroup
> > > directory is associated with a class of service id(closid). To map a
> > > task with closid during scheduling, this patch removes the closid field
> > > from task_struct and uses the already existing 'cgroups' field in
> > > task_struct.
> > > 
> > > The cgroup has a file 'l3_cbm' which represents the L3 cache capacity
> > > bitmask(CBM). The CBM is global for the whole system currently. The
> > > capacity bitmask needs to have only contiguous bits set and number of
> > > bits that can be set is less than the max bits that can be set. The
> > > tasks belonging to a cgroup get to fill in the L3 cache represented by
> > > the capacity bitmask of the cgroup. For ex: if the max bits in the CBM
> > > is 10 and the cache size is 10MB, each bit represents 1MB of cache
> > > capacity.
> > > 
> > > Root cgroup always has all the bits set in the l3_cbm. User can create
> > > more cgroups with mkdir syscall. By default the child cgroups inherit
> > > the capacity bitmask(CBM) from parent. User can change the CBM specified
> > > in hex for each cgroup. Each unique bitmask is associated with a class
> > > of service ID and an -ENOSPC is returned once we run out of
> > > closids.
> > 
> > This is still the original crap. No, we are not introducing this
> > interface now just because we can. I explained in great length why
> > this is completely useless and what we really need.
> > 
> > Thanks,
> > 
> > 	tglx
> 
> Can you make a summary of the points, and enumerate them, please.
> (what are the problems of the current interface, and why such problems
> are fixed in the new interface?).

Marcelo, you participated on the discussions. We discussed why this
is a bad interface a *lot* in the v15 posting. There are two writeups
that summarize all the problems:

 - https://lkml.org/lkml/2015/11/18/637

 - http://lkml.iu.edu/hypermail/linux/kernel/1511.0/02375.html
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