linux-kernel - Re: [PATCH 3/7] x86/intel_rdt: Support cache bit mask for Intel CAT

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Date:	Fri, 27 Feb 2015 14:42:04 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Vikas Shivappa <vikas.shivappa@...el.com>
Cc:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	linux-kernel@...r.kernel.org, matt.fleming@...el.com,
	hpa@...or.com, tglx@...utronix.de, mingo@...nel.org,
	peterz@...radead.org, will.auld@...el.com, dave.hansen@...el.com,
	andi.kleen@...el.com, tony.luck@...el.com, kanaka.d.juvva@...el.com
Subject: Re: [PATCH 3/7] x86/intel_rdt: Support cache bit mask for Intel CAT

Hello, Vikas.

On Fri, Feb 27, 2015 at 11:34:16AM -0800, Vikas Shivappa wrote:
> This cgroup subsystem would basically let the user partition one of the
> Platform shared resource , the LLC cache. This could be extended in future

I suppose LLC means last level cache?  It'd be great if you can spell
out the full term when the abbreviation is first referenced in the
comments or documentation.

> to partition more shared resources when there is hardware support that way
> we may eventually have more files in the cgroup. RDT is a generic term for
> platform resource sharing.

> For more information you can refer to section 17.15 of Intel SDM.
> We did go through quite a bit of discussion on lkml regarding adding the
> cgroup interface for CAT and the patches were posted only after that.
> This cgroup would not interact with other cgroups in the sense would not
> modify or add any elements to existing cgroups - there was such a proposal
> but was removed as we did not get agreement on lkml.
>
> the original lkml thread is here from 10/2014 for your reference -
> https://lkml.org/lkml/2014/10/16/568

Yeap, I followed that thread and this being a separate controller
definitely makes a lot more sense.

>   I
> >take it that the feature implemented is too coarse to allow for weight
> >based distribution?
> >
> Could you please clarify more on this ? However there is a limitation from
> hardware that there have to be a minimum of 2 bits in the cbm if thats what
> you referred to. Otherwise the bits in the cbm directly map to the number of
> cache ways and hence the cache capacity ..

Right, so the granularity is fairly coarse and specifying things like
"distribute cache in 4:2:1 (or even in absolute bytes) to these three
cgroups" wouldn't work at all.

Thanks.

-- 
tejun
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