linux-kernel - Re: [RFC PATCH 00/20] Intel(R) Resource Director Technology Cache Pseudo-Locking enabling

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Date:   Sun, 14 Jan 2018 23:54:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
cc:     fenghua.yu@...el.com, tony.luck@...el.com,
        vikas.shivappa@...ux.intel.com, dave.hansen@...el.com,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/20] Intel(R) Resource Director Technology Cache
 Pseudo-Locking enabling

On Fri, 17 Nov 2017, Reinette Chatre wrote:

Sorry for the delay. You know why :)

> On 11/17/2017 4:48 PM, Thomas Gleixner wrote:
> > On Mon, 13 Nov 2017, Reinette Chatre wrote:
> > Did you compare that against the good old cache coloring mechanism,
> > e.g. palloc ?
> 

> I understand where your question originates. I have not compared against
> PALLOC for two reasons:
>
> 1) PALLOC is not upstream and while inquiring about the status of this
> work (please see https://github.com/heechul/palloc/issues/4 for details)
> we learned that one reason for this is that recent Intel processors are
> not well supported.

So if I understand Heechul correctly then recent CPUs cannot be supported
easily due to changes in the memory controllers and the cache. I assume the
latter is related to CAT.

> 2) The most recent kernel supported by PALLOC is v4.4 and also mentioned
> in the above link there is currently no plan to upstream this work for a
> less divergent comparison of PALLOC and the more recent RDT/CAT enabling
> on which Cache Pseudo-Locking is built.

Well, that's not a really good excuse for not trying. You at Intel should
be able to get to the parameters easy enough :)

> >> The cache pseudo-locking approach relies on generation-specific behavior
> >> of processors. It may provide benefits on certain processor generations,
> >> but is not guaranteed to be supported in the future.
> > 
> > Hmm, are you saying that the CAT mechanism might change radically in the
> > future so that access to cached data in an allocated area which does not
> > belong to the current executing context wont work anymore?
> 
> Most devices that publicly support CAT in the Linux mainline can take
> advantage of Cache Pseudo-Locking.  However, Cache Pseudo-Locking is a
> model-specific feature so there may be some variation in if, or to what
> extent, current and future devices can support Cache Pseudo-Locking. CAT
> remains architectural.

Sure, but that does NOT answer my question at all.

> >> It is not a guarantee that data will remain in the cache. It is not a
> >> guarantee that data will remain in certain levels or certain regions of
> >> the cache. Rather, cache pseudo-locking increases the probability that
> >> data will remain in a certain level of the cache via carefully
> >> configuring the CAT feature and carefully controlling application
> >> behavior.
> > 
> > Which kind of applications are you targeting with that?
> >
> > Are there real world use cases which actually can benefit from this and
> 
> To ensure I answer your question I will consider two views. First, the
>"carefully controlling application behavior" referred to above refers to
> applications/OS/VMs running after the pseudo-locked regions have been set
> up. These applications should take care to not do anything, for example
> call wbinvd, that would affect the Cache Pseudo-Locked regions. Second,
> what you are also asking about is the applications using these Cache
> Pseudo-Locked regions. We do see a clear performance benefit to
> applications using these pseudo-locked regions. Latency sensitive
> applications could relocate their code as well as data to pseudo-locked
> regions for improved performance.

This is again a marketing pitch and not answering my question about real
world use cases.

> > what are those applications supposed to do once the feature breaks with
> > future generations of processors?
> 
> This feature is model specific with a few platforms supporting it at this
> time. Only platforms known to support Cache Pseudo-Locking will expose
> its resctrl interface.

And you deliberately avoided to answer my question again.

Thanks,

	tglx