linux-kernel - Re: [HMM-v25 00/19] HMM (Heterogeneous Memory Management) v25

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Date:   Thu, 17 Aug 2017 18:02:40 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: Re: [HMM-v25 00/19] HMM (Heterogeneous Memory Management) v25

On Thu, Aug 17, 2017 at 02:59:20PM -0700, Dan Williams wrote:
> On Thu, Aug 17, 2017 at 2:55 PM, Jerome Glisse <jglisse@...hat.com> wrote:
> > On Thu, Aug 17, 2017 at 02:39:16PM -0700, Andrew Morton wrote:
> >> On Wed, 16 Aug 2017 20:05:29 -0400 J__r__me Glisse <jglisse@...hat.com> wrote:
> >>
> >> > Heterogeneous Memory Management (HMM) (description and justification)
> >>
> >> The patchset adds 55 kbytes to x86_64's mm/*.o and there doesn't appear
> >> to be any way of avoiding this overhead, or of avoiding whatever
> >> runtime overheads are added.
> >
> > HMM have already been integrated in couple of Red Hat kernel and AFAIK there
> > is no runtime performance issue reported. Thought the RHEL version does not
> > use static key as Dan asked.
> >
> >>
> >> It also adds 18k to arm's mm/*.o and arm doesn't support HMM at all.
> >>
> >> So that's all quite a lot of bloat for systems which get no benefit from
> >> the patchset.  What can we do to improve this situation (a lot)?
> >
> > I will look into why object file grow so much on arm. My guess is that the
> > new migrate code is the bulk of that. I can hide the new page migration code
> > behind a kernel configuration flag.
> 
> Shouldn't we completely disable all of it unless there is a driver in
> the kernel that selects it?

At one point people asked to be able to use the new migrate helper without
HMM and hence why it is not behind any HMM kconfig.

IIRC even ARM folks were interested pretty much all SOC have several DMA
engine that site idle and i think people where toying with the idea of using
this new helper to make use of them. But i can add a different kconfig to
hide this code and if people want to use it they will have to select it.

Jérôme