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

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Message-ID: <20170817215549.GD2872@redhat.com>
Date:   Thu, 17 Aug 2017 17:55:50 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.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 02:39:16PM -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.

Cheers,
Jérôme