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

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Date:   Thu, 17 Aug 2017 14:59:20 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.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 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?