linux-kernel - Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5

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Date:   Fri, 21 Jul 2017 11:22:04 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Bob Liu <liubo95@...wei.com>,
        "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>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5

On Thu, Jul 20, 2017 at 08:48:20PM -0700, Dan Williams wrote:
> On Thu, Jul 20, 2017 at 6:41 PM, Jerome Glisse <jglisse@...hat.com> wrote:
> > On Fri, Jul 21, 2017 at 09:15:29AM +0800, Bob Liu wrote:
> >> On 2017/7/20 23:03, Jerome Glisse wrote:
> >> > On Wed, Jul 19, 2017 at 05:09:04PM +0800, Bob Liu wrote:
> >> >> On 2017/7/19 10:25, Jerome Glisse wrote:
> >> >>> On Wed, Jul 19, 2017 at 09:46:10AM +0800, Bob Liu wrote:
> >> >>>> On 2017/7/18 23:38, Jerome Glisse wrote:
> >> >>>>> On Tue, Jul 18, 2017 at 11:26:51AM +0800, Bob Liu wrote:
> >> >>>>>> On 2017/7/14 5:15, Jérôme Glisse wrote:
> >
> > [...]
> >
> >> >> Then it's more like replace the numa node solution(CDM) with ZONE_DEVICE
> >> >> (type MEMORY_DEVICE_PUBLIC). But the problem is the same, e.g how to make
> >> >> sure the device memory say HBM won't be occupied by normal CPU allocation.
> >> >> Things will be more complex if there are multi GPU connected by nvlink
> >> >> (also cache coherent) in a system, each GPU has their own HBM.
> >> >>
> >> >> How to decide allocate physical memory from local HBM/DDR or remote HBM/
> >> >> DDR?
> >> >>
> >> >> If using numa(CDM) approach there are NUMA mempolicy and autonuma mechanism
> >> >> at least.
> >> >
> >> > NUMA is not as easy as you think. First like i said we want the device
> >> > memory to be isolated from most existing mm mechanism. Because memory
> >> > is unreliable and also because device might need to be able to evict
> >> > memory to make contiguous physical memory allocation for graphics.
> >> >
> >>
> >> Right, but we need isolation any way.
> >> For hmm-cdm, the isolation is not adding device memory to lru list, and many
> >> if (is_device_public_page(page)) ...
> >>
> >> But how to evict device memory?
> >
> > What you mean by evict ? Device driver can evict whenever they see the need
> > to do so. CPU page fault will evict too. Process exit or munmap() will free
> > the device memory.
> >
> > Are you refering to evict in the sense of memory reclaim under pressure ?
> >
> > So the way it flows for memory pressure is that if device driver want to
> > make room it can evict stuff to system memory and if there is not enough
> > system memory than thing get reclaim as usual before device driver can
> > make progress on device memory reclaim.
> >
> >
> >> > Second device driver are not integrated that closely within mm and the
> >> > scheduler kernel code to allow to efficiently plug in device access
> >> > notification to page (ie to update struct page so that numa worker
> >> > thread can migrate memory base on accurate informations).
> >> >
> >> > Third it can be hard to decide who win between CPU and device access
> >> > when it comes to updating thing like last CPU id.
> >> >
> >> > Fourth there is no such thing like device id ie equivalent of CPU id.
> >> > If we were to add something the CPU id field in flags of struct page
> >> > would not be big enough so this can have repercusion on struct page
> >> > size. This is not an easy sell.
> >> >
> >> > They are other issues i can't think of right now. I think for now it
> >>
> >> My opinion is most of the issues are the same no matter use CDM or HMM-CDM.
> >> I just care about a more complete solution no matter CDM,HMM-CDM or other ways.
> >> HMM or HMM-CDM depends on device driver, but haven't see a public/full driver to
> >> demonstrate the whole solution works fine.
> >
> > I am working with NVidia close source driver team to make sure that it works
> > well for them. I am also working on nouveau open source driver for same NVidia
> > hardware thought it will be of less use as what is missing there is a solid
> > open source userspace to leverage this. Nonetheless open source driver are in
> > the work.
> 
> Can you point to the nouveau patches? I still find these HMM patches
> un-reviewable without an upstream consumer.

I am still working on those, i hope i will be able to post them in 3 weeks or so.

Cheers,
Jérôme