Re: [PATCH 2/2] gpu/i915: use HMM mirror for userptr buffer object.

[Daniel Vetter]

On Thu, Nov 22, 2018 at 03:59:50PM +0200, Joonas Lahtinen wrote:
> Hi Jerome,
> 
> Bit late reply, but here goes :)
> 
> We're working quite hard to avoid pinning any pages unless they're in
> the GPU page tables. And when they are in the GPU page tables, they must
> be pinned for whole of that duration, for the reason that our GPUs can
> not take a fault. And to avoid thrashing GPU page tables, we do leave
> objects in page tables with the expectation that smart userspace
> recycles buffers.
> 
> So what I understand of your proposal, it wouldn't really make a
> difference for us in the amount of pinned pages (which I agree,
> we'd love to see going down). When we're unable to take a fault,
> the first use effectively forces us to pin any pages and keep them
> pinned to avoid thrashing GPU page tables.
> 
> So from i915 perspective, it just seems to be mostly an exchange of
> an API to an another for getting the pages. You already mentioned
> the fast path is being worked on, which is an obvious difference.
> But is there some other improvement one would be expecting, beyond
> the page pinning?
> 
> Also, is the requirement for a single non-file-backed VMA in the
> plans of being eliminated or is that inherent restriction of the
> HMM_MIRROR feature? We're currently not imposing such a limitation.

I think a clear plus for HMM would be if this helps us fix the deadlocks
and races we're seeing. But I have no idea whether this gets us any closer
here or not.
-Daniel

> 
> Regards, Joonas
> 
> Quoting jglisse@xxxxxxxxxx (2018-09-10 03:57:36)
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 
> > This replace existing code that rely on get_user_page() aka GUP with
> > code that now use HMM mirror to mirror a range of virtual address as
> > a buffer object accessible by the GPU. There is no functional changes
> > from userspace point of view.
> > 
> > From kernel point of view we no longer pin pages for userptr buffer
> > object which is a welcome change (i am assuming that everyone dislike
> > page pin as i do).
> > 
> > Another change, from kernel point of view, is that it does no longer
> > have a fast path with get_user_pages_fast() this can eventually added
> > back through HMM.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: David Airlie <airlied@xxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
> > Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> > Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> > Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> > Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch