Re: [PATCH 5/5] drm/amdgpu: implement amdgpu_gem_prime_move_notify v2

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Feb 18, 2020 at 9:17 PM Thomas Hellström (VMware)
<thomas_os@shipmail.org> wrote:
>
> On 2/17/20 6:55 PM, Daniel Vetter wrote:
> > On Mon, Feb 17, 2020 at 04:45:09PM +0100, Christian König wrote:
> >> Implement the importer side of unpinned DMA-buf handling.
> >>
> >> v2: update page tables immediately
> >>
> >> Signed-off-by: Christian König <christian.koenig@amd.com>
> >> ---
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 66 ++++++++++++++++++++-
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  |  6 ++
> >>   2 files changed, 71 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> index 770baba621b3..48de7624d49c 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> @@ -453,7 +453,71 @@ amdgpu_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
> >>      return ERR_PTR(ret);
> >>   }
> >>
> >> +/**
> >> + * amdgpu_dma_buf_move_notify - &attach.move_notify implementation
> >> + *
> >> + * @attach: the DMA-buf attachment
> >> + *
> >> + * Invalidate the DMA-buf attachment, making sure that the we re-create the
> >> + * mapping before the next use.
> >> + */
> >> +static void
> >> +amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
> >> +{
> >> +    struct drm_gem_object *obj = attach->importer_priv;
> >> +    struct ww_acquire_ctx *ticket = dma_resv_locking_ctx(obj->resv);
> >> +    struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
> >> +    struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
> >> +    struct ttm_operation_ctx ctx = { false, false };
> >> +    struct ttm_placement placement = {};
> >> +    struct amdgpu_vm_bo_base *bo_base;
> >> +    int r;
> >> +
> >> +    if (bo->tbo.mem.mem_type == TTM_PL_SYSTEM)
> >> +            return;
> >> +
> >> +    r = ttm_bo_validate(&bo->tbo, &placement, &ctx);
> >> +    if (r) {
> >> +            DRM_ERROR("Failed to invalidate DMA-buf import (%d))\n", r);
> >> +            return;
> >> +    }
> >> +
> >> +    for (bo_base = bo->vm_bo; bo_base; bo_base = bo_base->next) {
> >> +            struct amdgpu_vm *vm = bo_base->vm;
> >> +            struct dma_resv *resv = vm->root.base.bo->tbo.base.resv;
> >> +
> >> +            if (ticket) {
> > Yeah so this is kinda why I've been a total pain about the exact semantics
> > of the move_notify hook. I think we should flat-out require that importers
> > _always_ have a ticket attach when they call this, and that they can cope
> > with additional locks being taken (i.e. full EDEADLCK) handling.
> >
> > Simplest way to force that contract is to add a dummy 2nd ww_mutex lock to
> > the dma_resv object, which we then can take #ifdef
> > CONFIG_WW_MUTEX_SLOWPATH_DEBUG. Plus mabye a WARN_ON(!ticket).
> >
> > Now the real disaster is how we handle deadlocks. Two issues:
> >
> > - Ideally we'd keep any lock we've taken locked until the end, it helps
> >    needless backoffs. I've played around a bit with that but not even poc
> >    level, just an idea:
> >
> > https://cgit.freedesktop.org/~danvet/drm/commit/?id=b1799c5a0f02df9e1bb08d27be37331255ab7582
> >
> >    Idea is essentially to track a list of objects we had to lock as part of
> >    the ttm_bo_validate of the main object.
> >
> > - Second one is if we get a EDEADLCK on one of these sublocks (like the
> >    one here). We need to pass that up the entire callchain, including a
> >    temporary reference (we have to drop locks to do the ww_mutex_lock_slow
> >    call), and need a custom callback to drop that temporary reference
> >    (since that's all driver specific, might even be internal ww_mutex and
> >    not anything remotely looking like a normal dma_buf). This probably
> >    needs the exec util helpers from ttm, but at the dma_resv level, so that
> >    we can do something like this:
> >
> > struct dma_resv_ticket {
> >       struct ww_acquire_ctx base;
> >
> >       /* can be set by anyone (including other drivers) that got hold of
> >        * this ticket and had to acquire some new lock. This lock might
> >        * protect anything, including driver-internal stuff, and isn't
> >        * required to be a dma_buf or even just a dma_resv. */
> >       struct ww_mutex *contended_lock;
> >
> >       /* callback which the driver (which might be a dma-buf exporter
> >        * and not matching the driver that started this locking ticket)
> >        * sets together with @contended_lock, for the main driver to drop
> >        * when it calls dma_resv_unlock on the contended_lock. */
> >       void (drop_ref*)(struct ww_mutex *contended_lock);
> > };
> >
> > This is all supremely nasty (also ttm_bo_validate would need to be
> > improved to handle these sublocks and random new objects that could force
> > a ww_mutex_lock_slow).
> >
> Just a short comment on this:
>
> Neither the currently used wait-die or the wound-wait algorithm
> *strictly* requires a slow lock on the contended lock. For wait-die it's
> just very convenient since it makes us sleep instead of spinning with
> -EDEADLK on the contended lock. For wound-wait IIRC one could just
> immediately restart the whole locking transaction after an -EDEADLK, and
> the transaction would automatically end up waiting on the contended
> lock, provided the mutex lock stealing is not allowed. There is however
> a possibility that the transaction will be wounded again on another
> lock, taken before the contended lock, but I think there are ways to
> improve the wound-wait algorithm to reduce that probability.
>
> So in short, choosing the wound-wait algorithm instead of wait-die and
> perhaps modifying the ww mutex code somewhat would probably help passing
> an -EDEADLK up the call chain without requiring passing the contended
> lock, as long as each locker releases its own locks when receiving an
> -EDEADLK.

Hm this is kinda tempting, since rolling out the full backoff tricker
across driver boundaries is going to be real painful.

What I'm kinda worried about is the debug/validation checks we're
losing with this. The required backoff has this nice property that
ww_mutex debug code can check that we've fully unwound everything when
we should, that we've blocked on the right lock, and that we're
restarting everything without keeling over. Without that I think we
could end up with situations where a driver in the middle feels like
handling the EDEADLCK, which might go well most of the times (the
deadlock will probably be mostly within a given driver, not across).
Right up to the point where someone creates a deadlock across drivers,
and the lack of full rollback will be felt.

So not sure whether we can still keep all these debug/validation
checks, or whether this is a step too far towards clever tricks.

But definitely a neat idea ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch