Re: [PATCH 01/24] dma-buf: add dma_resv_replace_fences

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 03, 2022 at 11:48:25AM +0100, Christian König wrote:
> Am 22.12.21 um 22:05 schrieb Daniel Vetter:
> > On Tue, Dec 07, 2021 at 01:33:48PM +0100, Christian König wrote:
> > > This function allows to replace fences from the shared fence list when
> > > we can gurantee that the operation represented by the original fence has
> > > finished or no accesses to the resources protected by the dma_resv
> > > object any more when the new fence finishes.
> > > 
> > > Then use this function in the amdkfd code when BOs are unmapped from the
> > > process.
> > > 
> > > Signed-off-by: Christian König <christian.koenig@amd.com>
> > > ---
> > >   drivers/dma-buf/dma-resv.c                    | 43 ++++++++++++++++
> > >   .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c  | 49 +++----------------
> > >   include/linux/dma-resv.h                      |  2 +
> > >   3 files changed, 52 insertions(+), 42 deletions(-)
> > > 
> > > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> > > index 4deea75c0b9c..a688dbded3d3 100644
> > > --- a/drivers/dma-buf/dma-resv.c
> > > +++ b/drivers/dma-buf/dma-resv.c
> > > @@ -284,6 +284,49 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
> > >   }
> > >   EXPORT_SYMBOL(dma_resv_add_shared_fence);
> > > +/**
> > > + * dma_resv_replace_fences - replace fences in the dma_resv obj
> > > + * @obj: the reservation object
> > > + * @context: the context of the fences to replace
> > > + * @replacement: the new fence to use instead
> > > + *
> > > + * Replace fences with a specified context with a new fence. Only valid if the
> > > + * operation represented by the original fences is completed or has no longer
> > > + * access to the resources protected by the dma_resv object when the new fence
> > > + * completes.
> > > + */
> > > +void dma_resv_replace_fences(struct dma_resv *obj, uint64_t context,
> > > +			     struct dma_fence *replacement)
> > > +{
> > > +	struct dma_resv_list *list;
> > > +	struct dma_fence *old;
> > > +	unsigned int i;
> > > +
> > > +	dma_resv_assert_held(obj);
> > > +
> > > +	write_seqcount_begin(&obj->seq);
> > > +
> > > +	old = dma_resv_excl_fence(obj);
> > > +	if (old->context == context) {
> > > +		RCU_INIT_POINTER(obj->fence_excl, dma_fence_get(replacement));
> > > +		dma_fence_put(old);
> > > +	}
> > > +
> > > +	list = dma_resv_shared_list(obj);
> > > +	for (i = 0; list && i < list->shared_count; ++i) {
> > > +		old = rcu_dereference_protected(list->shared[i],
> > > +						dma_resv_held(obj));
> > > +		if (old->context != context)
> > > +			continue;
> > > +
> > > +		rcu_assign_pointer(list->shared[i], dma_fence_get(replacement));
> > > +		dma_fence_put(old);
> > Since the fences are all guaranteed to be from the same context, maybe we
> > should have a WARN_ON(__dma_fence_is_later()); here just to be safe?
> 
> First of all happy new year!

Happy new year to you too!

Also I'm only still catching up.

> Then to answer your question, no :)
> 
> This here is the case where we replace an preemption fence with a VM page
> table update fence. So both fences are not from the same context.
> 
> But since you ask that means that I somehow need to improve the
> documentation.

Hm yeah then I'm confused, since right above you have the context check.
And I thought if the contexts are equal, then the fences must be ordered,
and since you're adding a new one it must be a later fences.

But now you're saying this is to replace a fence with a totally different
context one (which can totally make sense for the special fences compute
mode contexts all need), but then I honestly don't get why you even check
for the context.

Maybe more docs help explain what's going on, or maybe we should have the
is_later check only if the new fences is from the same context. amdkfd
might not benefit, but this is a new generic interface and other drivers
might horrendously screw this up :-) Plus then a big comment that if it's
a different fence timeline context the driver must guarantee that the new
fence is guaranteed to signal after anything we're replacing here.

I think it might also be good to just include the specific amdkfd use case
with a short intro to wth are preempt-ctx and page table fences, to
explain when this function is actually useful.

It's definitely a very special case function, and I'm worried driver
authors might come up with creative abuses for it that cause trouble.
-Daniel

> 
> Regards,
> Christian.
> 
> > 
> > With that added:
> > 
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > > +	}
> > > +
> > > +	write_seqcount_end(&obj->seq);
> > > +}
> > > +EXPORT_SYMBOL(dma_resv_replace_fences);
> > > +
> > >   /**
> > >    * dma_resv_add_excl_fence - Add an exclusive fence.
> > >    * @obj: the reservation object
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> > > index 71acd577803e..b558ef0f8c4a 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> > > @@ -236,53 +236,18 @@ void amdgpu_amdkfd_release_notify(struct amdgpu_bo *bo)
> > >   static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
> > >   					struct amdgpu_amdkfd_fence *ef)
> > >   {
> > > -	struct dma_resv *resv = bo->tbo.base.resv;
> > > -	struct dma_resv_list *old, *new;
> > > -	unsigned int i, j, k;
> > > +	struct dma_fence *replacement;
> > >   	if (!ef)
> > >   		return -EINVAL;
> > > -	old = dma_resv_shared_list(resv);
> > > -	if (!old)
> > > -		return 0;
> > > -
> > > -	new = kmalloc(struct_size(new, shared, old->shared_max), GFP_KERNEL);
> > > -	if (!new)
> > > -		return -ENOMEM;
> > > -
> > > -	/* Go through all the shared fences in the resevation object and sort
> > > -	 * the interesting ones to the end of the list.
> > > +	/* TODO: Instead of block before we should use the fence of the page
> > > +	 * table update and TLB flush here directly.
> > >   	 */
> > > -	for (i = 0, j = old->shared_count, k = 0; i < old->shared_count; ++i) {
> > > -		struct dma_fence *f;
> > > -
> > > -		f = rcu_dereference_protected(old->shared[i],
> > > -					      dma_resv_held(resv));
> > > -
> > > -		if (f->context == ef->base.context)
> > > -			RCU_INIT_POINTER(new->shared[--j], f);
> > > -		else
> > > -			RCU_INIT_POINTER(new->shared[k++], f);
> > > -	}
> > > -	new->shared_max = old->shared_max;
> > > -	new->shared_count = k;
> > > -
> > > -	/* Install the new fence list, seqcount provides the barriers */
> > > -	write_seqcount_begin(&resv->seq);
> > > -	RCU_INIT_POINTER(resv->fence, new);
> > > -	write_seqcount_end(&resv->seq);
> > > -
> > > -	/* Drop the references to the removed fences or move them to ef_list */
> > > -	for (i = j; i < old->shared_count; ++i) {
> > > -		struct dma_fence *f;
> > > -
> > > -		f = rcu_dereference_protected(new->shared[i],
> > > -					      dma_resv_held(resv));
> > > -		dma_fence_put(f);
> > > -	}
> > > -	kfree_rcu(old, rcu);
> > > -
> > > +	replacement = dma_fence_get_stub();
> > > +	dma_resv_replace_fences(bo->tbo.base.resv, ef->base.context,
> > > +				replacement);
> > > +	dma_fence_put(replacement);
> > >   	return 0;
> > >   }
> > > diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> > > index eebf04325b34..e0be34265eae 100644
> > > --- a/include/linux/dma-resv.h
> > > +++ b/include/linux/dma-resv.h
> > > @@ -457,6 +457,8 @@ void dma_resv_init(struct dma_resv *obj);
> > >   void dma_resv_fini(struct dma_resv *obj);
> > >   int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences);
> > >   void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence);
> > > +void dma_resv_replace_fences(struct dma_resv *obj, uint64_t context,
> > > +			     struct dma_fence *fence);
> > >   void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence);
> > >   int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **pfence_excl,
> > >   			unsigned *pshared_count, struct dma_fence ***pshared);
> > > -- 
> > > 2.25.1
> > > 
> 

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