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Date:   Thu, 31 Aug 2023 18:53:01 +0200
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Subject: Re: [PATCH drm-misc-next 2/3] drm/gpuva_mgr: generalize
 dma_resv/extobj handling and GEM validation
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To:     Danilo Krummrich <dakr@redhat.com>
Cc:     airlied@gmail.com, daniel@ffwll.ch, matthew.brost@intel.com,
        thomas.hellstrom@linux.intel.com, sarah.walker@imgtec.com,
        donald.robson@imgtec.com, boris.brezillon@collabora.com,
        christian.koenig@amd.com, faith.ekstrand@collabora.com,
        bskeggs@redhat.com, Liam.Howlett@oracle.com,
        nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org
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From:   =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
In-Reply-To: <ZPB26A0/oLHTmyqk@cassiopeiae>
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Hi,

On 8/31/23 13:18, Danilo Krummrich wrote:
> On Thu, Aug 31, 2023 at 11:04:06AM +0200, Thomas Hellström (Intel) wrote:
>> Hi!
>>
>> On 8/30/23 17:00, Danilo Krummrich wrote:
>>> On Wed, Aug 30, 2023 at 03:42:08PM +0200, Thomas Hellström (Intel) wrote:
>>>> On 8/30/23 14:49, Danilo Krummrich wrote:
>>>>> Hi Thomas,
>>>>>
>>>>> thanks for having a look!
>>>>>
>>>>> On Wed, Aug 30, 2023 at 09:27:45AM +0200, Thomas Hellström (Intel) wrote:
>>>>>> Hi, Danilo.
>>>>>>
>>>>>> Some quick comments since I'm doing some Xe work in this area. Will probably
>>>>>> get back with more.
>>>>>>
>>>>>> On 8/20/23 23:53, Danilo Krummrich wrote:
> <snip>
>
>>>>>>> diff --git a/include/drm/drm_gpuva_mgr.h b/include/drm/drm_gpuva_mgr.h
>>>>>>> index ed8d50200cc3..693e2da3f425 100644
>>>>>>> --- a/include/drm/drm_gpuva_mgr.h
>>>>>>> +++ b/include/drm/drm_gpuva_mgr.h
>>>>>>> @@ -26,12 +26,16 @@
>>>>>>>       */
>>>>>>>      #include <linux/list.h>
>>>>>>> +#include <linux/dma-resv.h>
>>>>>>> +#include <linux/maple_tree.h>
>>>>>>>      #include <linux/rbtree.h>
>>>>>>>      #include <linux/types.h>
>>>>>>>      #include <drm/drm_gem.h>
>>>>>>> +#include <drm/drm_exec.h>
>>>>>>>      struct drm_gpuva_manager;
>>>>>>> +struct drm_gpuva_gem;
>>>>>>>      struct drm_gpuva_fn_ops;
>>>>>>>      /**
>>>>>>> @@ -140,7 +144,7 @@ struct drm_gpuva {
>>>>>>>      int drm_gpuva_insert(struct drm_gpuva_manager *mgr, struct drm_gpuva *va);
>>>>>>>      void drm_gpuva_remove(struct drm_gpuva *va);
>>>>>>> -void drm_gpuva_link(struct drm_gpuva *va);
>>>>>>> +void drm_gpuva_link(struct drm_gpuva *va, struct drm_gpuva_gem *vm_bo);
>>>>>>>      void drm_gpuva_unlink(struct drm_gpuva *va);
>>>>>>>      struct drm_gpuva *drm_gpuva_find(struct drm_gpuva_manager *mgr,
>>>>>>> @@ -240,15 +244,137 @@ struct drm_gpuva_manager {
>>>>>>>      	 * @ops: &drm_gpuva_fn_ops providing the split/merge steps to drivers
>>>>>>>      	 */
>>>>>>>      	const struct drm_gpuva_fn_ops *ops;
>>>>>>> +
>>>>>>> +	/**
>>>>>>> +	 * @d_obj: Dummy GEM object; used internally to pass the GPU VMs
>>>>>>> +	 * dma-resv to &drm_exec.
>>>>>>> +	 */
>>>>>>> +	struct drm_gem_object d_obj;
>>>>>>> +
>>>>>>> +	/**
>>>>>>> +	 * @resv: the &dma_resv for &drm_gem_objects mapped in this GPU VA
>>>>>>> +	 * space
>>>>>>> +	 */
>>>>>>> +	struct dma_resv *resv;
>>>>>>> +
>>>>>>> +	/**
>>>>>>> +	 * @exec: the &drm_exec helper to lock external &drm_gem_objects
>>>>>>> +	 */
>>>>>>> +	struct drm_exec exec;
>>>>>>> +
>>>>>>> +	/**
>>>>>>> +	 * @mt_ext: &maple_tree storing external &drm_gem_objects
>>>>>>> +	 */
>>>>>>> +	struct maple_tree mt_ext;
>>>>>> Why are you using a maple tree here? Insertion and removal is O(log(n))
>>>>>> instead of O(1) for a list?
>>>>>>
>>>>> Having a list of drm_gem_objects directly wouldn't work, as multiple GPU-VMs
>>>>> could have mappings of the same extobj.
>>>>>
>>>>> I considered using the VM_BO abstraction (struct drm_gpuva_gem) as list entry
>>>>> instead, which also seems to be the obvious choice. However, there is a locking
>>>>> conflict.
>>>>>
>>>>> A drm_gem_object keeps a list of drm_gpuva_gems, while each drm_gpuva_gem keeps
>>>>> a list of drm_gpuvas. Both lists are either protected with the dma-resv lock of
>>>>> the corresponding drm_gem_object, or with an external lock provided by the
>>>>> driver (see drm_gem_gpuva_set_lock()). The latter is used by drivers performing
>>>>> changes on the GPUVA space directly from the fence signalling path.
>>>>>
>>>>> Now, similar to what drm_gpuva_link() and drm_gpuva_unlink() are doing already,
>>>>> we'd want to add a drm_gpuva_gem to the extobj list for the first mapping being
>>>>> linked and we'd want to remove it for the last one being unlinked.
>>>>>
>>>>> (Actually we'd want to add the drm_gpuva_gem object to the extobj list even
>>>>> before, because otherwise we'd not acquire it's dma-resv lock of this GEM object
>>>>> through drm_gpuva_manager_lock(). But that's trival, we could do that when we
>>>>> create the drm_gpuva_gem, which we need to do anyways.)
>>>>>
>>>>> Anyway, we'd probably want to keep removing the drm_gpuva_gem from the extobj
>>>>> list from drm_gpuva_unlink() when the last mapping of this BO is unlinked. In
>>>>> order to do so, we'd (as discussed above) either need to hold the outer GPU-VM
>>>>> lock or the GPU-VMs dma-resv lock. Both would be illegal in the case
>>>>> drm_gpuva_unlink() is called from within the fence signalling path. For drivers
>>>>> like XE or Nouveau, we'd at least need to make sure to not mess up the locking
>>>>> hierarchy of GPU-VM lock and dma-resv lock of the corresponding BO.
>>>>>
>>>>> Considering all that, I thought it's probably better to track extobjs separate
>>>>> from the drm_gpuva_gem, hence the maple tree choice.
>>>> Hm. OK, in Xe we're having a list of the xe_vmas (drm_gpuvas) that point to
>>>> external objects, or in the case of multiple mappings to the same gem
>>>> object, only one of the drm_gpuvas is in the list. These are protected by
>>>> the GPU-VM lock. I don't see a problem with removing those from the fence
>>>> signalling path, though?
>>> I intentionally tried to avoid keeping a list of drm_gpuvas to track extobjs,
>>> since this is generic code I don't know how much mappings of an external object
>>> the corresponding driver potentially creates. This could become a pretty large
>>> list to iterate. Another reason was, that I want to keep the drm_gpuva structure
>>> as small as possible, hence avoiding another list_head.
>> Yes, the list might be pretty large, but OTOH you never iterate to access a
>> single list element. When you need to iterate the whole list you need to do
>> that regardless of the data structure used. As for the list head, it might
>> perhaps be aliased (union) with an upcoming userptr list head?
>>
> Oh, I did not mean that I'm concerned about the size of a list of extobjs in
> general, that would indeed be the same for every data structure chosen. But I
> would be concerned about keeping a list of *all* mappings being backed by an
> extobj.
>
>>> Now, it sounds like in XE you're doing some kind of optimization just keeping a
>>> single mapping of an extobj in the list? How do you know when to remove it? What
>>> if the mapping from the extobj list gets unmapped, but there is still another
>>> one left in the GPU-VM being backed by the same BO?
>> When removing from the lists, we iterate through the object's list of vmas,
>> and if there is one matching the same vm, we replace the old one with the
>> new one. A similar iteration is done when adding to avoid adding one that is
>> already on the list.
> I see, but wouldn't this be O(n) on insertion and O(m) on removal of an extobj,
> while using the maple tree is O(log(n))?

No, insertion and removal is O(m) where m is the number of vms the 
object is currently bound to. Typically a very small number.

>
>>> Although assuming that's a no-go for GPUVA wouldn't an XArray be a better
>>> choice, keeping O(1)?
>>> When tracking extobjs, the address of the drm_gem_object is the key while the
>>> reference count is the value. I was thinking of an XArray as well, but I was
>>> worried that the corresponding indices could be too much distributed for an
>>> XArray to still be efficient. Now that I think about it, it's probably not that
>>> bad.
>>>
>>> Btw., while I agree trying to make things as efficient as possible, what is the
>>> magnitue for extobjs to be tracked, do we need to worry about the O(log(n))?
>> Not sure yet, TBH, but I think one of our UMDs can only use external object,
>> because they don't know at creation time which ones need exporting. However
>> if this turns out to be too bad, there are various flavours of "clever but
>> complicated" optimizations that we could think of to reduce the list size.
>> Still in our case, we opted for the vma list head for now.
> Considering the above, I would guess that if your current approach is good
> enough, a maple tree will work as well.

Hmm, Yeah it's probably a bikeshed since each drm_exec builds a 
realloced array of all external objects on each exec.

>
> Otherwise, if you want, I could do some experiments with Xarray and see how
> that works out compared to using a maple tree.
>
> Btw. another nice thing about using Xarray or maple tree for that is that
> drivers updating the VA space from the fence signalling path don't need to
> hold a GPU-VM lock to update the extobj list. Actually, they might not need
> a GPU-VM lock at all.

I still don't follow why drivers would want to do that. Isn't the VA 
space / fence object list always updated sync from the IOCTL?

/Thomas


>
>> /Thomas
>>
>>
>>>>>>> +
>>>>>>> +	/**
>>>>>>> +	 * @evict: structure holding the evict list and evict list lock
>>>>>>> +	 */
>>>>>>> +	struct {
>>>>>>> +		/**
>>>>>>> +		 * @list: &list_head storing &drm_gem_objects currently being
>>>>>>> +		 * evicted
>>>>>>> +		 */
>>>>>>> +		struct list_head list;
>>>>>>> +
>>>>>>> +		/**
>>>>>>> +		 * @lock: spinlock to protect the evict list against concurrent
>>>>>>> +		 * insertion / removal of different &drm_gpuva_gems
>>>>>>> +		 */
>>>>>>> +		spinlock_t lock;
>>>>>>> +	} evict;
>>>>>>>      };
>>>>>>>      void drm_gpuva_manager_init(struct drm_gpuva_manager *mgr,
>>>>>>> +			    struct drm_device *drm,
>>>>>>>      			    const char *name,
>>>>>>>      			    u64 start_offset, u64 range,
>>>>>>>      			    u64 reserve_offset, u64 reserve_range,
>>>>>>>      			    const struct drm_gpuva_fn_ops *ops);
>>>>>>>      void drm_gpuva_manager_destroy(struct drm_gpuva_manager *mgr);
>>>>>>> +/**
>>>>>>> + * DRM_GPUVA_EXEC - returns the &drm_gpuva_managers &drm_exec instance
>>>>>>> + * @mgr: the &drm_gpuva_managers to return the &drm_exec instance for
>>>>>>> + */
>>>>>>> +#define DRM_GPUVA_EXEC(mgr)	&(mgr)->exec
>>>>>> A struct ww_acquire_ctx and thus a drm_exec is fundamentally per task and
>>>>>> should typically be allocated on the stack. Otherwise you'd need to protect
>>>>>> the mgr->exec member with an exclusive lock throughout the locking process,
>>>>>> and that's not what we want.
>>>>> Oh, good point. I think it works in Nouveau, because there it's implicitly
>>>>> protected with the job submission lock.
>>>>>
>>>>>> Did you consider subclassing a drm_exec for drm_gpuva purposes and add
>>>>>> needed ops to it: Like so:
>>>>> That's a good idea, will take this into V2.
>>>> Actually, I'm not fully sure that was a good idea: I've now have a working
>>>> version of Xe ported over to drm_exec, having these helpers in mind and with
>>>> the intention to start using them as they mature. What I found, though is
>>>> that open-coding the drm_exec loop is not all that bad, but that building
>>>> blocks that can be called from within the loop are useful:
>>>>
>>>> Like the drm_gpuva_prepare_objects() and an imaginary
>>>> drm_gpuva_prepare_gpuva() that locks the vm resv and the resv of the object
>>>> (if different and the gpuva points to the object. And
>>>> drm_gpuva_prepare_array() although we don't use it within Xe. That means you
>>>> can use these building blocks like helpers and avoid the fn() callback by
>>>> instead open-coding.
>>>>
>>>> But I guess YMMV.
>>> That's exactly why those building blocks are exported, I already had in mind
>>> that there might be drivers which still want to open-code the drm_exec loop,
>>> while others might just want a simple interface to lock everything.
>>>
>>> I still think it is a good idea, but I'd keep that as simple as possible. And
>>> for everything else just let the driver open-code it and use the "building
>>> blocks" - will also expand the bulding blocks to what you mentioned above.
>>>
>>>>>> struct drm_gpuva_exec_ops {
>>>>>>        int (*fn) (struct drm_gpuva_exec *exec, int num_fences);
>>>>> Is this the fn argument from drm_gpuva_manager_lock_extra()?
>>>>>
>>>>>>        int (*bo_validate) (struct drm_gpuva_exec *exec, struct drm_gem_object
>>>>>> *obj);
>>>>> I guess we could also keep that within the drm_gpuva_fn_ops? This should always
>>>>> be the same callback, right?
>>>>>
>>>>>> };
>>>>>>
>>>>>> struct drm_gpuva_exec {
>>>>>>        const struct drm_gpuva_exec_ops *ops;
>>>>>>        struct drm_exec exec;
>>>>>>        struct drm_gpuva_manager *mgr;
>>>>>> };
>>>>>>
>>>>>> Although I'd actually expect bo_validate to be part of fn in the typical
>>>>>> case. The drm_gpuva_exec would then be allocated by the caller on the stack.
>>>>> This doesn't sound like my assumption about fn() above is correct.
>>>> Well one important thing in our conversion is that ttm_bo_validate () needs
>>>> to be in the until_all_locked() loop. We want to be able soon to use
>>>> sleeping locks for eviction, so a xe_bo_validate() would, at least
>>>> temporarily, add locked objects to the drm_exec list of locked objects. That
>>>> means everything that may end up calling validate deep within the call chain
>>>> needs to be part of the until_all_locked() loop, so our
>>>> drm_gpuva_manager_lock_extra() fn callback would include those validates and
>>>> look different all the time. Hence that's why open-coding isn't all that
>>>> bad...
>>> Oh, I see. You indeed want to call validate() from within until_all_locked().
>>>
>>>> /Thomas
>>>>
>>>>
> <snip>