Re: [PATCH v3 01/12] drm: Add dummy page per device or GEM object

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

On 1/11/21 11:15 AM, Daniel Vetter wrote:
> On Mon, Jan 11, 2021 at 05:13:56PM +0100, Daniel Vetter wrote:
>> On Fri, Jan 08, 2021 at 04:49:55PM +0000, Grodzovsky, Andrey wrote:
>>> Ok then, I guess I will proceed with the dummy pages list implementation then.
>>>
>>> Andrey
>>>
>>> ________________________________
>>> From: Koenig, Christian <Christian.Koenig@amd.com>
>>> Sent: 08 January 2021 09:52
>>> To: Grodzovsky, Andrey <Andrey.Grodzovsky@amd.com>; Daniel Vetter <daniel@ffwll.ch>
>>> Cc: amd-gfx@lists.freedesktop.org <amd-gfx@lists.freedesktop.org>; dri-devel@lists.freedesktop.org <dri-devel@lists.freedesktop.org>; daniel.vetter@ffwll.ch <daniel.vetter@ffwll.ch>; robh@kernel.org <robh@kernel.org>; l.stach@pengutronix.de <l.stach@pengutronix.de>; yuq825@gmail.com <yuq825@gmail.com>; eric@anholt.net <eric@anholt.net>; Deucher, Alexander <Alexander.Deucher@amd.com>; gregkh@linuxfoundation.org <gregkh@linuxfoundation.org>; ppaalanen@gmail.com <ppaalanen@gmail.com>; Wentland, Harry <Harry.Wentland@amd.com>
>>> Subject: Re: [PATCH v3 01/12] drm: Add dummy page per device or GEM object
>>>
>>> Mhm, I'm not aware of any let over pointer between TTM and GEM and we
>>> worked quite hard on reducing the size of the amdgpu_bo, so another
>>> extra pointer just for that corner case would suck quite a bit.
>> We have a ton of other pointers in struct amdgpu_bo (or any of it's lower
>> things) which are fairly single-use, so I'm really not much seeing the
>> point in making this a special case. It also means the lifetime management
>> becomes a bit iffy, since we can't throw away the dummy page then the last
>> reference to the bo is released (since we don't track it there), but only
>> when the last pointer to the device is released. Potentially this means a
>> pile of dangling pages hanging around for too long.
> Also if you really, really, really want to have this list, please don't
> reinvent it since we have it already. drmm_ is exactly meant for resources
> that should be freed when the final drm_device reference disappears.
> -Daniel


Can you elaborate ? We still need to actually implement the list but you want me 
to use
drmm_add_action for it's destruction instead of explicitly doing it (like I'm 
already doing from  ttm_bo_device_release) ?

Andrey


>   
>> If you need some ideas for redundant pointers:
>> - destroy callback (kinda not cool to not have this const anyway), we
>>    could refcount it all with the overall gem bo. Quite a bit of work.
>> - bdev pointer, if we move the device ttm stuff into struct drm_device, or
>>    create a common struct ttm_device, we can ditch that
>> - We could probably merge a few of the fields and find 8 bytes somewhere
>> - we still have 2 krefs, would probably need to fix that before we can
>>    merge the destroy callbacks
>>
>> So there's plenty of room still, if the size of a bo struct is really that
>> critical. Imo it's not.
>>
>>
>>> Christian.
>>>
>>> Am 08.01.21 um 15:46 schrieb Andrey Grodzovsky:
>>>> Daniel had some objections to this (see bellow) and so I guess I need
>>>> you both to agree on the approach before I proceed.
>>>>
>>>> Andrey
>>>>
>>>> On 1/8/21 9:33 AM, Christian König wrote:
>>>>> Am 08.01.21 um 15:26 schrieb Andrey Grodzovsky:
>>>>>> Hey Christian, just a ping.
>>>>> Was there any question for me here?
>>>>>
>>>>> As far as I can see the best approach would still be to fill the VMA
>>>>> with a single dummy page and avoid pointers in the GEM object.
>>>>>
>>>>> Christian.
>>>>>
>>>>>> Andrey
>>>>>>
>>>>>> On 1/7/21 11:37 AM, Andrey Grodzovsky wrote:
>>>>>>> On 1/7/21 11:30 AM, Daniel Vetter wrote:
>>>>>>>> On Thu, Jan 07, 2021 at 11:26:52AM -0500, Andrey Grodzovsky wrote:
>>>>>>>>> On 1/7/21 11:21 AM, Daniel Vetter wrote:
>>>>>>>>>> On Tue, Jan 05, 2021 at 04:04:16PM -0500, Andrey Grodzovsky wrote:
>>>>>>>>>>> On 11/23/20 3:01 AM, Christian König wrote:
>>>>>>>>>>>> Am 23.11.20 um 05:54 schrieb Andrey Grodzovsky:
>>>>>>>>>>>>> On 11/21/20 9:15 AM, Christian König wrote:
>>>>>>>>>>>>>> Am 21.11.20 um 06:21 schrieb Andrey Grodzovsky:
>>>>>>>>>>>>>>> Will be used to reroute CPU mapped BO's page faults once
>>>>>>>>>>>>>>> device is removed.
>>>>>>>>>>>>>> Uff, one page for each exported DMA-buf? That's not
>>>>>>>>>>>>>> something we can do.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> We need to find a different approach here.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Can't we call alloc_page() on each fault and link them together
>>>>>>>>>>>>>> so they are freed when the device is finally reaped?
>>>>>>>>>>>>> For sure better to optimize and allocate on demand when we reach
>>>>>>>>>>>>> this corner case, but why the linking ?
>>>>>>>>>>>>> Shouldn't drm_prime_gem_destroy be good enough place to free ?
>>>>>>>>>>>> I want to avoid keeping the page in the GEM object.
>>>>>>>>>>>>
>>>>>>>>>>>> What we can do is to allocate a page on demand for each fault
>>>>>>>>>>>> and link
>>>>>>>>>>>> the together in the bdev instead.
>>>>>>>>>>>>
>>>>>>>>>>>> And when the bdev is then finally destroyed after the last
>>>>>>>>>>>> application
>>>>>>>>>>>> closed we can finally release all of them.
>>>>>>>>>>>>
>>>>>>>>>>>> Christian.
>>>>>>>>>>> Hey, started to implement this and then realized that by
>>>>>>>>>>> allocating a page
>>>>>>>>>>> for each fault indiscriminately
>>>>>>>>>>> we will be allocating a new page for each faulting virtual
>>>>>>>>>>> address within a
>>>>>>>>>>> VA range belonging the same BO
>>>>>>>>>>> and this is obviously too much and not the intention. Should I
>>>>>>>>>>> instead use
>>>>>>>>>>> let's say a hashtable with the hash
>>>>>>>>>>> key being faulting BO address to actually keep allocating and
>>>>>>>>>>> reusing same
>>>>>>>>>>> dummy zero page per GEM BO
>>>>>>>>>>> (or for that matter DRM file object address for non imported
>>>>>>>>>>> BOs) ?
>>>>>>>>>> Why do we need a hashtable? All the sw structures to track this
>>>>>>>>>> should
>>>>>>>>>> still be around:
>>>>>>>>>> - if gem_bo->dma_buf is set the buffer is currently exported as
>>>>>>>>>> a dma-buf,
>>>>>>>>>>      so defensively allocate a per-bo page
>>>>>>>>>> - otherwise allocate a per-file page
>>>>>>>>> That exactly what we have in current implementation
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Or is the idea to save the struct page * pointer? That feels a
>>>>>>>>>> bit like
>>>>>>>>>> over-optimizing stuff. Better to have a simple implementation
>>>>>>>>>> first and
>>>>>>>>>> then tune it if (and only if) any part of it becomes a problem
>>>>>>>>>> for normal
>>>>>>>>>> usage.
>>>>>>>>> Exactly - the idea is to avoid adding extra pointer to
>>>>>>>>> drm_gem_object,
>>>>>>>>> Christian suggested to instead keep a linked list of dummy pages
>>>>>>>>> to be
>>>>>>>>> allocated on demand once we hit a vm_fault. I will then also
>>>>>>>>> prefault the entire
>>>>>>>>> VA range from vma->vm_end - vma->vm_start to vma->vm_end and map
>>>>>>>>> them
>>>>>>>>> to that single dummy page.
>>>>>>>> This strongly feels like premature optimization. If you're worried
>>>>>>>> about
>>>>>>>> the overhead on amdgpu, pay down the debt by removing one of the
>>>>>>>> redundant
>>>>>>>> pointers between gem and ttm bo structs (I think we still have
>>>>>>>> some) :-)
>>>>>>>>
>>>>>>>> Until we've nuked these easy&obvious ones we shouldn't play "avoid 1
>>>>>>>> pointer just because" games with hashtables.
>>>>>>>> -Daniel
>>>>>>>
>>>>>>> Well, if you and Christian can agree on this approach and suggest
>>>>>>> maybe what pointer is
>>>>>>> redundant and can be removed from GEM struct so we can use the
>>>>>>> 'credit' to add the dummy page
>>>>>>> to GEM I will be happy to follow through.
>>>>>>>
>>>>>>> P.S Hash table is off the table anyway and we are talking only
>>>>>>> about linked list here since by prefaulting
>>>>>>> the entire VA range for a vmf->vma i will be avoiding redundant
>>>>>>> page faults to same VMA VA range and so
>>>>>>> don't need to search and reuse an existing dummy page but simply
>>>>>>> create a new one for each next fault.
>>>>>>>
>>>>>>> Andrey
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&amp;data=04%7C01%7Candrey.grodzovsky%40amd.com%7C4b581c55df204ca3d07408d8b64c1db8%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637459785321798393%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=EvvAip8vs9fzVRS1rb0r5ODiBMngxPuI9GKR2%2F%2B2LzE%3D&amp;reserved=0
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