On Tue, Jan 12, 2021 at 9:12 AM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 11.01.21 um 17:13 schrieb Daniel Vetter: > > 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@xxxxxxx> > >> Sent: 08 January 2021 09:52 > >> To: Grodzovsky, Andrey <Andrey.Grodzovsky@xxxxxxx>; Daniel Vetter <daniel@xxxxxxxx> > >> Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx <dri-devel@xxxxxxxxxxxxxxxxxxxxx>; daniel.vetter@xxxxxxxx <daniel.vetter@xxxxxxxx>; robh@xxxxxxxxxx <robh@xxxxxxxxxx>; l.stach@xxxxxxxxxxxxxx <l.stach@xxxxxxxxxxxxxx>; yuq825@xxxxxxxxx <yuq825@xxxxxxxxx>; eric@xxxxxxxxxx <eric@xxxxxxxxxx>; Deucher, Alexander <Alexander.Deucher@xxxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx <gregkh@xxxxxxxxxxxxxxxxxxx>; ppaalanen@xxxxxxxxx <ppaalanen@xxxxxxxxx>; Wentland, Harry <Harry.Wentland@xxxxxxx> > >> 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. > > Yeah, all of them are already on my TODO list, but see below. > > > 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. > > The bigger problems is that TTM based drivers are using the destroy > callback pointer to distinct ghost objects from real ones. > > We first need to get rid of those. I already have a plan for that and > ~20% of it implemented, but it is more complicated because of the driver > specific backends in Nouveau, Amdgpu and vmwgfx. > > > - bdev pointer, if we move the device ttm stuff into struct drm_device, or > > create a common struct ttm_device, we can ditch that > > Yes, exactly that's what my device structure rename patch set is aiming > for :) Hm already on the list and did I miss it? > > - We could probably merge a few of the fields and find 8 bytes somewhere > > Please point out where. Flags and bool deleted looked compressible at a glance. Not sure that's worth it. > > - we still have 2 krefs, would probably need to fix that before we can > > merge the destroy callbacks > > Yes, already on my TODO list as well. But the last time I looked into > this I was blocked by the struct_mutex once more. Uh struct_mutex, I thought we've killed that for good. How is it getting in the way? > > So there's plenty of room still, if the size of a bo struct is really that > > critical. Imo it's not. > > It is. See we had a size of struct amdgpu_bo of over 1500 bytes because > we stopped caring for that, no we are down to 816 at the moment. > > We really need to get rid of this duplication of functionality and > structure between TTM and GEM. Yeah, and if you have patches nag me, happy to review them anytime really. Cheers, Daniel > > Christian. > > > -Daniel > > > > > >> 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 http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
