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@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. 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 > 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 > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
