On Thu, Oct 18, 2012 at 06:43:40PM +0200, Thomas Hellstrom wrote: > On 10/18/2012 04:45 PM, Maarten Lankhorst wrote: > >Op 18-10-12 13:55, Thomas Hellstrom schreef: > >>On 10/18/2012 01:38 PM, Maarten Lankhorst wrote: > >>>Op 18-10-12 13:02, Thomas Hellstrom schreef: > >>>>On 10/18/2012 10:37 AM, Maarten Lankhorst wrote: > >>>>>Hey, > >>>>> > >>>>>Op 18-10-12 09:59, Thomas Hellstrom schreef: > >>>>>>On 10/18/2012 09:28 AM, Thomas Hellstrom wrote: > >>>>>>>Hi, Maarten, > >>>>>>> > >>>>>>>As you know I have been having my doubts about this change. > >>>>>>>To me it seems insane to be forced to read the fence pointer under a > >>>>>>>reserved lock, simply because when you take the reserve lock, another > >>>>>>>process may have it and there is a substantial chance that that process > >>>>>>>will also be waiting for idle while holding the reserve lock. > >>>>>I think it makes perfect sense, the only times you want to read the fence > >>>>>is when you want to change the members protected by the reservation. > >>>>No, that's not true. A typical case (or the only case) > >>>>is where you want to do a map with no_wait semantics. You will want > >>>>to be able to access a buffer's results even if the eviction code > >>>>is about to schedule an unbind from the GPU, and have the buffer > >>>>reserved? > >>>Well either block on reserve or return -EBUSY if reserved, presumably the > >>>former.. > >>> > >>>ttm_bo_vm_fault does the latter already, anyway > >>ttm_bo_vm_fault only trylocks to avoid a deadlock with mmap_sem, it's really > >>a waiting reserve but for different reasons. Typically a user-space app will keep > >>asynchronous maps to TTM during a buffer lifetime, and the buffer lifetime may > >>be long as user-space apps keep caches. > >>That's not the same as accessing a buffer after the GPU is done with it. > >Ah indeed. > >>>You don't need to hold the reservation while performing the wait itself though, > >>>you could check if ttm_bo_wait(no_wait_gpu = true) returns -EBUSY, and if so > >>>take a ref to the sync_obj member and then wait after unreserving. You won't > >>>reset sync_obj member to NULL in that case, but that should be harmless. > >>>This will allow you to keep the reservations fast and short. Maybe a helper > >>>would be appropriate for this since radeon and nouveau both seem to do this. > >>> > >>The problem is that as long as other users are waiting for idle with reservation > >>held, for example to switch GPU engine or to delete a GPU bind, waiting > >>for reserve will in many case mean wait for GPU. > >This example sounds inefficient, I know nouveau can do this, but this essentially > >just stalls the gpu entirely. I think guess you mean functions like nouveau_gem_object_close? > >It wouldn't surprise me if performance in nouveau could be improved simply by > >fixing those cases up instead, since it stalls the application completely too for other uses. > > > Please see the Nouveau cpu_prep patch as well. > > While there are a number of cases that can be fixed up, also in > Radeon, there's no way around that reservation > is a heavyweight lock that, particularly on simpler hardware without > support for fence ordering > with barriers and / or "semaphores" and accelerated eviction will be > held while waiting for idle. > > As such, it is unsuitable to protect read access to the fence > pointer. If the intention is to keep a single fence pointer > I think the best solution is to allow reading the fence pointer > outside reservation, but make sure this can be done > atomically. If the intention is to protect an array or list of fence > pointers, I think a spinlock is the better solution. > > /Thomas Just wanted to point out that like Thomas i am concern about having to have object reserved when waiting on its associated fence. I fear it will hurt us somehow. I will try to spend couple days stress testing your branch on radeon trying to see if it hurts performance anyhow with current use case. Cheers, Jerome _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
