On Thu, Aug 15, 2019 at 4:26 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > On Thu, Aug 15, 2019 at 12:48 PM Matthew Auld > <matthew.william.auld@xxxxxxxxx> wrote: > > > > On Tue, 30 Jul 2019 at 17:26, Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > > > On Thu, Jun 27, 2019 at 09:55:59PM +0100, Matthew Auld wrote: > > > > Support basic eviction for regions. > > > > > > > > Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx> > > > > Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx> > > > > Cc: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxxxxxxxx> > > > > > > So from a very high level this looks like it was largely modelled after > > > i915_gem_shrink.c and not i915_gem_evict.c (our other "make room, we're > > > running out of stuff" code). Any specific reasons? > > > > IIRC I think it was originally based on the patches that exposed > > stolen-memory to userspace from a few years ago. Forgot to add this: Yeah, I guess those have been best-effort to at most. Or at least I never really looked at them seriously since the open source userspace never went anywhere. -Daniel > > > I think i915_gem_evict is a lot closer match for what we want for vram (it > > > started out to manage severely limitted GTT on gen2/3/4) after all. With > > > the complication that we'll have to manage physical memory with multiple > > > virtual mappings of it on top, so unfortunately we can't just reuse the > > > locking patter Chris has come up with in his struct_mutex-removal branch. > > > But at least conceptually it should be a lot closer. > > > > When you say make it more like i915_gem_evict, what does that mean? > > Are you talking about the eviction roster stuff, or the > > placement/locking of the eviction logic, with it being deep down in > > get_pages? > > So there's kinda two aspects here that I meant. > > First is the high-level approach of the shrinker, which is a direct > reflection of core mm low memory handling principles: Core mm just > tries to equally shrink everyone when there's low memory, which is > managed by watermarks, and a few other tricks. This is all only > best-effort, and if multiple threads want a lot of memory at the same > time then it's all going to fail with ENOMEM. > > On gpus otoh, and what we do in i915_gem_eviction.c for gtt (and very > much needed with the tiny gtt for everything in gen2/3/4/5) is that > when we run out of space, we stall, throw out everyone else, and have > exclusive access to the entire gpu space. Then the next batchbuffer > goes through the same dance. With this you guarantee that if you have > a series of batchbuffers which all need e.g. 60% of lmem, they will > all be able to execute. With the shrinker-style of low-memory handling > eventually you're unlucky, both threads will only get up to 50%, fail > with ENOSPC, and userspace crashes. Which is not good. > > The other bit is locking. Since we need to free pages from the > shrinker there's tricky locking rules involved. Worse, we cannot back > off from the shrinker down to e.g. the kmalloc or alloc_pages called > that put us into reclaim. Which means the usual deadlock avoidance > trick of having a slowpath where you first drop all the locks, then > reacquire them in the right order doesn't work - in some cases the > caller of kmalloc or alloc_pages could be holding a lock that we'd > need to unlock first. Hence why the shrinker uses the > best-effort-might-fail solution of trylocks, encoded in shrinker_lock. > > But for lmem we don't have such an excuse, because it's all our own > code. The locking design can (and should!) assume that it can get out > of any deadlock and always acquire all the locks it needs. Without > that you can't achive the first part about guaranteeing execution of > batches which collectively need more than 100% of lmem, but > individually all fit. As an example if you look at the amdgpu command > submission ioctl, that passes around ttm_operation_ctx which tracks a > few things about locks and other bits, and if they hit a possible > deadlock situation they can unwind the entire CS and restart by taking > the locks in the right order. > > I thought I typed that up somewhere, but I guess it got lost ... > > Cheers, Daniel > > > > > > > > > But I might be entirely off the track with reconstructing how this code > > > came to be, so please elaborate a bit. > > > > > > Thanks, Daniel > > > > -- > Daniel Vetter > Software Engineer, Intel Corporation > +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
