> -----Original Message----- > From: Intel-gfx [mailto:intel-gfx-boun...@lists.freedesktop.org] On Behalf > Of Daniel Vetter > Sent: Thursday, August 15, 2019 7:27 AM > To: Matthew Auld <matthew.william.a...@gmail.com> > Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>; Auld, > Matthew <matthew.a...@intel.com> > Subject: Re: [Intel-gfx] [PATCH v2 03/37] drm/i915/region: support basic > eviction > > On Thu, Aug 15, 2019 at 12:48 PM Matthew Auld > <matthew.william.a...@gmail.com> wrote: > > > > On Tue, 30 Jul 2019 at 17:26, Daniel Vetter <dan...@ffwll.ch> 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.a...@intel.com> > > > > Cc: Joonas Lahtinen <joonas.lahti...@linux.intel.com> > > > > Cc: Abdiel Janulgue <abdiel.janul...@linux.intel.com> > > > > > > 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. > > > > > > > > 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.
Thank you for the explanation. What does our 'struct_mutex' protect for exactly? As example, I see when blitter engine functions are called, we hold 'struct_mutex" first. Can we replace 'struct_mutex' with some fine-grain locks so that we can lock obj->mm.lock first, and then lock these fine-grain locks? I need some background info about 'struct_mutex' design. --CQ > > 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 > _______________________________________________ > Intel-gfx mailing list > Intel-gfx@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/intel-gfx _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
