> -----Original Message----- > From: Daniel Vetter [mailto:daniel@xxxxxxxx] > Sent: Thursday, August 15, 2019 9:21 AM > To: Tang, CQ <cq.tang@xxxxxxxxx> > Cc: Matthew Auld <matthew.william.auld@xxxxxxxxx>; Intel Graphics > Development <intel-gfx@xxxxxxxxxxxxxxxxxxxxx>; Auld, Matthew > <matthew.auld@xxxxxxxxx> > Subject: Re: [PATCH v2 03/37] drm/i915/region: support basic > eviction > > On Thu, Aug 15, 2019 at 4:58 PM Tang, CQ <cq.tang@xxxxxxxxx> wrote: > > > > > > > > > -----Original Message----- > > > From: Intel-gfx [mailto:intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx] On > > > Behalf Of Daniel Vetter > > > Sent: Thursday, August 15, 2019 7:27 AM > > > To: Matthew Auld <matthew.william.auld@xxxxxxxxx> > > > Cc: Intel Graphics Development <intel-gfx@xxxxxxxxxxxxxxxxxxxxx>; > > > Auld, Matthew <matthew.auld@xxxxxxxxx> > > > Subject: Re: [PATCH v2 03/37] drm/i915/region: support > > > basic eviction > > > > > > 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. > > > > > > > > > > > > > > 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? > > Sure. With lots of efforts. > > > I need some background info about 'struct_mutex' design. > > There's not really a design behind it, it's just 10+ years of evolution. Yes, in old days, a big coarse-grain lock was OK. Now with so many engines in new hardware for new computation workloads, we might need to do fine-grain locks --CQ > -Daniel > > > --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@xxxxxxxxxxxxxxxxxxxxx > > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx > > > > -- > 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
