On Thu, Aug 29, 2024 at 11:53:58AM +0200, Thomas Hellström wrote: > But as Sima pointed out in private communication, exhaustive eviction > is not really needed for faulting to make (crawling) progress. > Watermarks and VRAM trylock shrinking should suffice, since we're > strictly only required to service a single gpu page granule at a time. > > However, ordinary bo-based jobs would still like to be able to > completely evict SVM vram. Whether that is important enough to strive > for is ofc up for discussion. My take is that you don't win anything for exhaustive eviction by having the dma_resv somewhere in there for svm allocations. Roughly for split lru world, where svm ignores bo/dma_resv: When evicting vram from the ttm side we'll fairly switch between selecting bo and throwing out svm pages. With drm_exec/ww_acquire_ctx selecting bo will eventually succeed in vacuuming up everything (with a few retries perhaps, if we're not yet at the head of the ww ticket queue). svm pages we need to try to evict anyway - there's no guarantee, becaue the core mm might be holding temporary page references (which block migration) or have the page locked (which also block the migration). But as long as those two steps succeed, we'll win and get the pages. There might be some thrashing against concurrent svm faults stealing them again, but they have a disadvantage since they can't steal dma_resv_locked bo. And if it's still too much we can stall them in the page allocator. So it's not entirely reliable, but should be close enough. Now for bo based svm the picture isn't any different, because holding dma_resv is not actually enough to migrate svm mappings. We still need to hope there's no temporary page references around, and we still need to succeed at locking the page. And the migration code only does trylocks, because that's it's deadlock prevent algorithm if different migrations needing the same set of pages, but acquiring them in a different order. So we win nothing. Worse, if dma_resv does actually hold up svm migration and reclaim, then we potentially deadlock because that lock is for a bigger range than individual pages (or folios). And the core mm assumes that it can get out of a deadlock bind by (at least stochastically) eventually succeeding in acquiring/locking down a single page. This means we cannot use dma_resv tricks to give the ttm world an advantage in exhaustive eviction against concurrent svm faults. Or at least not more than we can do without by just stalling svm faults that need to allocate gpu memory (but that must happen without holding locks or we're busted). So the only benefit I'm seeing is the unified lru, which I'm not sure is worth it. There's also a bit a lru design tension here, because for the bo world we want objects that are locked to stay on the lru, so that the competing processes can figure out who has the winning ww ticket. The core mm design otoh does isolate pages and remove them from the lru when they're acquired, so that they don't gunk up other processes from trying to make forward progress and are better hidden. Which reduces temporary page references (from lru walk) preventing migration and stuff like that. Cheers, Sima -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
