From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx> commit fe52c649438b8489c9456681d93a9b3de3d38263 upstream. One alternative to the fix Christian proposed in https://lore.kernel.org/dri-devel/20241024124159.4519-3-christian.koenig@xxxxxxx/ is to replace the rather complex open coded sorting loops with the kernel standard sort followed by a context squashing pass. Proposed advantage of this would be readability but one concern Christian raised was that there could be many fences, that they are typically mostly sorted, and so the kernel's heap sort would be much worse by the proposed algorithm. I had a look running some games and vkcube to see what are the typical number of input fences. Tested scenarios: 1) Hogwarts Legacy under Gamescope 450 calls per second to __dma_fence_unwrap_merge. Percentages per number of fences buckets, before and after checking for signalled status, sorting and flattening: N Before After 0 0.91% 1 69.40% 2-3 28.72% 9.4% (90.6% resolved to one fence) 4-5 0.93% 6-9 0.03% 10+ 2) Cyberpunk 2077 under Gamescope 1050 calls per second, amounting to 0.01% CPU time according to perf top. N Before After 0 1.13% 1 52.30% 2-3 40.34% 55.57% 4-5 1.46% 0.50% 6-9 2.44% 10+ 2.34% 3) vkcube under Plasma 90 calls per second. N Before After 0 1 2-3 100% 0% (Ie. all resolved to a single fence) 4-5 6-9 10+ In the case of vkcube all invocations in the 2-3 bucket were actually just two input fences.
