From ad765fae792e16ce3c1d0b69ce939e3f7dba40ab Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Fri, 8 Jul 2022 16:20:11 +0200 Subject: [PATCH] drm/i915/gem: Look for waitboosting across the whole object prior to individual waits We employ a "waitboost" heuristic to detect when userspace is stalled waiting for results from earlier execution. Under latency sensitive work mixed between the gpu/cpu, the GPU is typically under-utilised and so RPS sees that low utilisation as a reason to downclock the frequency, causing longer stalls and lower throughput. The user left waiting for the results is not impressed. On applying commit 047a1b877ed4 ("dma-buf & drm/amdgpu: remove dma_resv workaround") it was observed that deinterlacing h264 on Haswell performance dropped by 2-5x. The reason being that the natural workload was not intense enough to trigger RPS (using HW evaluation intervals) to upclock, and so it was depending on waitboosting for the throughput. Commit 047a1b877ed4 ("dma-buf & drm/amdgpu: remove dma_resv workaround") changes the composition of dma-resv from keeping a single write fence + multiple read fences, to a single array of multiple write and read fences (a maximum of one pair of write/read fences per context). The iteration order was also changed implicitly from all-read fences then the single write fence, to a mix of write fences followed by read fences. It is that ordering change that belied the fragility of waitboosting. Currently, a waitboost is inspected at the point of waiting on an outstanding fence. If the GPU is backlogged such that we haven't yet stated the request we need to wait on, we force the GPU to upclock until the completion of that request. By changing the order in which we waited upon requests, we ended up waiting on those requests in sequence and as such we saw that each request was already started and so not a suitable candidate for waitboosting. Instead of asking whether to boost each fence in turn, we can look at whether boosting is required for the dma-resv ensemble prior to waiting on any fence, making the heuristic more robust to the order in which fences are stored in the dma-resv. Reported-by: Thomas Voegtle Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6284 Fixes: 047a1b877ed4 ("dma-buf & drm/amdgpu: remove dma_resv workaround") Signed-off-by: Chris Wilson Cc: Tvrtko Ursulin Signed-off-by: Karolina Drobnik Tested-by: Thomas Voegtle Reviewed-by: Andi Shyti Acked-by: Rodrigo Vivi Signed-off-by: Rodrigo Vivi Link: https://patchwork.freedesktop.org/patch/msgid/07e05518d9f6620d20cc1101ec1849203fe973f9.1657289332.git.karolina.drobnik@intel.com (cherry picked from commit 394e2b57a989113de494c52d4683444bcb02d4e1) Signed-off-by: Rodrigo Vivi --- drivers/gpu/drm/i915/gem/i915_gem_wait.c | 34 ++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/drivers/gpu/drm/i915/gem/i915_gem_wait.c b/drivers/gpu/drm/i915/gem/i915_gem_wait.c index 319936f..e6e01c2 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_wait.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_wait.c @@ -9,6 +9,7 @@ #include #include "gt/intel_engine.h" +#include "gt/intel_rps.h" #include "i915_gem_ioctls.h" #include "i915_gem_object.h" @@ -31,6 +32,37 @@ i915_gem_object_wait_fence(struct dma_fence *fence, timeout); } +static void +i915_gem_object_boost(struct dma_resv *resv, unsigned int flags) +{ + struct dma_resv_iter cursor; + struct dma_fence *fence; + + /* + * Prescan all fences for potential boosting before we begin waiting. + * + * When we wait, we wait on outstanding fences serially. If the + * dma-resv contains a sequence such as 1:1, 1:2 instead of a reduced + * form 1:2, then as we look at each wait in turn we see that each + * request is currently executing and not worthy of boosting. But if + * we only happen to look at the final fence in the sequence (because + * of request coalescing or splitting between read/write arrays by + * the iterator), then we would boost. As such our decision to boost + * or not is delicately balanced on the order we wait on fences. + * + * So instead of looking for boosts sequentially, look for all boosts + * upfront and then wait on the outstanding fences. + */ + + dma_resv_iter_begin(&cursor, resv, + dma_resv_usage_rw(flags & I915_WAIT_ALL)); + dma_resv_for_each_fence_unlocked(&cursor, fence) + if (dma_fence_is_i915(fence) && + !i915_request_started(to_request(fence))) + intel_rps_boost(to_request(fence)); + dma_resv_iter_end(&cursor); +} + static long i915_gem_object_wait_reservation(struct dma_resv *resv, unsigned int flags, @@ -40,6 +72,8 @@ i915_gem_object_wait_reservation(struct dma_resv *resv, struct dma_fence *fence; long ret = timeout ?: 1; + i915_gem_object_boost(resv, flags); + dma_resv_iter_begin(&cursor, resv, dma_resv_usage_rw(flags & I915_WAIT_ALL)); dma_resv_for_each_fence_unlocked(&cursor, fence) { -- 2.7.4