texture_cache: keep the allocator's block slack out of the usable VRAM

The collector's usage leaves out the unused part of VMA's blocks, but
the driver counts it and a heap at its budget cannot give it to a new
image. On an RX 7800 XT that slack was 1.06 GiB, the whole 1 GiB margin:
the heap sat at its budget for a minute while the collector saw room,
then images stopped fitting and the process died. Reserve the slack
(capped at 1.5 GiB) on top of the margin: at once when it grows, slowly
when it shrinks, so eviction leaving partly used blocks doesn't chase
the threshold down.

Assisted-by: Claude Sonnet 5.5 (diagnosis and code)
(cherry picked from commit 73fd5a489ea28226fa16a55f5008e622d631b54c)

2 files changed, 22 insertions, 2 deletions

diff --git a/src/video_core/texture_cache/texture_cache.h b/src/video_core/texture_cache/texture_cache.h
index cf042c1488..45bc35dee6 100644
--- a/src/video_core/texture_cache/texture_cache.h
+++ b/src/video_core/texture_cache/texture_cache.h
@@ -313,14 +313,33 @@ void TextureCache<P>::TickFrame() {
                 if (budget > 2_GiB) {
                     const u64 old_critical = critical_memory;
                     const u64 margin = (std::min)(budget / 8, 1_GiB);
-                    critical_memory = budget - (std::min)(margin + pressure_margin, budget / 2);
+                    if constexpr (requires { runtime.GetDeviceMemorySlack(); }) {
+                        // The usage counted here leaves out the unused part of the allocator's
+                        // blocks, which the driver counts and a full heap cannot hand to a new
+                        // image (RX 7800 XT: 1.06 GiB of it, the whole margin, with the heap at
+                        // its budget for a minute before images stopped fitting). Reserve it
+                        // too: at once when it grows, slowly when it shrinks, so evicting images
+                        // (which leaves partly used blocks) does not chase the threshold down.
+                        const u64 slack = (std::min)(runtime.GetDeviceMemorySlack(), 1536_MiB);
+                        slack_margin = slack >= slack_margin
+                                           ? slack
+                                           : slack_margin - (std::min)(slack_margin - slack, 16_MiB);
+                    }
+                    critical_memory =
+                        budget - (std::min)(margin + pressure_margin + slack_margin, budget / 2);
                     expected_memory = critical_memory - (std::min)(margin, critical_memory / 2);
                     // Log the first values and real changes (the budget moves by a few MiB all
                     // the time).
                     const u64 change = critical_memory > old_critical
                                            ? critical_memory - old_critical
                                            : old_critical - critical_memory;
-                    ApplyThresholdOverrides(budget_check_frames == 1 || change >= 256_MiB);
+                    const bool log = budget_check_frames == 1 || change >= 256_MiB;
+                    ApplyThresholdOverrides(log);
+                    if (log) {
+                        LOG_INFO(HW_GPU,
+                                 "Memory margins: base {} MiB, out of memory {} MiB, block slack {} MiB",
+                                 margin >> 20, pressure_margin >> 20, slack_margin >> 20);
+                    }
                 }
             }
         }
diff --git a/src/video_core/texture_cache/texture_cache_base.h b/src/video_core/texture_cache/texture_cache_base.h
index 1a6931d18a..24ebaf0689 100644
--- a/src/video_core/texture_cache/texture_cache_base.h
+++ b/src/video_core/texture_cache/texture_cache_base.h
@@ -139,6 +139,7 @@ class TextureCache : public VideoCommon::ChannelSetupCaches<TextureCacheChannelI
     u32 seen_pressure_events{};   ///< Allocator pressure events already answered
     u64 pressure_margin{};        ///< Extra room under the budget after images did not fit
     u64 pressure_quiet_frames{};  ///< Frames since the last such image
+    u64 slack_margin{};           ///< Extra room for allocator block space the driver counts as used
     u64 astc_opaque_images{};
     u64 astc_alpha_images{};
     std::vector<u8> astc_scan_buffer;