Kernel Trace Analysis

Kernel Trace Analysis

Profile and analyze GPU kernel ATT traces to identify stall hotspots and produce an optimization plan.

Arguments

Analyzer Scripts

• scripts/hotspot_analyzer.py — reads a ui_output_agent_*_dispatch_* ATT directory; reports top-K stall hotspots, stall-type breakdown, and occupancy (combined-VGPR-pool model, reads accum/LDS/SGPR from out_kernel_trace.csv).
• scripts/pmc_l2_analyzer.py — reads rocprofv3 PMC counter CSV(s); reports L2 hit rate, HBM 32B-partial fraction, and over-fetch ratio. Use when a kernel is memory-bound and you need to know why (ATT has no cache counters). See "L2 / HBM efficiency analysis" under Step 5.

Workflow

Mode A: Analyze existing dispatch directory

If the user provides --dir <path> or already has a ui_output_agent_*_dispatch_* directory:

# Write hotspot_analyzer.py (see above), then:
python /tmp/hotspot_analyzer.py <dispatch_dir> --topk 15 --mode both
python /tmp/hotspot_analyzer.py <dispatch_dir> --topk 5 --mode src --detail --context 4

Skip to Step 4: Interpret Results.

Mode B: Full collection workflow

Step 1: Kernel Discovery

touch /tmp/trace_ts
rocprofv3 --stats --kernel-trace -f csv -- <CMD> 2>&1
find . -maxdepth 3 -name "*stats*" -newer /tmp/trace_ts -type f 2>/dev/null

Parse the stats CSV and present a kernel table:

Ask the user which kernel to trace if not obvious.

Prefer results.db if available — use sqlite3 for structured queries:

sqlite3 results.db "
SELECT ks.KernelName, COUNT(*) calls,
       ROUND(AVG(kd.end-kd.start)/1000.0,1) avg_us
FROM rocpd_kernel_dispatch kd
JOIN rocpd_info_kernel_symbol ks ON kd.kernel_symbol_id=ks.id
GROUP BY ks.KernelName ORDER BY avg_us DESC LIMIT 20;"

Step 2: Configure input.yaml

cp ~/Documents/input.yaml /tmp/trace_input.yaml

Edit /tmp/trace_input.yaml:

jobs:
   -
       kernel_include_regex: <KERNEL_NAME_PATTERN>
       kernel_iteration_range: "[1, [3-4]]"
       output_file: out
       output_directory: kernel_trace_output
       output_format: [csv]
       truncate_kernels: true
       sys_trace: true
       advanced_thread_trace: true
       att_target_cu: 1
       att_shader_engine_mask: "0xf"
       att_simd_select: "0xf"
       att_buffer_size: "0x6000000"

Key notes:

• kernel_iteration_range: "[1, [3-4]]" skips warmup, traces dispatches 3-4
• att_buffer_size: 96MB per SE; increase to "0xC000000" if truncated
• att_target_cu: 1: single CU keeps output manageable

Step 3: Collect ATT Trace

FLYDSL_DEBUG_ENABLE_DEBUG_INFO=1 rocprofv3 -i /tmp/trace_input.yaml -- <CMD> 2>&1
find . -type d -name "ui_output_agent_*" -newer /tmp/trace_ts 2>/dev/null

If rocprof-trace-decoder library is missing:

wget -q https://github.com/ROCm/rocprof-trace-decoder/releases/download/0.1.6/rocprof-trace-decoder-manylinux-2.28-0.1.6-Linux.sh
chmod +x rocprof-trace-decoder-manylinux-2.28-0.1.6-Linux.sh
./rocprof-trace-decoder-manylinux-2.28-0.1.6-Linux.sh --skip-license --prefix=/tmp/rtd-install
find /tmp/rtd-install -name '*.so*' -exec cp -a {} /opt/rocm/lib/ \;
ldconfig

Output structure:

ui_output_agent_<PID>_dispatch_<N>/
├── code.json          ← PRIMARY: per-instruction stall/cycle data
├── snapshots.json     ← source file path mapping (virtual → local filename)
├── source_0_*.py      ← embedded source files
├── filenames.json     ← wave file index
├── occupancy.json     ← occupancy timeline
└── se*_sm*_sl*_wv*.json  ← per-wave raw traces

Step 4: Run hotspot_analyzer.py

Write the script (see above), then run:

# Full report
python /tmp/hotspot_analyzer.py <dispatch_dir> --topk 15 --mode both

# Source-level with code context (best for optimization)
python /tmp/hotspot_analyzer.py <dispatch_dir> --topk 5 --mode src --detail --context 4

# ASM-only for instruction-level detail
python /tmp/hotspot_analyzer.py <dispatch_dir> --mode asm --topk 20

Step 5: Interpret Results

code.json field reference

Each row in code["code"] is:

[asm, _, pc_index, source_loc, _, pc_addr, exec_count, total_cycles, stall_cycles, issue_cycles]
  0   1     2          3       4     5          6            7              8             9

• col[8] stall_cycles: cycles the instruction was blocked from issuing — primary hotspot metric
• col[7] total_cycles: total cycles charged to this instruction across all waves
• col[3] source_loc: "/path/to/file.py:LINE" — virtual path resolved via snapshots.json
• col[6] exec_count: number of wave-threads that executed this instruction

snapshots.json: resolving source paths

snapshots.json encodes a nested dict tree mapping virtual paths to local filenames:

{"/": {"FlyDSL": {"kernels": {"pa_decode_sw_fp8_ps.py": "source_0_pa_decode_sw_fp8_ps.py"}}}}

Flatten recursively: /FlyDSL/kernels/pa_decode_sw_fp8_ps.py → source_0_pa_decode_sw_fp8_ps.py

Stall type classification

Common hotspot patterns

Pattern 1: V/K loads inside MFMA loop → very high stall rate (80–95%)

# BAD: load and MFMA alternate — only 1 MFMA of hiding time
for k_step in range_constexpr(QKHELOOP * 2):
    if k_step % 2 == 0:
        v_data = buffer_ops.buffer_load(...)   # stall_rate ~92%
    acc = rocdl.mfma_f32_16x16x32_fp8_fp8(...)

# GOOD: batch all loads before the MFMA loop
for td in range_constexpr(TLOOP):
    v_prefetch[td] = [buffer_ops.buffer_load(...) for _ in range_constexpr(QKHELOOP)]

for td in range_constexpr(TLOOP):
    for k_step in range_constexpr(QKHELOOP * 2):
        acc = rocdl.mfma_f32_16x16x32_fp8_fp8(...)   # entire QK MFMA hides VMEM latency
    v_results[td] = v_prefetch[td]   # already in registers

Pattern 2: Sequential loads with no compute → VMEM queue saturation

# BAD: all loads back-to-back, no compute interleaved
for td in range_constexpr(TLOOP):
    for qkhe in range_constexpr(QKHELOOP):
        k4 = buffer_ops.buffer_load(k_rsrc, ka_dw, ...)   # queue fills up

# GOOD: prefetch next tile's K loads during current tile's MFMA computation

Pattern 3: LDS prob reads immediately before PV MFMA → lgkmcnt stall

# BAD: LDS reads and MFMA in same loop
for vhe in ...:
    for vt in ...:
        p_i64 = lds_read(...)    # issued here
        tmp = mfma(v_i64, p_i64, ...)   # immediately consumed → lgkmcnt stall

# GOOD: batch all LDS reads first, then all MFMAs
for vhe in ...:
    for vt in ...:
        p_i64s.append(lds_read(...))    # all LDS reads issued first

for vhe in ...:
    for vt in ...:
        tmp = mfma(v_i64s[...], p_i64s[...], ...)   # LDS data already ready

Pattern 4: Scale loads too close to usage

# BAD: scale load and usage separated by only TLOOP MFMAs
for td in range_constexpr(TLOOP):
    k_scale = buffer_ops.buffer_load(ks_rsrc, ...)   # issued here
# ... small compute gap ...
    result = acc * k_scale   # used too soon → stall

# GOOD: issue scale loads at the very beginning of the block,
# before K loads, to maximise latency hiding distance

Pattern 5: Hotspot attributed to kernel entry line

When @flyc.kernel / kernel decorator line appears as the top hotspot with a mix of VMEM-wait + barrier stall types — this is a debug info aggregation artifact. MLIR/compiler-generated instructions (address arithmetic, cndmask, prologue setup) map to the outermost scope line. Ignore this line; focus on lines with explicit user ops.

Register pressure check (architecture-aware)

hotspot_analyzer.py auto-detects the GPU architecture from ISA instruction patterns and computes occupancy (waves/SIMD) as the minimum across every resource limiter:

occupancy = min(vgpr_limit, lds_limit, sgpr_limit, hw_max=8)
  vgpr_limit = 512 // (arch_vgpr_alloc + accum_vgpr_alloc)              # per SIMD
  lds_limit  = (LDS_total // lds_per_wg) * waves_per_wg // 4_SIMDs      # per SIMD
  sgpr_limit = 800 // sgpr_alloc                                        # per SIMD

VGPR is a combined 512-entry pool on BOTH gfx942 and gfx950. CDNA2 (gfx90a) unified the arch (256) and accum (256) VGPR files into one 512 budget per SIMD, and gfx942/gfx950 inherit that. Occupancy from VGPR is 512 / (arch + accum) on both — NOT 256 / max(arch, accum). (The separate-pool 256/max model only applied to gfx908 / CDNA1, where accum VGPRs were a distinct file accessible only by MFMA.)

What actually changed in CDNA4 vs CDNA3 is the LDS size (64KB→160KB) and the LDS alloc granularity — not the VGPR pooling model.

Reading the real counts. code.json only holds the (often single-CU, often vgpr-form) disassembly, so it cannot reveal accum_vgpr / LDS / SGPR / workgroup size — an AGPR-form-blind ISA scan reports accum=0 and gets occupancy badly wrong. The analyzer reads out_kernel_trace.csv (staged next to the dispatch dir) for the authoritative Accum_VGPR_Count / LDS_Block_Size / SGPR_Count / Workgroup_Size_*. arch_vgpr is taken as max(ISA_scan, CSV) so a bogus-low CSV VGPR_Count field can't under-report. If no CSV is found it falls back to ISA-only and prints a warning.

Auto-detection: gfx950-specific instructions (v_mfma_scale_f32_*, v_mfma_f32_16x16x128_*, v_mfma_f32_32x32x64_*) indicate CDNA4. Absence indicates CDNA3.

sqlite3 results.db "
SELECT ks.KernelName, ki.arch_vgpr_count, ki.accum_vgpr_count, ki.lds_size
FROM rocpd_kernel_dispatch kd
JOIN rocpd_info_kernel_symbol ks ON kd.kernel_symbol_id=ks.id
JOIN rocpd_info_kernel ki ON kd.kernel_id=ki.id LIMIT 5;"

Worked example (PA decode, gfx942): arch 144 + accum 136 = 280 combined → 512//280 = 1 wave/SIMD, VGPR-bound (LDS allows 5, SGPR allows 7). Reaching 2 waves needs combined ≤ 256, e.g. freeing ~24 VGPRs.

Warning: maxnreg forcing accum_vgpr=0 doubles occupancy but causes MFMA spills through arch_vgpr — measured 4.5x GPU slowdown. Do not use maxnreg for MFMA-heavy kernels.

L2 / HBM efficiency analysis (PMC, not ATT)

When the ATT hotspots are dominated by VMEM-load at high stall rate (e.g. 40-50% of stall, ~94% per-load), the kernel is memory-bound and the next question is why — and ATT cannot answer it (it has no cache counters). Capture PMC counters (see capture-kernel-trace "PMC Mode") and analyze with scripts/pmc_l2_analyzer.py:

python scripts/pmc_l2_analyzer.py \
    /tmp/pmc_out/pass_1/pmc_l2_counter_collection.csv \
    /tmp/pmc_ea_out/pass_1/pmc_ea_counter_collection.csv \
    --kernel <kernel> --ideal-gb <bytes_per_dispatch_GB> --ea-channels 2

Three metrics, three decisions:

Decision tree for a memory-bound decode kernel:

1. L2 hit rate < 5% → pure streaming, no reuse. This is expected and correct for decode with independent per-sequence paged KV — each KV byte is read once; the GQA (×heads) and MTP (×seq) reuse is captured in registers/LDS, never re-reads L2. "Improving L2 hit rate" is a non-goal. The only thing that raises it is real KV reuse = shared-prefix serving (a workload/scheduling property, not a kernel change).
2. 32B fraction ≈ 0% → full 64B cache lines, no spatial-locality waste. Nothing to fix at the line level. (High 32B% would point to scattered/ misaligned access worth restructuring.)
3. over-fetch ≈ 1.0x → the kernel reads exactly the data it needs. The achieved bandwidth (compute as ideal_bytes / kernel_time) is then the real ceiling for this access pattern. 50-60% of theoretical HBM peak is normal even for clean streaming; paged-gather decode living at ~54% with 0% partial + ~1.0x over-fetch is healthy, not a defect.

Worked example (PA decode, gfx942, bs=16, ctx=131072, batch=256): L2 hit 1.7%, 32B 0%, over-fetch 1.04x, 2.85 TB/s = 54% peak. Conclusion: the memory subsystem is clean; there is no KV-load optimization left — verified by also testing block_size 16→64 (regressed +7.8%) and confirming dwordx8 doesn't exist on CDNA3 (dwordx4 / 16B is the max single vector load).

Counter-capture caveat: keep each PMC job to ≤ ~4 TCC counters (single hardware pass). Multi-pass collection has triggered a GPU Hang on gfx942 — see capture-kernel-trace.

MFMA latency reference (cycles = pipeline depth)

MFMA dependency NOPs (CDNA4, from ISA reference Table 38)

These are the minimum independent instructions (or s_nop counts) required between MFMA result production and consumption. The values vary by MFMA variant:

Wait counts for "5, 8, 12, or 20" depend on MFMA variant:

• 5 waits: 16x16 4-block variants (8 cycle MFMAs)
• 8 waits: 16x16x16 F16/BF16 etc. (16 cycle MFMAs)
• 12 waits: 32x32x8, 16x16x4 F32, etc. (32 cycle MFMAs)
• 20 waits: 32x32x4 F32, 32x32x2 F32 (64 cycle MFMAs)

Step 6: Optimization Plan

After running hotspot_analyzer.py --detail, produce a prioritized plan:

## Stall Summary
- Total stalls: X cycles (Y% of kernel)
- Top stall type: VMEM-load (Z%)

## Hotspot Analysis

### #1 :LINE  stall=XK (N%)  VMEM-load  stall_rate=92%
Root cause: buffer_load inside QK MFMA loop — only 1 MFMA of hiding time.
Fix: Move all V loads before the QK MFMA loop.
Estimated gain: ~20% kernel cycle reduction.

### #2 :LINE  stall=XK (N%)  VMEM-load  stall_rate=80%
Root cause: K loads sequential with no compute interleaved.
Fix: Prefetch next tile's K during current tile's MFMA (double-buffer pattern).
See /prefetch-data-load skill.

### #3 ...

## Priority Order
1. [HIGH]  Fix V-load position (24% of all stalls, easy refactor)
2. [HIGH]  K-load cross-tile prefetch (8% of stalls, needs _process_block restructure)
3. [MED]   Move scale loads earlier (8% of stalls, trivial move)
4. [LOW]   Batch LDS reads before PV MFMA (4% of stalls, loop split)

Error Handling