27feb94a49
Closes defect #5 prerequisites for the GPU-pure step_with_lobsim that lands in R6. Replaces the host Thompson + host argmax + host log-softmax loops the flawed Phase F shipped in step_with_lobsim (which violated feedback_cpu_is_read_only with 5 DtoH copies + host per-action loops + HtoD action upload per training step). Three new kernels: 1. rl_action_kernel.cu — Thompson sampler over the C51 atom distribution. One block per batch, N_ACTIONS=9 threads. Each thread softmaxes its action's Q_N_ATOMS=21 atoms, samples one atom via CDF walk, writes sampled return to shared mem. Thread 0 argmaxes over per-action sampled returns + writes actions[b] + advances per-batch PRNG state. PRNG: per-batch xorshift32 state in prng_state_d (allocated + host-seeded from cfg.dqn_seed via ChaCha8 at trainer init per pearl_scoped_init_seed_for_reproducibility, with .max(1) guard since xorshift32 freezes at 0). Each per-action thread XORs its action index (golden-ratio mixed) into a thread-local copy of the per-batch state — no inter-thread race, reproducible by (cfg.dqn_seed, b_size, step_count). No cuRAND dep. 2. argmax_expected_q.cu — Bellman-target argmax over expected Q per action. Same layout as rl_action_kernel but deterministic (no PRNG). Per pearl_thompson_for_distributional_action_selection: Thompson for rollout (rl_action_kernel), argmax for Bellman target (this kernel) — distinct kernels, distinct ISV consumers. 3. log_pi_at_action.cu — per-batch log π(actions[b] | s_b) via log-softmax + lookup. One thread per batch entry (N_ACTIONS=9 is small enough for a per-thread sequential loop). Feeds the PPO importance ratio in R6. IntegratedTrainer gains: - 3 cubin includes (rl_action_kernel, argmax_expected_q, log_pi_at_action) - 3 module/function field pairs - 2 new device buffers populated at init: prng_state_d: CudaSlice<u32> of length n_batch atom_supports_d: CudaSlice<f32> of length Q_N_ATOMS=21, values [Q_V_MIN, Q_V_MIN + step, …, Q_V_MAX] = linspace(-1, +1, 21) - 3 launcher methods: launch_rl_action_kernel(q_logits_d, actions_d, b_size) launch_argmax_expected_q(q_logits_d, next_actions_d, b_size) launch_log_pi_at_action(pi_logits_d, actions_d, log_pi_out_d, b_size) GPU-oracle tests in tests/r4_action_kernels.rs (per feedback_no_cpu_test_fallbacks every oracle is analytical, not a CPU reference): R4.1: Thompson under sharp distribution (action 5 has logit=20 on atom 20 / support +1.0; others have logit=20 on atom 0 / support −1.0) collapses to argmax — per-action dominant-atom probability ≈ 1 − 4e-8, so 100/100 trials should pick action 5. Assert ≥99/100 (tolerates one fp-rounding edge near u ≈ 1.0 in CDF walk). R4.2: argmax_expected_q picks the rewarded action under the same sharp distribution. Negative invariant: swap dominant atom to action 2 → next_action follows. R4.3: log_pi_at_action with π logits dominant at action 3 (logit=20, others=0) → log π(3) ≈ 0 within 1e-4. Negative invariant: log π(other action) ≈ −20 within 1e-3. Build cache-bust v27. cargo check + cargo build --tests on ml-alpha green (heads_bit_equiv pre-existing failure persists). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
56 lines
2.1 KiB
Plaintext
56 lines
2.1 KiB
Plaintext
// log_pi_at_action.cu — GPU-resident per-batch log π(action | s)
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// computation for the PPO importance-ratio path (Phase R4 of the
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// integrated RL trainer rebuild;
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// see docs/superpowers/plans/2026-05-23-integrated-rl-trainer-rebuild.md).
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//
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// Replaces the host-side `pi_logits[action] − log_sum_exp(pi_logits)`
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// loop the flawed Phase F shipped in step_with_lobsim — which
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// violated `feedback_cpu_is_read_only` by DtoH-copying π logits and
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// computing log-softmax on CPU.
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//
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// Per-batch: log_pi[b] = pi_logits[b, actions[b]] − log Σ_a exp(pi_logits[b, a]).
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// Numerically stabilised via max-shift before exp.
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//
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// One thread per batch entry; element-wise. No reduction across
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// batches, no atomics. N_ACTIONS = 9 fits in a per-thread sequential
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// loop comfortably.
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#define N_ACTIONS 9
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// Inputs:
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// pi_logits [b_size, N_ACTIONS] row-major
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// actions [b_size] index into [0, N_ACTIONS)
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// Outputs:
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// log_pi_out [b_size]
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extern "C" __global__ void log_pi_at_action(
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const float* __restrict__ pi_logits,
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const int* __restrict__ actions,
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float* __restrict__ log_pi_out,
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int b_size
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) {
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const int b = blockIdx.x * blockDim.x + threadIdx.x;
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if (b >= b_size) return;
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const int row_off = b * N_ACTIONS;
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float max_l = -INFINITY;
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#pragma unroll
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for (int i = 0; i < N_ACTIONS; ++i) {
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const float l = pi_logits[row_off + i];
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if (l > max_l) max_l = l;
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}
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float sum_exp = 0.0f;
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#pragma unroll
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for (int i = 0; i < N_ACTIONS; ++i) {
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sum_exp += expf(pi_logits[row_off + i] - max_l);
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}
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// log-sum-exp identity: log Σ exp(x) = max + log Σ exp(x − max)
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const float log_norm = max_l + logf(sum_exp);
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const int chosen = actions[b];
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// No bounds check on `chosen` per `feedback_no_quickfixes`: the
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// upstream Thompson kernel (rl_action_kernel) writes into
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// [0, N_ACTIONS) by construction. A bounds check here would mask
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// an upstream contract violation; let it crash visibly.
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log_pi_out[b] = pi_logits[row_off + chosen] - log_norm;
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}
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