[BREAKING][vllm, fsdp] feat: add Rollout-Training Mismatch Fix -- Truncated importance sampling (volcengine#2953)

yaof20 · zdhNarsil · gemini-code-assist[bot] · fabianlim · commit f149c46d24c1 · 2025-10-04T15:48:37.000-04:00
Support [vLLM-FSDP off-policy importance sampling correction](https://fengyao.notion.site/off-policy-rl) using Truncated Importance Sampling (TIS): <img width="859" height="382" alt="TIS" src="https://github.com/user-attachments/assets/adc8f797-aa14-4b29-b265-a682c281d08e" /> - [ ] Search for similar PRs. Paste at least one query link here: ... - [ ] Format the PR title as `[{modules}] {type}: {description}` (This will be checked by the CI) - `{modules}` include `fsdp`, `megatron`, `sglang`, `vllm`, `rollout`, `trainer`, `ci`, `training_utils`, `recipe`, `hardware`, `deployment`, `ray`, `worker`, `single_controller`, `misc`, `perf`, `model`, `algo`, `env`, `tool`, `ckpt`, `doc`, `data` - If this PR involves multiple modules, separate them with `,` like `[megatron, fsdp, doc]` - `{type}` is in `feat`, `fix`, `refactor`, `chore`, `test` - If this PR breaks any API (CLI arguments, config, function signature, etc.), add `[BREAKING]` to the beginning of the title. - Example: `[BREAKING][fsdp, megatron] feat: dynamic batching` > For changes that can not be tested by CI (e.g., algorithm implementation, new model support), validate by experiment(s) and show results like training curve plots, evaluation results, etc. > Demonstrate how the API changes if any, and provide usage example(s) if possible. ```python python3 -m verl.trainer.main_ppo \ algorithm.adv_estimator=gae \ data.train_files="$train_files" \ data.val_files="$test_files" \ data.train_batch_size=1024 \ data.max_prompt_length=1024 \ data.max_response_length=1024 \ data.filter_overlong_prompts=True \ data.truncation='error' \ actor_rollout_ref.model.path=Qwen/Qwen2.5-32B-Instruct \ actor_rollout_ref.model.enable_gradient_checkpointing=False \ actor_rollout_ref.actor.optim.lr=1e-6 \ actor_rollout_ref.model.use_remove_padding=True \ actor_rollout_ref.actor.ppo_mini_batch_size=256 \ actor_rollout_ref.actor.ppo_micro_batch_size_per_gpu=8 \ actor_rollout_ref.model.enable_gradient_checkpointing=True \ actor_rollout_ref.actor.fsdp_config.param_offload=False \ actor_rollout_ref.actor.fsdp_config.optimizer_offload=False \ actor_rollout_ref.actor.use_kl_loss=False \ actor_rollout_ref.rollout.log_prob_micro_batch_size_per_gpu=16 \ actor_rollout_ref.rollout.tensor_model_parallel_size=4 \ actor_rollout_ref.rollout.name=vllm \ actor_rollout_ref.rollout.gpu_memory_utilization=0.5 \ critic.optim.lr=1e-5 \ critic.model.use_remove_padding=True \ critic.model.path=Qwen/Qwen2.5-32B-Instruct \ critic.model.enable_gradient_checkpointing=False \ critic.ppo_micro_batch_size_per_gpu=8 \ critic.model.fsdp_config.param_offload=False \ critic.model.fsdp_config.optimizer_offload=False \ algorithm.use_kl_in_reward=False \ trainer.critic_warmup=0 \ trainer.logger='["console","wandb"]' \ trainer.project_name='verl_example' \ trainer.experiment_name='Qwen2.5-32B-Instruct_function_rm' \ trainer.n_gpus_per_node=8 \ trainer.nnodes=4 \ trainer.save_freq=20 \ trainer.test_freq=10 \ trainer.total_epochs=15 \ actor_rollout_ref.rollout.calculate_log_probs=True \ # add this config to return rollout prob +actor_rollout_ref.actor.behav_imp_weight_cap=10.0$@ # add this config to set up C value in TIS ``` > Demonstrate the high-level design if this PR is complex, and list the specific changes. > [!IMPORTANT] > Please check all the following items before requesting a review, otherwise the reviewer might deprioritize this PR for review. - [ ] Read the [Contribute Guide](https://github.com/volcengine/verl/blob/main/CONTRIBUTING.md). - [ ] Apply [pre-commit checks](https://github.com/volcengine/verl/blob/main/CONTRIBUTING.md#code-linting-and-formatting): `pre-commit install && pre-commit run --all-files --show-diff-on-failure --color=always` - [ ] Add / Update [the documentation](https://github.com/volcengine/verl/tree/main/docs). - [ ] Add unit or end-to-end test(s) to [the CI workflow](https://github.com/volcengine/verl/tree/main/.github/workflows) to cover all the code. If not feasible, explain why: ... - [ ] Once your PR is ready for CI, send a message in [the `ci-request` channel](https://verl-project.slack.com/archives/C091TCESWB1) in [the `verl` Slack workspace](https://join.slack.com/t/verl-project/shared_invite/zt-3855yhg8g-CTkqXu~hKojPCmo7k_yXTQ). (If not accessible, please try [the Feishu group (飞书群)](https://applink.larkoffice.com/client/chat/chatter/add_by_link?link_token=772jd4f1-cd91-441e-a820-498c6614126a).) --------- Co-authored-by: Narsil-Dinghuai Zhang 张鼎怀 <dinghuai233@gmail.com> Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com> Co-authored-by: LiyuanLucasLiu <llychinalz@gmail.com>
diff --git a/docs/algo/grpo.md b/docs/algo/grpo.md
@@ -46,7 +46,7 @@ Instead of adding KL penalty in the reward, GRPO regularizes by directly adding
 
 - `actor_rollout_ref.actor.kl_loss_coef`: The coefficient of kl loss. Default is 0.001.
 
-- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
+- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. Appending "+" in the end (e.g., 'k1+' and 'k3+') would apply straight through to employ k2 for unbiased gradient estimation, regardless of the kl value estimation (see https://github.com/volcengine/verl/pull/2953#issuecomment-3162113848 for more details). How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
 
 ## Advanced Extensions
 
diff --git a/docs/algo/ppo.md b/docs/algo/ppo.md
@@ -59,7 +59,7 @@ Options to use KL loss for KL divergence control:
 
 - `actor_rollout_ref.actor.kl_loss_coef`: The coefficient of kl loss. Default is 0.001.
 
-- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
+- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. Appending "+" in the end (e.g., 'k1+' and 'k3+') would apply straight through to employ k2 for unbiased gradient estimation, regardless of the kl value estimation (see https://github.com/volcengine/verl/pull/2953#issuecomment-3162113848 for more details). How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
 
 Options to use KL penalty in the reward:
 
diff --git a/docs/examples/config.rst b/docs/examples/config.rst
@@ -118,6 +118,7 @@ Actor/Rollout/Reference Policy
       clip_ratio: 0.2
       entropy_coeff: 0.0
       use_kl_loss: False # True for GRPO
+      tis_imp_ratio_cap: -1 # set to positive values for Truncated Importance Sampling (requires setting `rollout.calculate_log_probs` as True)
       use_torch_compile: True # False to disable torch compile
       kl_loss_coef: 0.001 # for grpo
       kl_loss_type: low_var_kl # for grpo
@@ -188,6 +189,7 @@ Actor/Rollout/Reference Policy
           attention_backend: null # null means use the engine default value, available options: flashinfer, triton, flashmla
 
       n: 1 # for each prompt, sample n responses (i.e. num sample times). set it to values > 1 for grpo, rloo
+      calculate_log_probs: False # set to True for computing log probs via rollouts
       val_kwargs:
         # sampling parameters for validation
         top_k: -1 # 0 for hf rollout, -1 for vllm rollout
@@ -289,7 +291,7 @@ Actor/Rollout/Reference Policy
 
 - ``actor_rollout_ref.actor.kl_loss_coef``: The coefficient of kl loss. Default is 0.001. 
 
-- ``actor_rollout_ref.actor.kl_loss_type``: Support ``kl`` (``k1``), ``abs``, ``mse`` (``k2``), ``low_var_kl`` (``k3``) and ``full``. How to calculate the kl divergence between actor and reference policy. For specific options, refer to `kl_penalty()` in `core_algos.py <https://github.com/volcengine/verl/blob/main/verl/trainer/ppo/core_algos.py>`_ . See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
+- ``actor_rollout_ref.actor.kl_loss_type``: Support ``kl`` (``k1``), ``abs``, ``mse`` (``k2``), ``low_var_kl`` (``k3``) and ``full``. Appending ``+`` in the end (e.g., ``k1+`` and ``k3+``) would use straight-through to employ ``k2`` for unbiased gradient estimation, regardless of the kl value estimation (see https://github.com/volcengine/verl/pull/2953#issuecomment-3162113848 for more details). How to calculate the kl divergence between actor and reference policy. For specific options, refer to `kl_penalty()` in `core_algos.py <https://github.com/volcengine/verl/blob/main/verl/trainer/ppo/core_algos.py>`_ . See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
 
 - ``actor_rollout_ref.actor.checkpoint``: The configurations of checkpoint function in actor
 
diff --git a/examples/grpo_trainer/README.md b/examples/grpo_trainer/README.md
@@ -44,7 +44,7 @@ Instead of adding KL penalty in the reward, GRPO regularizes by directly adding
 
 - `actor_rollout_ref.actor.kl_loss_coef`: The coefficient of kl loss. Default is 0.001.
 
-- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
+- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. Appending "+" in the end (e.g., 'k1+' and 'k3+') would apply straight through to employ k2 for unbiased gradient estimation, regardless of the kl value estimation (see https://github.com/volcengine/verl/pull/2953#issuecomment-3162113848 for more details). How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
 
 ## Advanced Extensions
 
diff --git a/examples/ppo_trainer/README.md b/examples/ppo_trainer/README.md
@@ -57,7 +57,7 @@ Options to use KL loss for KL divergence control:
 
 - `actor_rollout_ref.actor.kl_loss_coef`: The coefficient of kl loss. Default is 0.001.
 
-- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
+- `actor_rollout_ref.actor.kl_loss_type`: Support kl(k1), abs, mse(k2), low_var_kl(k3) and full. Appending "+" in the end (e.g., 'k1+' and 'k3+') would apply straight through to employ k2 for unbiased gradient estimation, regardless of the kl value estimation (see https://github.com/volcengine/verl/pull/2953#issuecomment-3162113848 for more details). How to calculate the kl divergence between actor and reference policy. See this blog post for detailed analysis: http://joschu.net/blog/kl-approx.html
 
 Options to use KL penalty in the reward:
 
diff --git a/recipe/dapo/run_dapo_qwen2.5_32b_tis.sh b/recipe/dapo/run_dapo_qwen2.5_32b_tis.sh
@@ -0,0 +1,144 @@
+#!/usr/bin/env bash
+set -xeuo pipefail
+
+project_name='DAPO'
+exp_name='DAPO-Qwen2.5-32B-TIS'  # Truncated Importance Sampling (TIS) -> https://fengyao.notion.site/off-policy-rl
+
+adv_estimator=grpo
+
+use_kl_in_reward=False
+kl_coef=0.0
+use_kl_loss=False
+kl_loss_coef=0.0
+tis_imp_ratio_cap=2.0
+
+clip_ratio_low=0.2
+clip_ratio_high=0.28
+
+max_prompt_length=$((1024 * 2))
+max_response_length=$((1024 * 20))
+enable_overlong_buffer=True
+overlong_buffer_len=$((1024 * 4))
+overlong_penalty_factor=1.0
+
+loss_agg_mode="token-mean"
+
+enable_filter_groups=True
+filter_groups_metric=acc
+max_num_gen_batches=10
+train_prompt_bsz=512
+gen_prompt_bsz=$((train_prompt_bsz * 3))
+n_resp_per_prompt=16
+train_prompt_mini_bsz=32
+
+# Ray
+RAY_ADDRESS=${RAY_ADDRESS:-"http://localhost:8265"}
+WORKING_DIR=${WORKING_DIR:-"${PWD}"}
+RUNTIME_ENV=${RUNTIME_ENV:-"${WORKING_DIR}/verl/trainer/runtime_env.yaml"}
+NNODES=${NNODES:-16}
+# Paths
+RAY_DATA_HOME=${RAY_DATA_HOME:-"${HOME}/verl"}
+MODEL_PATH=${MODEL_PATH:-"${RAY_DATA_HOME}/models/Qwen2.5-32B"}
+CKPTS_DIR=${CKPTS_DIR:-"${RAY_DATA_HOME}/ckpts/${project_name}/${exp_name}"}
+TRAIN_FILE=${TRAIN_FILE:-"${RAY_DATA_HOME}/data/dapo-math-17k.parquet"}
+TEST_FILE=${TEST_FILE:-"${RAY_DATA_HOME}/data/aime-2024.parquet"}
+
+# Algorithm
+temperature=1.0
+top_p=1.0
+top_k=-1 # 0 for HF rollout, -1 for vLLM rollout
+val_top_p=0.7
+
+# Performance Related Parameter
+sp_size=8
+use_dynamic_bsz=True
+actor_ppo_max_token_len=$((max_prompt_length + max_response_length))
+infer_ppo_max_token_len=$((max_prompt_length + max_response_length))
+offload=True
+gen_tp=4
+
+
+# Truncated Importance Sampling (TIS) -> https://fengyao.notion.site/off-policy-rl
+
+# Please note that server mode(agent loop) hasn't return rollout_log_probs for now.
+# so currently, server mode is not supported for TIS.
+
+# To turn on TIS, you need to set the following parameters. Note 2.0 is a hyper-parameter and can be tuned.
+#   actor_rollout_ref.actor.tis_imp_ratio_cap=2.0
+#   actor_rollout_ref.rollout.calculate_log_probs=True
+
+ray job submit --no-wait --runtime-env="${RUNTIME_ENV}" \
+    --working-dir "${WORKING_DIR}" \
+    -- python3 -m recipe.dapo.main_dapo \
+    data.train_files="${TRAIN_FILE}" \
+    data.val_files="${TEST_FILE}" \
+    data.prompt_key=prompt \
+    data.truncation='left' \
+    data.max_prompt_length=${max_prompt_length} \
+    data.max_response_length=${max_response_length} \
+    data.gen_batch_size=${gen_prompt_bsz} \
+    data.train_batch_size=${train_prompt_bsz} \
+    actor_rollout_ref.rollout.n=${n_resp_per_prompt} \
+    algorithm.adv_estimator=${adv_estimator} \
+    algorithm.use_kl_in_reward=${use_kl_in_reward} \
+    algorithm.kl_ctrl.kl_coef=${kl_coef} \
+    actor_rollout_ref.actor.use_kl_loss=${use_kl_loss} \
+    actor_rollout_ref.actor.kl_loss_coef=${kl_loss_coef} \
+    actor_rollout_ref.actor.clip_ratio_low=${clip_ratio_low} \
+    actor_rollout_ref.actor.clip_ratio_high=${clip_ratio_high} \
+    actor_rollout_ref.actor.clip_ratio_c=10.0 \
+    algorithm.filter_groups.enable=${enable_filter_groups} \
+    algorithm.filter_groups.max_num_gen_batches=${max_num_gen_batches} \
+    algorithm.filter_groups.metric=${filter_groups_metric} \
+    actor_rollout_ref.model.use_remove_padding=True \
+    actor_rollout_ref.actor.use_dynamic_bsz=${use_dynamic_bsz} \
+    actor_rollout_ref.ref.log_prob_use_dynamic_bsz=${use_dynamic_bsz} \
+    actor_rollout_ref.rollout.log_prob_use_dynamic_bsz=${use_dynamic_bsz} \
+    actor_rollout_ref.actor.ppo_max_token_len_per_gpu=${actor_ppo_max_token_len} \
+    actor_rollout_ref.ref.log_prob_max_token_len_per_gpu=${infer_ppo_max_token_len} \
+    actor_rollout_ref.rollout.log_prob_max_token_len_per_gpu=${infer_ppo_max_token_len} \
+    actor_rollout_ref.model.path="${MODEL_PATH}" \
+    actor_rollout_ref.model.enable_gradient_checkpointing=True \
+    actor_rollout_ref.actor.optim.lr=1e-6 \
+    actor_rollout_ref.actor.optim.lr_warmup_steps=10 \
+    actor_rollout_ref.actor.optim.weight_decay=0.1 \
+    actor_rollout_ref.actor.ppo_mini_batch_size=${train_prompt_mini_bsz} \
+    actor_rollout_ref.actor.fsdp_config.param_offload=${offload} \
+    actor_rollout_ref.actor.fsdp_config.optimizer_offload=${offload} \
+    actor_rollout_ref.actor.entropy_coeff=0 \
+    actor_rollout_ref.actor.grad_clip=1.0 \
+    actor_rollout_ref.actor.loss_agg_mode=${loss_agg_mode} \
+    actor_rollout_ref.actor.ulysses_sequence_parallel_size=${sp_size} \
+    actor_rollout_ref.actor.tis_imp_ratio_cap=${tis_imp_ratio_cap} \
+    actor_rollout_ref.rollout.calculate_log_probs=True \
+    actor_rollout_ref.rollout.gpu_memory_utilization=0.80 \
+    actor_rollout_ref.rollout.tensor_model_parallel_size=${gen_tp} \
+    actor_rollout_ref.rollout.enable_chunked_prefill=True \
+    actor_rollout_ref.rollout.max_num_batched_tokens=$((max_prompt_length + max_response_length)) \
+    actor_rollout_ref.rollout.temperature=${temperature} \
+    actor_rollout_ref.rollout.top_p=${top_p} \
+    actor_rollout_ref.rollout.top_k="${top_k}" \
+    actor_rollout_ref.rollout.val_kwargs.temperature=${temperature} \
+    actor_rollout_ref.rollout.val_kwargs.top_p=${val_top_p} \
+    actor_rollout_ref.rollout.val_kwargs.top_k=${top_k} \
+    actor_rollout_ref.rollout.val_kwargs.do_sample=True \
+    actor_rollout_ref.rollout.val_kwargs.n=1 \
+    actor_rollout_ref.rollout.name=vllm \
+    actor_rollout_ref.ref.fsdp_config.param_offload=${offload} \
+    actor_rollout_ref.ref.ulysses_sequence_parallel_size=${sp_size} \
+    actor_rollout_ref.actor.fsdp_config.fsdp_size=-1 \
+    reward_model.reward_manager=dapo \
+    reward_model.overlong_buffer.enable=${enable_overlong_buffer} \
+    reward_model.overlong_buffer.len=${overlong_buffer_len} \
+    reward_model.overlong_buffer.penalty_factor=${overlong_penalty_factor} \
+    trainer.logger='["console","wandb"]' \
+    trainer.project_name="${project_name}" \
+    trainer.experiment_name="${exp_name}" \
+    trainer.n_gpus_per_node=8 \
+    trainer.nnodes="${NNODES}" \
+    trainer.val_before_train=True \
+    trainer.test_freq=5 \
+    trainer.save_freq=5 \
+    trainer.total_epochs=1 \
+    trainer.default_local_dir="${CKPTS_DIR}" \
+    trainer.resume_mode=auto 
diff --git a/verl/trainer/config/_generated_ppo_megatron_trainer.yaml b/verl/trainer/config/_generated_ppo_megatron_trainer.yaml
@@ -26,6 +26,7 @@ actor_rollout_ref:
     clip_ratio_c: 3.0
     loss_agg_mode: token-mean
     entropy_coeff: 0
+    tis_imp_ratio_cap: -1
     use_kl_loss: false
     use_torch_compile: true
     kl_loss_coef: 0.001
diff --git a/verl/trainer/config/_generated_ppo_trainer.yaml b/verl/trainer/config/_generated_ppo_trainer.yaml
@@ -26,6 +26,7 @@ actor_rollout_ref:
     clip_ratio_c: 3.0
     loss_agg_mode: token-mean
     entropy_coeff: 0
+    tis_imp_ratio_cap: -1
     use_kl_loss: false
     use_torch_compile: true
     kl_loss_coef: 0.001
diff --git a/verl/trainer/config/actor/actor.yaml b/verl/trainer/config/actor/actor.yaml
@@ -71,6 +71,10 @@ loss_agg_mode: token-mean
 # Entropy regularization coefficient in PPO loss
 entropy_coeff: 0
 
+# Truncated Importance Sampling (TIS): https://fengyao.notion.site/off-policy-rl
+# the truncation value C of truncated Importance Sampling (-1 for disable TIS)
+tis_imp_ratio_cap: -1
+
 # Whether to use KL loss instead of KL reward penalty. True for GRPO
 use_kl_loss: false
 
diff --git a/verl/trainer/config/rollout/rollout.yaml b/verl/trainer/config/rollout/rollout.yaml
@@ -174,7 +174,8 @@ multi_turn:
   format: hermes
 
 # support logging rollout prob for debugging purpose
-calculate_log_probs: False
+# "Truncated importance sampling" requires rollout log probs, set to True when turning on Truncated importance sampling
+calculate_log_probs: False 
 
 # [Experimental] agent loop based rollout configs
 agent:
diff --git a/verl/trainer/ppo/core_algos.py b/verl/trainer/ppo/core_algos.py
@@ -820,6 +820,7 @@ def compute_policy_loss_vanilla(
     response_mask: torch.Tensor,
     loss_agg_mode: str = "token-mean",
     config: Optional[DictConfig | AlgoConfig] = None,
+    rollout_log_probs=None,
 ) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]:
     """
     Compute the clipped policy objective and related metrics for PPO.
@@ -838,6 +839,10 @@ def compute_policy_loss_vanilla(
             Mask indicating which tokens to include in the loss, shape (batch_size, response_length).
         loss_agg_mode (str, optional):
             Aggregation mode for `agg_loss`. Defaults to "token-mean".
+        config: `(verl.trainer.config.ActorConfig)`:
+            config for the actor.
+        rollout_log_probs: `(torch.Tensor)`:
+            log probabilities of actions under the rollout policy, shape (batch_size, response_length).
     """
 
     assert config is not None
@@ -884,6 +889,13 @@ def compute_policy_loss_vanilla(
     )
 
     pg_losses = torch.where(advantages < 0, clip_pg_losses2, clip_pg_losses1)
+
+    if config.tis_imp_ratio_cap > 0 and rollout_log_probs is not None:
+        # Apply truncated importance sampling -> https://fengyao.notion.site/off-policy-rl
+        tis_imp_ratio = torch.exp(old_log_prob - rollout_log_probs)
+        tis_imp_ratio = torch.clamp(tis_imp_ratio, max=config.tis_imp_ratio_cap)
+        pg_losses = pg_losses * tis_imp_ratio
+
     pg_loss = agg_loss(loss_mat=pg_losses, loss_mask=response_mask, loss_agg_mode=loss_agg_mode)
 
     return pg_loss, pg_clipfrac, ppo_kl, pg_clipfrac_lower
@@ -1270,6 +1282,32 @@ def compute_value_loss(
 
 
 def kl_penalty(logprob: torch.FloatTensor, ref_logprob: torch.FloatTensor, kl_penalty) -> torch.FloatTensor:
+    """Compute KL divergence given logprob and ref_logprob. Optionally using straight through to bind k2 on other
+    kl penalty compute method for unbiased KL gradient estimation.
+    See more description in http://joschu.net/blog/kl-approx.html
+
+    Args:
+        logprob:
+        ref_logprob:
+
+    Returns:
+        kl_estimate
+    """
+    forward_score = kl_penalty_forward(logprob, ref_logprob, kl_penalty)
+    if not kl_penalty.endswith("+") or kl_penalty in ("mse", "k2"):
+        return forward_score
+
+    """
+    The expectation of k1 and k3 estimator is the expectaed value of KL, but the expected gradient of k1 and k3
+    estimator is not the expectaed gradient of KL. On the other hand k2 estimator gives right gradient estimator, 
+    so we use a straight through trick here if the kl_penalty method ends with '+', .e.g., k3+. 
+    """
+    backward_score = 0.5 * (logprob - ref_logprob).square()
+
+    return backward_score - backward_score.detach() + forward_score.detach()
+
+
+def kl_penalty_forward(logprob: torch.FloatTensor, ref_logprob: torch.FloatTensor, kl_penalty) -> torch.FloatTensor:
     """Compute KL divergence given logprob and ref_logprob.
     Copied from https://github.com/huggingface/trl/blob/main/trl/trainer/ppo_trainer.py#L1104
     See more description in http://joschu.net/blog/kl-approx.html
@@ -1279,7 +1317,7 @@ def kl_penalty(logprob: torch.FloatTensor, ref_logprob: torch.FloatTensor, kl_pe
         ref_logprob:
 
     Returns:
-
+        kl_estimate
     """
     if kl_penalty in ("kl", "k1"):
         return logprob - ref_logprob
diff --git a/verl/workers/actor/dp_actor.py b/verl/workers/actor/dp_actor.py
@@ -376,6 +376,13 @@ def update_policy(self, data: DataProto):
         ]
         if self.config.use_kl_loss:
             select_keys.append("ref_log_prob")
+        if self.config.tis_imp_ratio_cap > 0:
+            assert "rollout_log_probs" in data.batch.keys(), (
+                "Truncated Importance Sampling (TIS) requires to configure "
+                "`actor_rollout_ref.rollout.calculate_log_probs=True` "
+                "and is not currently supported in Server mode (agent loop)."
+            )
+            select_keys.append("rollout_log_probs")
 
         has_multi_modal_inputs = "multi_modal_inputs" in data.non_tensor_batch.keys()
         non_tensor_select_keys = ["multi_modal_inputs"] if has_multi_modal_inputs else []
@@ -406,6 +413,7 @@ def update_policy(self, data: DataProto):
                     model_inputs = {**micro_batch.batch, **micro_batch.non_tensor_batch}
                     response_mask = model_inputs["response_mask"]
                     old_log_prob = model_inputs["old_log_probs"]
+                    rollout_log_probs = model_inputs["rollout_log_probs"] if self.config.tis_imp_ratio_cap > 0 else None
                     advantages = model_inputs["advantages"]
 
                     entropy_coeff = self.config.entropy_coeff
@@ -444,6 +452,7 @@ def update_policy(self, data: DataProto):
                         response_mask=response_mask,
                         loss_agg_mode=loss_agg_mode,
                         config=self.config,
+                        rollout_log_probs=rollout_log_probs,
                     )
 
                     if use_adaptive_entropy_adjustment:
diff --git a/verl/workers/config/actor.py b/verl/workers/config/actor.py
@@ -101,6 +101,7 @@ class ActorConfig(BaseConfig):
     clip_ratio_c: float = 3.0
     loss_agg_mode: str = "token-mean"
     entropy_coeff: float = 0
+    tis_imp_ratio_cap: float = -1
     use_kl_loss: bool = False
     use_torch_compile: bool = True
     target_entropy: float = 0.5
