[Bugfix] Format kernel-import errors eagerly so warning_once does not retain them by zwang86 · Pull Request #54098 · vllm-project/vllm

zwang86 · 2026-08-27T19:28:33Z

Purpose

logger.warning_once caches its arguments forever (lru_cache). The kernel import fallbacks passed the live ImportError as a lazy format argument, so the cache retained the exception — and through __traceback__ every frame and local on the raising path. import_kernels runs inside LLM.__init__, so on platforms without compiled vllm._C kernels the pinned locals included the LLM instance under construction: it could never be garbage collected, its EngineCore subprocess only exited with the interpreter, and on accelerator plugins the device stayed held by the leaked engine. Each exception instance was also a distinct cache key, defeating the once-deduplication.

Per review feedback, this fixes the specific call sites and nothing else: format the exception eagerly at the five kernel import sites (repr(e) with %s renders byte-identical output to the previous %r of the live exception). No logger changes, no new tests.

Test Plan

Block the vllm._C import with a meta-path finder, keep a weakref to the raised exception, call Platform.import_kernels() twice, then gc.collect():

import gc, sys, weakref
from vllm.platforms.interface import Platform

class TrackedError(ModuleNotFoundError):  # plain ModuleNotFoundError is not weakref-able
    pass

refs = []
class Blocker:
    def find_spec(self, name, path=None, target=None):
        if name in ("vllm._C", "vllm._moe_C_stable_libtorch"):
            e = TrackedError(f"No module named '{name}'")
            refs.append(weakref.ref(e))
            raise e
        return None

for m in ("vllm._C", "vllm._moe_C_stable_libtorch"):
    sys.modules.pop(m, None)
sys.meta_path.insert(0, Blocker())

Platform.import_kernels()
Platform.import_kernels()  # second call: shows dedup behavior
gc.collect()
print("exception retained after gc:", any(r() is not None for r in refs))

Test Result

Output before this fix (main):

WARNING 09-14 21:12:50 [interface.py:368] Failed to import from vllm._C: TrackedError("No module named 'vllm._C'")
WARNING 09-14 21:12:50 [interface.py:368] Failed to import from vllm._C: TrackedError("No module named 'vllm._C'")
exception retained after gc: True

The warning fires on every call ("once" is defeated because each exception instance is a new cache key), and the exception is retained forever with its traceback.

Output after this fix:

WARNING 09-14 21:10:57 [interface.py:368] Failed to import from vllm._C: TrackedError("No module named 'vllm._C'")
exception retained after gc: False

The warning text is byte-identical, fires once, and nothing is retained.

End-to-end: on a Neuron (OOT platform) stack with this fix applied, del llm shuts the EngineCore down within 2s; previously the engine survived until interpreter exit and held the device.

claude

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github-actions · 2026-08-27T19:28:42Z

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yewentao256

Thanks for reporting this!

When the exception is raised inside LLM.init — e.g. the vllm._C ImportError

I think in this case, we should fix the specific bug we have instead of doing updates to logs. I don't think it worth the complexity we introduce here. Something raised in LLM.__init__ is severe

… retain them logger.warning_once caches its arguments forever (lru_cache). The kernel import fallbacks passed the live ImportError as a lazy format argument, so the cache retained the exception -- and through __traceback__ every frame and local on the raising path. import_kernels runs inside LLM.__init__, so on platforms without compiled vllm._C kernels the pinned locals included the LLM instance under construction: it could never be garbage collected, its EngineCore subprocess only exited with the interpreter, and on accelerator plugins the device stayed held by the leaked engine. Each exception instance was also a distinct cache key, defeating the once-deduplication. Format the exception eagerly (repr(e) with %s renders byte-identical output to the previous %r of the live exception) at the five kernel import call sites, and add a regression test that the ImportError is collectable after Platform.import_kernels. Signed-off-by: Zeyu Wang <zwang86@users.noreply.github.com>

zwang86 · 2026-08-27T22:23:17Z

Thanks for reporting this!

When the exception is raised inside LLM.init — e.g. the vllm._C ImportError

I think in this case, we should fix the specific bug we have instead of doing updates to logs. I don't think it worth the complexity we introduce here. Something raised in LLM.__init__ is severe

@yewentao256 Thanks for the feedback, reworked it.
The PR now only touches the five kernel-import call sites (format the exception eagerly with repr(e); log output is byte-identical) and leaves the logger untouched. Added a targeted regression test that the ImportError is collectable after import_kernels.

yewentao256

Thanks for the work!

yewentao256 · 2026-09-11T23:36:15Z

+def test_import_kernels_does_not_retain_the_import_error(monkeypatch):
+    """The vllm._C ImportError must be collectable after import_kernels.
+
+    ``logger.warning_once`` caches its arguments forever (``lru_cache``). If
+    the live ImportError is passed as a format argument, the cache retains it
+    and — through ``__traceback__`` — every frame on the raising path.
+    ``import_kernels`` runs inside ``LLM.__init__``, so on platforms without
+    compiled ``vllm._C`` kernels this pinned the ``LLM`` instance itself: it
+    could never be garbage collected and its EngineCore only exited with the
+    interpreter. See #54096.
+    """
+    import gc
+    import weakref
+
+    from vllm.platforms.interface import Platform
+
+    blocked = ("vllm._C", "vllm._moe_C_stable_libtorch")
+    exc_refs = []
+
+    class _TrackedModuleNotFoundError(ModuleNotFoundError):
+        pass  # python-level subclass; supports weak references
+
+    class _BlockKernelImports:
+        def find_spec(self, name, path=None, target=None):
+            if name in blocked:
+                exc = _TrackedModuleNotFoundError(f"No module named '{name}' (test)")
+                exc_refs.append(weakref.ref(exc))
+                raise exc
+            return None
+
+    for mod in blocked:
+        monkeypatch.delitem(sys.modules, mod, raising=False)
+    monkeypatch.setattr(sys, "meta_path", [_BlockKernelImports()] + sys.meta_path)
+
+    Platform.import_kernels()
+
+    assert exc_refs, "the blocked import was never attempted"
+    gc.collect()
+    leaked = [ref for ref in exc_refs if ref() is not None]
+    assert not leaked, (
+        "ImportError retained after import_kernels; a cached reference "
+        "(e.g. a live exception passed to logger.warning_once) pins its "
+        "traceback and every local on the raising path"
+    )
+
+


Suggested change

def test_import_kernels_does_not_retain_the_import_error(monkeypatch):

"""The vllm._C ImportError must be collectable after import_kernels.

``logger.warning_once`` caches its arguments forever (``lru_cache``). If

the live ImportError is passed as a format argument, the cache retains it

and — through ``__traceback__`` — every frame on the raising path.

``import_kernels`` runs inside ``LLM.__init__``, so on platforms without

compiled ``vllm._C`` kernels this pinned the ``LLM`` instance itself: it

could never be garbage collected and its EngineCore only exited with the

interpreter. See #54096.

"""

import gc

import weakref

from vllm.platforms.interface import Platform

blocked = ("vllm._C", "vllm._moe_C_stable_libtorch")

exc_refs = []

class _TrackedModuleNotFoundError(ModuleNotFoundError):

pass # python-level subclass; supports weak references

class _BlockKernelImports:

def find_spec(self, name, path=None, target=None):

if name in blocked:

exc = _TrackedModuleNotFoundError(f"No module named '{name}' (test)")

exc_refs.append(weakref.ref(exc))

raise exc

return None

for mod in blocked:

monkeypatch.delitem(sys.modules, mod, raising=False)

monkeypatch.setattr(sys, "meta_path", [_BlockKernelImports()] + sys.meta_path)

Platform.import_kernels()

assert exc_refs, "the blocked import was never attempted"

gc.collect()

leaked = [ref for ref in exc_refs if ref() is not None]

assert not leaked, (

"ImportError retained after import_kernels; a cached reference "

"(e.g. a live exception passed to logger.warning_once) pins its "

"traceback and every local on the raising path"

)

We don't need a specific test for this small update

removed the test

yewentao256 · 2026-09-11T23:36:38Z

+            # Format eagerly: warning_once caches its arguments forever, and a
+            # live exception would pin its traceback (and every local on the
+            # raising path, including the LLM under construction).
+            logger.warning_once("Failed to import from vllm._C: %s", repr(e))


Suggested change

# Format eagerly: warning_once caches its arguments forever, and a

# live exception would pin its traceback (and every local on the

# raising path, including the LLM under construction).

logger.warning_once("Failed to import from vllm._C: %s", repr(e))

logger.warning_once("Failed to import from vllm._C: %s", repr(e))

Comment not needed

removed the comment

Signed-off-by: Zeyu Wang <zwang86@users.noreply.github.com>

yewentao256

Please also update the PR description, what's the output before/after this fix

zwang-amazon · 2026-09-14T21:58:30Z

Please also update the PR description, what's the output before/after this fix

Thank you @yewentao256 , updated the PR description as well.

yewentao256

LGTM, thanks for the work!

github-actions · 2026-09-14T22:17:47Z

✅ @zwang86, CI is now available for this PR.

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Your branch must contain every commit currently on its upstream target branch. Merge or rebase onto the latest target branch, then rerun the command. Append --allow-stale to a run command to test an outdated branch at your own risk.
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zwang86 · 2026-09-14T23:03:39Z

/ci run

github-actions · 2026-09-14T23:03:57Z

✅ Triggered Buildkite CI #88947 for commit 8e656af14827.

zwang86 · 2026-09-15T18:20:00Z

/ci run

github-actions · 2026-09-15T18:20:18Z

❌ This PR is 3 commits behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

zwang86 · 2026-09-15T18:21:06Z

/ci run

github-actions · 2026-09-15T18:21:26Z

✅ Triggered Buildkite CI #89148 for commit f82ca61a48c7.

… retain them (vllm-project#54098) Signed-off-by: Zeyu Wang <zwang86@users.noreply.github.com> Co-authored-by: Zeyu Wang <zwang86@users.noreply.github.com> Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

claude Bot reviewed Aug 27, 2026

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mergify Bot added the bug Something isn't working label Aug 27, 2026

yewentao256 reviewed Aug 27, 2026

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zwang86 force-pushed the fix-warning-once-exception-leak branch from bc8b025 to 356cb47 Compare August 27, 2026 22:13

zwang86 requested review from bigPYJ1151 and xuechendi as code owners August 27, 2026 22:13

mergify Bot added the nvidia label Aug 27, 2026

github-project-automation Bot added this to NVIDIA Aug 27, 2026

zwang86 changed the title ~~[Bugfix] Prevent logger *_once from retaining exception tracebacks~~ [Bugfix] Format kernel-import errors eagerly so warning_once does not retain them Aug 27, 2026

mergify Bot added the cpu Related to CPU backends label Aug 27, 2026

Merge branch 'main' into fix-warning-once-exception-leak

b9ba25c

zwang86 requested a review from yewentao256 August 27, 2026 22:23

yewentao256 reviewed Sep 11, 2026

View reviewed changes

zwang86 added 2 commits September 11, 2026 23:48

Remove explanatory comment per review

d7f6b55

Signed-off-by: Zeyu Wang <zwang86@users.noreply.github.com>

Remove regression test per review

5d69171

Signed-off-by: Zeyu Wang <zwang86@users.noreply.github.com>

yewentao256 reviewed Sep 12, 2026

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yewentao256 approved these changes Sep 14, 2026

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github-project-automation Bot moved this to Ready in NVIDIA Sep 14, 2026

yewentao256 added the ready ONLY add when PR is ready to merge/full CI is needed label Sep 14, 2026

Merge branch 'main' into fix-warning-once-exception-leak

8e656af

Merge branch 'main' into fix-warning-once-exception-leak

393bf4f

Merge branch 'main' into fix-warning-once-exception-leak

f82ca61

yewentao256 merged commit b6ce714 into vllm-project:main Sep 15, 2026
119 checks passed

github-project-automation Bot moved this from Ready to Done in NVIDIA Sep 15, 2026

rahul-steiger-nv mentioned this pull request Oct 22, 2026

[Bugfix] Avoid retaining tensors in Quack FP8 fallback logging vllm-project/vllm-omni#9262

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