[KV Offload] Fix failed-load livelock by marking the lookup verdict as a miss - #49328
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@varun-sundar-rabindranath can you please also take a look?
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This pull request has merge conflicts that must be resolved before it can be |
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@orozery @varun-sundar-rabindranath ready for another look when you have a moment. All the inline points are addressed and the agreed lean version is pushed: access-based lookup (no stat), the C helper back to upstream verbatim, a failed load marks the verdict False (single-try, best-effort) instead of dropping it, and delete only on a provable short read. 320 tiering tests pass, and I've replied on each thread. Thanks for the reviews. |
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@varun-sundar-rabindranath please review also the core changes
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@varun-sundar-rabindranath done. Rebased onto main (with #49152) and pushed. The delete-on-error is now narrowed to a provable short read only, in both the C Since loads are batched now, one bad block fails the whole batch and marks all its keys negative (single-try, best-effort). The request recomputes them and the verdict clears on request finish, so intact blocks re-HIT for later requests. I added a test for that blast-radius behaviour plus the C/Python delete split. Also folded in the style points: merged the tests you and @orozery flagged, trimmed the verbose comments, and dropped the repeated #49176 refs. The drain_results guard stays, since it's defensive as you guessed. 342 tiering tests pass, 352 with the fs_io_C extension built. Two notes for transparency: the C-path test variants only run when CI builds fs_io_C (they skip locally, so I compiled and exercised the C path directly to verify it), and there's a pre-existing EINTR-partial edge in the C |
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✅ @RobbieJ, CI is now available for this PR.
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Hi @RobbieJ, the pre-commit checks have failed. Please run: uv pip install pre-commit>=4.5.1
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pre-commit run --all-filesThen, commit the changes and push to your branch. For future commits, |
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/ci run |
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✅ Triggered Buildkite CI #82899 for commit |
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/ci run |
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✅ Triggered Buildkite CI #82914 for commit |
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Hi @RobbieJ, the pre-commit checks have failed. Please run: uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-filesThen, commit the changes and push to your branch. For future commits, |
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/ci run |
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✅ Triggered Buildkite CI #82990 for commit |
…s a miss A failed secondary-tier load livelocks the requesting request (vllm-project#49176): the async lookup cache records a positive verdict per key and nothing corrects it on load failure, so the scheduler re-issues the same doomed promotion every step until the request is aborted. - On a failed load the owning tier marks the cached lookup verdict as a miss (mark_miss), from get_finished_jobs on the scheduler thread. A cached miss is returned on subsequent lookups without re-probing, so the request recomputes on the GPU and never retries it, which structurally cannot loop. The tier already learns of its own failed loads, so no new SecondaryTierManager API is needed. A key is enqueued for probing exactly once, so drain_results asserts that invariant, keeping a late or duplicate result from resurrecting a corrected verdict. - Loads are batched. The C loader reports how many blocks were read before the first failure, so the tier fills JobResult.successful_keys (vllm-project#50321) and marks only the failed block onward as a miss. The manager keeps the blocks that did load in the primary tier, so they stay a hit; only the failed tail is recomputed, and the miss clears when the request finishes. - load_block removes the block file only on a provable short read, in both the C (_load_block) and Python paths. Stores are atomic, so a too-short existing file is genuine corruption; removing it makes future requests miss instead of repeating the failed load. Open failures and read errors leave the file untouched, and a harmless close after a full read no longer fails the load, so a transient host hiccup is not turned into permanent data loss or a spurious miss. This narrows the delete-on-any-error added in vllm-project#49152. Lookup keeps upstream's access-based existence check. Tests: per-tier livelock regressions, partial-batch keep (loaded blocks stay a hit, only the failed tail misses), delete-on-short-read and transient-leaves-file across the C and Python paths, and mark_miss unit tests including the enqueue-once invariant. 345 tiering tests pass (356 with the fs_io_C extension built). Fixes vllm-project#49176 Signed-off-by: Robbie J <RobbieJ@users.noreply.github.com>
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✅ Triggered Buildkite CI #83012 for commit |
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CI status All 246 jobs pass on 071f98a. That includes v1-core-plus-kv-plus-metrics on both x86 and AMD, and all three lm-eval-kv-offload jobs. Build #83012 still shows as failed. That is one flaky job, kernels-core-operation-test, which failed on its first attempt and passed on retry. Buildkite does not recompute the build state after a retry, so the build stays red while every job in it is green. What was failing v1-core-plus-kv-plus-metrics was failing on both x86 and AMD. The cause was a test this PR adds, not the code that test covers. test_transient_load_failure_leaves_file checks that a transient open() error does not delete the block file. It made open() fail by setting the file mode to 000, which normally gives EACCES. CI runs as root, and root ignores file permission bits, so open() succeeded, the load succeeded, and the assertion that the load had failed did not hold. The test passed on my machines because a normal user does get EACCES there. The fix The test now makes open() fail with ELOOP, by pointing the path at a symlink loop. ELOOP is not permission based, so it fails the same way for every user, root included. The code path under test is unchanged: open() fails, the job fails, and the file must not be unlinked. Checked three ways: it passes on macOS and on Linux, and it still fails if the loader is mutated to delete on any error. The branch is also rebased onto current main. @orozery ready for another look. |
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LGTM!
Thanks @RobbieJ and @varun-sundar-rabindranath !
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/ci retry |
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✅ Queued 2 failed job(s) for retry in Buildkite CI #83012. |
…s a miss (vllm-project#49328) Signed-off-by: Robbie J <RobbieJ@users.noreply.github.com>
3 participants
Purpose
Fixes #49176.
A failed secondary-tier load can wedge a request forever. When a promotion (secondary → primary load) fails because the block file is truncated, deleted, or unreadable, the async lookup cache keeps the positive verdict it recorded earlier and never learns about the failure. The scheduler sees a hit, schedules the promotion, the load fails, and next step it sees the same hit again: a busy-loop of wasted I/O and scheduler work that only ends when the request is aborted. Because the cache is refcounted per block, the stale verdict is served to every request overlapping those blocks, so one bad file wedges more than the request that first touched it, and it's silent.
I ran into this bringing KV offloading up on the vLLM Metal backend (Apple Silicon / MLX), where the filesystem tier gets a real workout and occasional torn block files made it reproducible. It isn't Metal-specific though; it lives in the shared tiering layer and reproduces on plain CPU with no GPU (see the regression test).
The fix, shaped by review, is deliberately lean and best-effort:
mark_miss), fromget_finished_jobson the scheduler thread. A cached miss is returned without re-probing, so the request recomputes on the GPU and never retries that block: single-try, and structurally it can't loop. The tier already learns of its own failed loads, so there's no newSecondaryTierManagerAPI.drain_resultsasserts the enqueue-once invariant, so a late or duplicate result can't resurrect a corrected verdict.load_blockremoves the block file only on a provable short read, in both the C_load_blockand the Python fallback. Stores are atomic, so a too-short file is genuine corruption. Open failures, read errors, and a harmless close after a full read leave the file untouched, so a transient hiccup isn't turned into permanent data loss or a spurious miss. This narrows the delete-on-any-error that [KV-offload][FS] : Batch store/load_block in C #49152 introduced.Loads are batched (#49152), so one bad block fails the whole batch and marks all its keys a miss; the request recomputes them, and the verdict clears when the request finishes, so intact blocks still hit for later requests.
Test Plan
Per-tier livelock regressions, the batched-failure blast radius, delete-on-short-read and transient-leaves-file across both the C and Python paths, and
mark_missunit tests including the enqueue-once invariant.Test Result
Mutation-checked rather than trusting a green run: neutralizing
mark_missmakes the livelock tests fail (hit instead of miss), and reverting the delete-narrowing or the invariant assert fails their respective tests.