[EPLB] Enable nixl eplb communicator for elastic ep

[ilmarkov]

Purpose

Enable NixlEplbCommunicator for elastic EP, allowing async EPLB during elastic scale-up/down.

Key changes:

1. Drain-before-scale. Consume all pending async transfers before groups are replaced. Wait for all layers to finish rather than stop in the middle to avoid cross-rank and cross-thread races.
2. Start async thread after scale-up on new workers.
3. Deferred NIXL remote setup. Postpone collective metadata
   exchange to first set_transfer_context() to avoid deadlocks.
4. Replace monitored_barrier with all_reduce + wait(timeout) for stateless groups in Nixl EPLB communicator.
5. Prefer NIXL in auto-selection for all EP modes.

Test Plan

tests/distributed/test_elastic_ep.py. Test params tuned (step_interval=10, window_size=5) to actually trigger EPLB.

• tests/distributed/test_eplb_execute.py — new
  test_nixl_deferred_init verifying the deferred-init path end-to-end.

Test Result

EPLB and elastic EP tests pass (both sync and async variants).

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ilmarkov.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

Hi @ilmarkov, the pre-commit checks have failed. Please run:

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[mergify]

Documentation preview: https://vllm--45013.org.readthedocs.build/en/45013/

[itayalroy]

Shouldn't this be removed?

[ilmarkov]

Yes, missed during the merge. Thanks!

[itayalroy]

We’ve been investigating this internally in NIXL team, and I think you may also be missing an update to the EPLB group after scale-up/down, see this reference: rtourgeman@e0d4e1c.

Without it, EPLB after scale up will hit an assert and no longer run

[ilmarkov]

@itayalroy The fix makes sense. We also need to update the tests to actually run EPLB, as they didn't catch the issue due to probably too high step_interval.

[ilmarkov]

Update of the tests reveals a bunch of bugs in elastic EP + async EPLB. For example, we don't track if the async EPLB transfer is finished and we can start scaling up/down while the transfers are still in fly. We probably need to update AsyncEplbLayerResult.

[itayalroy]

Update of the tests reveals a bunch of bugs in elastic EP + async EPLB. For example, we don't track if the async EPLB transfer is finished and we can start scaling up/down while the transfers are still in fly. We probably need to update AsyncEplbLayerResult.

Right, elastic EP suppressed the sync EPLB by flipping some boolean, relying on that EPLB runs on the same thread as the elastic state machine. This is no longer true with async EPLB, I think we might need "suppress eplb" to flush and stop the EPLB thread, and "resume eplb" to restart it

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ilmarkov.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[ilmarkov]

@itayalroy Now async EPLB + elastic EP seems to work. PTAL

[itayalroy]

Solution looks good, left a few comments reg. implementation

[ilmarkov]

@itayalroy The comments are addressed. Please, give another round of review.

[itayalroy]

@ilmarkov thanks for the work! :)