[Bugfix] [Core] don't schedule prefill if freeing kv cache

[toslunar]

FIX #5578

This PR makes the priority "running > swapped > waiting" strict. The previous code has chance to schedule new prefills even if preemption/swap happens or there's a swapped request, which (unintentionally) changes the order of requests.

#3853 says

1. Schedule all decodes
2. Schedule all chunked prefills (running prefills)
3. Schedule swapped
4. Schedule new prefills

and it has been implemented that (3.) is skipped if (1.) or (2.) fails to schedule all. The new change is to skip (4.) if any previous step fails to schedule all.

Re-adding preempted requests to chunked prefill may be OK but it seems better to implement "partial preemption" of a sequence (i.e. freeing some tail part of KV cache), which is out of scope of the PR.

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[toslunar]

The testcase test_running_prefill_prioritized_over_swap is failing. The scenario has to be rewritten because it's still important to test the priority Step 2 > Step 3.

The current failure is at setup and caused by prioritizing swapped seq group (id: "1") over waiting seq group (id: "2"), i.e. Step 3 over Step 4, which is expected by the change of the algorithm.

[toslunar]

The PR is ready for review. Each CI passes at least once before or after the empty commit.

[rkooo567]

sorry will need 1 more day to review!

[toslunar]

The technique 231ec60#diff-2c6af6e25b8d1074f25ef5ad2901121b30bc1528de74d2b3625636fcb8181624R386-R389 to run test_running_prefill_prioritized_over_swap no longer works for some reason after the merge.

[toslunar]

Is there anything I'm supposed to do? Removing the test is my suggestion, rather than indicating I'm working in progress. I'm sorry that it's unclear from my last comment.

[comaniac]

Can you fix the tests to match the desired behavior instead of removing them?

[mergify]

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

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

[hmellor]

@toslunar do you plan to continue this work?

[toslunar]

Yes! It seems test_running_prefill_prioritized_over_swap has been removed (by #9569).

But I'll recreate a PR with a squashed commit in order to visibly keep the CI results here, while the commits have to be signed off now.

[hmellor]

Ok, thanks for the update! Shall I close this PR and wait for the recreated one?

[toslunar]

Thank you. I created the new PR.