Fix priority preemption regression test in scheduler (#37051)

Signed-off-by: HarshRathva <harshrathvaai@gmail.com>
Co-authored-by: Or Ozeri <oro@il.ibm.com>

tests/v1/core/test_scheduler.py

@@ -2040,91 +2040,103 @@ def test_priority_scheduling_mixed_priority_and_arrival():
     assert scheduled_req_ids == ["3", "2", "1", "0"]
 
 
-# This test had previously been passing due to its use of duplicate
-# request ids which resulted in incorrect behavior.
-# Now that the duplicate req ids had been fixed it fails and
-# investigation is needed into whether the priority scheduling
-# preemption logic is working as designed or not.
-@pytest.mark.skip("needs investigation")
 def test_priority_scheduling_preemption():
-    """Test that priority scheduling preempts
-    lower priority requests when memory is constrained."""
-    # Create scheduler with very limited memory to force preemption
+    """Test that under KV block pressure the scheduler preempts the
+    lowest-priority *running* request, not the highest-priority one.
+
+    A low-priority request starts running first. Then a high-priority
+    request arrives and is admitted to running.  When block pressure
+    builds, the scheduler preempts the low-priority running request
+    while keeping the high-priority one.
+
+    Block math
+    ----------
+    block_size = 16, num_blocks = 6 (1 null → 5 usable).
+
+    Phase 1: lo1 (priority 5, 32 tokens) → 2 blocks.  3 free.
+             Decode → lo1 has 33 tokens (needs 3rd block on next schedule).
+    Phase 2: hi1 (priority 0, 32 tokens) arrives.
+             schedule() allocates lo1's 3rd block (3 used) and admits
+             hi1 (2 blocks) → 5 used, 0 free. Both running.
+             Decode → lo1 34 tokens, hi1 33 tokens.
+    Phase 3: schedule() → hi1 needs 3rd block, 0 free → preemption.
+             lo1 (priority 5) is preempted, hi1 (priority 0) survives.
+    """
+    block_size = 16
+    num_blocks = 6  # 1 null block → 5 usable
+    num_tokens = block_size * 2  # 32 tokens = exactly 2 blocks
+
     scheduler = create_scheduler_with_priority(
-        max_num_seqs=3,  # Allow multiple requests
+        max_num_seqs=3,
         max_num_batched_tokens=200,
-        num_blocks=6,  # Very limited blocks to force memory pressure
-        block_size=16,  # Standard block size
-    )
-
-    # Create initial low-priority requests that will consume most memory
-    low_priority_requests = create_requests_with_priority(
-        num_requests=2,
-        priorities=[5, 5],  # Low priority
-        arrival_times=[1.0, 2.0],
-        num_tokens=30,  # Large enough to consume significant memory,
-        req_ids=["lo1", "lo2"],
+        num_blocks=num_blocks,
+        block_size=block_size,
     )
 
-    # Add and schedule low priority requests
-    for request in low_priority_requests:
-        scheduler.add_request(request)
+    # --- Phase 1: low-priority request starts running ---
+    lo1 = create_requests_with_priority(
+        num_requests=1,
+        priorities=[5],
+        arrival_times=[1.0],
+        num_tokens=num_tokens,
+        req_ids=["lo1"],
+    )[0]
+    scheduler.add_request(lo1)
 
     output = scheduler.schedule()
-    assert len(output.scheduled_new_reqs) == 2
+    assert len(output.scheduled_new_reqs) == 1
 
-    # Simulate model execution to move requests to running state
+    # Decode: lo1 now has 33 tokens (crosses 32-token boundary).
     model_output = ModelRunnerOutput(
-        req_ids=[req.request_id for req in low_priority_requests],
-        req_id_to_index={
-            req.request_id: i for i, req in enumerate(low_priority_requests)
-        },
-        sampled_token_ids=[[100] for _ in low_priority_requests],
+        req_ids=["lo1"],
+        req_id_to_index={"lo1": 0},
+        sampled_token_ids=[[100]],
         logprobs=None,
         prompt_logprobs_dict={},
         pooler_output=[],
     )
     scheduler.update_from_output(output, model_output)
 
-    # Verify both requests are running
-    assert len(scheduler.running) == 2
-
-    # Now add a high-priority request that requires memory allocation
-    # This should trigger preemption due to memory constraints
-    high_priority_request = create_requests_with_priority(
+    # --- Phase 2: high-priority request arrives AFTER lo1 is running ---
+    hi1 = create_requests_with_priority(
         num_requests=1,
-        priorities=[0],  # High priority
-        arrival_times=[3.0],
-        num_tokens=30,  # Large enough to require significant memory
+        priorities=[0],
+        arrival_times=[2.0],
+        num_tokens=num_tokens,
         req_ids=["hi1"],
     )[0]
+    scheduler.add_request(hi1)
 
-    scheduler.add_request(high_priority_request)
-
-    # Schedule again - this should trigger
-    # preemption when trying to allocate memory
+    # schedule(): lo1 gets its 3rd block (3 used), hi1 admitted (5 used,
+    # 0 free).  Both are now running.
     output = scheduler.schedule()
+    assert any(r.req_id == "hi1" for r in output.scheduled_new_reqs)
+    assert len(scheduler.running) == 2
+
+    # Decode: lo1 → 34 tokens, hi1 → 33 tokens.
+    model_output = ModelRunnerOutput(
+        req_ids=["lo1", "hi1"],
+        req_id_to_index={"lo1": 0, "hi1": 1},
+        sampled_token_ids=[[101], [100]],
+        logprobs=None,
+        prompt_logprobs_dict={},
+        pooler_output=[],
+    )
+    scheduler.update_from_output(output, model_output)
 
-    # Due to the scheduler's design, if preemption happens
-    # during running request scheduling,
-    # waiting requests won't be scheduled in the same step
-    # Let's check if preemption occurred by looking at the waiting queue
+    # --- Phase 3: preemption with mixed-priority running requests ---
+    # hi1 needs a 3rd block but 0 are free.  The scheduler picks the
+    # lowest-priority running request to preempt:
+    #   max(running, key=(priority, arrival_time)) → lo1 (5 > 0).
+    output = scheduler.schedule()
 
-    # If preemption happened, we should see requests in the
-    # waiting queue
-    if len(scheduler.waiting) > 1:  # high priority + preempted request
-        # Preemption occurred - verify the high priority request
-        # gets scheduled next
-        output2 = scheduler.schedule()
-        assert len(output2.scheduled_new_reqs) == 1
-        # High priority request
-        assert output2.scheduled_new_reqs[0].req_id == "hi1"
-    else:
-        # No preemption needed - all requests fit
-        # This is also valid behavior if memory allows
-        assert len(output.scheduled_new_reqs) == 1
-        # High priority request
-        assert output.scheduled_new_reqs[0].req_id == "hi1"
+    lo1_req = scheduler.requests["lo1"]
+    assert lo1_req.status == RequestStatus.PREEMPTED, (
+        "Expected low-priority 'lo1' to be preempted"
+    )
+    assert any(req.request_id == "hi1" for req in scheduler.running), (
+        "High-priority 'hi1' should still be running"
+    )
 
 
 def test_priority_scheduling_no_preemption_when_space_available():