[Core] Update `_earliest_arrival_time` calculation of the waiting seq_groups

[Felix-Zhenghao]

When Scheduler._schedule_prefills() is called, we assume the waiting queue is sorted. Therefore, we can directly access the sequence group coming earliest through waiting_queue[0] in Scheduler._schedule_prefills().

However, in the Scheduler._passed_delay() implementation, we access the earliest_arrival_time by iterating the whole self.waiting, which is unnecessary if we assume self.waiting is sorted.

This PR changes the Scheduler._passed_delay() implementation to make sure that it can benefits from the "sorted" assumption as well.

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[rkooo567]

also lint failure!

[Felix-Zhenghao]

Btw, this is always not 100% guaranteed upon preemption (because we always appendleft preempted requests). Do you think this can cause any issues in this case?

Thanks for pointing it out. I think that's not a problem.

Since in _schedule_prefills(), WAITING seq_groups are always scheduled to RUNNING in time order. Thus for those preempted seq_groups, their arrival time is always earlier than those seq_groups in self.waiting list. The only question left is whether the preempted list is sorted.

When _schedule_running(), the running seq_groups are sorted. Each preemption is done to the lowest-prority seq_group. Newly preempted seq_group is appended to the preempted list. Thus, the the preempted list is sorted as well.

As stated above, self.waiting[0].metrics.arrival_time can correctly get the earliest arrival time and will not have problem even there exist preempted requests. @rkooo567

also lint failure!

yapf lint failure has been fixed.

[rkooo567]

Hmm actually, we may need to reverse this order to guarantee the ordering? https://github.com/vllm-project/vllm/blob/190bc838e17196733526896bf2861f8d05bd3f43/vllm/core/scheduler.py#L772

the reason is we append to preempted https://github.com/vllm-project/vllm/blob/190bc838e17196733526896bf2861f8d05bd3f43/vllm/core/scheduler.py#L433, meaning preempted list is sorted in the reverse order (from low -> high priority). And when we do appendleft, it means the lower priority will be in the top of the list?