**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

airflow/jobs/scheduler_job.py
if self._process is None:
            raise AirflowException("Tried to kill process before starting!")
        if self._process.is_alive() and self._process.pid:
            self.log.warning("Killing DAGFileProcessorProcess (PID=%d)", self._process.pid)
            os.kill(self._process.pid, signal.SIGKILL)
        if self._parent_channel:
            self._parent_channel.close()
    @property
    def pid(self) -> int:
        """
        :return: the PID of the process launched to process the given file
        :rtype: int
        """
        if self._process is None or self._process.pid is None:
            raise AirflowException("Tried to get PID before starting!")
        return self._process.pid
    @property
    def exit_code(self) -> Optional[int]:
        """
        After the process is finished, this can be called to get the return code
        :return: the exit code of the process
        :rtype: int
        """
        if self._process is None:
            raise AirflowException("Tried to get exit code before starting!")
        if not self._done:
            raise AirflowException("Tried to call retcode before process was finished!")
        return self._process.exitcode
    @property
    def done(self) -> bool: