[RFC] sched/eevdf: sched feature to dismiss lag on wakeup

[Luis Machado]

Hi Tobias,

On 2/28/24 16:10, Tobias Huschle wrote:
> The previously used CFS scheduler gave tasks that were woken up an
> enhanced chance to see runtime immediately by deducting a certain value
> from its vruntime on runqueue placement during wakeup.
> 
> This property was used by some, at least vhost, to ensure, that certain
> kworkers are scheduled immediately after being woken up. The EEVDF
> scheduler, does not support this so far. Instead, if such a woken up
> entitiy carries a negative lag from its previous execution, it will have
> to wait for the current time slice to finish, which affects the
> performance of the process expecting the immediate execution negatively.
> 
> To address this issue, implement EEVDF strategy #2 for rejoining
> entities, which dismisses the lag from previous execution and allows
> the woken up task to run immediately (if no other entities are deemed
> to be preferred for scheduling by EEVDF).
> 
> The vruntime is decremented by an additional value of 1 to make sure,
> that the woken up tasks gets to actually run. This is of course not
> following strategy #2 in an exact manner but guarantees the expected
> behavior for the scenario described above. Without the additional
> decrement, the performance goes south even more. So there are some
> side effects I could not get my head around yet.
> 
> Questions:
> 1. The kworker getting its negative lag occurs in the following scenario
>    - kworker and a cgroup are supposed to execute on the same CPU
>    - one task within the cgroup is executing and wakes up the kworker
>    - kworker with 0 lag, gets picked immediately and finishes its
>      execution within ~5000ns
>    - on dequeue, kworker gets assigned a negative lag
>    Is this expected behavior? With this short execution time, I would
>    expect the kworker to be fine.

That strikes me as a bit odd as well. Have you been able to determine how a negative lag
is assigned to the kworker after such a short runtime?

I was looking at a different thread (https://lore.kernel.org/lkml/20240226082349.302363-1-yu.c.chen@intel.com/) that
uncovers a potential overflow in the eligibility calculation. Though I don't think that is the case for this particular
vhost problem.