Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

[Vincent Guittot <vincent.guittot@linaro.org>]

On Tue, 28 Mar 2023 at 13:06, Peter Zijlstra <peterz@infradead.org> wrote:
>
> In the case where (due to latency-nice) there are different request
> sizes in the tree, the smaller requests tend to be dominated by the
> larger. Also note how the EEVDF lag limits are based on r_max.
>
> Therefore; add a heuristic that for the mixed request size case, moves
> smaller requests to placement strategy #2 which ensures they're
> immidiately eligible and and due to their smaller (virtual) deadline
> will cause preemption.
>
> NOTE: this relies on update_entity_lag() to impose lag limits above
> a single slice.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/fair.c     |   14 ++++++++++++++
>  kernel/sched/features.h |    1 +
>  kernel/sched/sched.h    |    1 +
>  3 files changed, 16 insertions(+)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -616,6 +616,7 @@ avg_vruntime_add(struct cfs_rq *cfs_rq,
>         s64 key = entity_key(cfs_rq, se);
>
>         cfs_rq->avg_vruntime += key * weight;
> +       cfs_rq->avg_slice += se->slice * weight;
>         cfs_rq->avg_load += weight;
>  }
>
> @@ -626,6 +627,7 @@ avg_vruntime_sub(struct cfs_rq *cfs_rq,
>         s64 key = entity_key(cfs_rq, se);
>
>         cfs_rq->avg_vruntime -= key * weight;
> +       cfs_rq->avg_slice -= se->slice * weight;
>         cfs_rq->avg_load -= weight;
>  }
>
> @@ -4832,6 +4834,18 @@ place_entity(struct cfs_rq *cfs_rq, stru
>                 lag = se->vlag;
>
>                 /*
> +                * For latency sensitive tasks; those that have a shorter than
> +                * average slice and do not fully consume the slice, transition
> +                * to EEVDF placement strategy #2.
> +                */
> +               if (sched_feat(PLACE_FUDGE) &&
> +                   cfs_rq->avg_slice > se->slice * cfs_rq->avg_load) {
> +                       lag += vslice;
> +                       if (lag > 0)
> +                               lag = 0;

By using different lag policies for tasks, doesn't this create
unfairness between tasks ?

I wanted to stress this situation with a simple use case but it seems
that even without changing the slice, there is a fairness problem:

Task A always run
Task B loops on : running 1ms then sleeping 1ms
default nice and latency nice prio bot both
each task should get around 50% of the time.

The fairness is ok with tip/sched/core
but with eevdf, Task B only gets around 30%

I haven't identified the problem so far


> +               }
> +
> +               /*
>                  * If we want to place a task and preserve lag, we have to
>                  * consider the effect of the new entity on the weighted
>                  * average and compensate for this, otherwise lag can quickly
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -5,6 +5,7 @@
>   * sleep+wake cycles. EEVDF placement strategy #1, #2 if disabled.
>   */
>  SCHED_FEAT(PLACE_LAG, true)
> +SCHED_FEAT(PLACE_FUDGE, true)
>  SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
>
>  /*
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -559,6 +559,7 @@ struct cfs_rq {
>         unsigned int            idle_h_nr_running; /* SCHED_IDLE */
>
>         s64                     avg_vruntime;
> +       u64                     avg_slice;
>         u64                     avg_load;
>
>         u64                     exec_clock;
>
>