On Thu, 22 Aug 2019 at 04:18, Rik van Riel <r...@surriel.com> wrote:
>
> The idea behind __sched_period makes sense, but the results do not always.
>
> When a CPU has one high priority task and a large number of low priority
> tasks, __sched_period will return a value larger than sysctl_sched_latency,
> and the one high priority task may end up getting a timeslice all for itself
> that is also much larger than sysctl_sched_latency.
note that unless you enable sched_feat(HRTICK), the sched_slice is
mainly used to decide how fast we preempt running task at tick but a
newly wake up task can preempt it before
>
> The low priority tasks will have their time slices rounded up to
> sysctl_sched_min_granularity, resulting in an even larger scheduling
> latency than targeted by __sched_period.
Will this not break the fairness between a always running task and a
short sleeping one with this changes ?
>
> Simplify the code by simply ripping out __sched_period and always taking
> fractions of sysctl_sched_latency.
>
> If a high priority task ends up getting a "too small" time slice compared
> to low priority tasks, the vruntime scaling ensures that it will simply
> get scheduled more frequently than low priority tasks.
Will you not increase the number of context switch ?
>
> Signed-off-by: Rik van Riel <r...@surriel.com>
> ---
> kernel/sched/fair.c | 18 +-----------------
> 1 file changed, 1 insertion(+), 17 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8f8c85c6da9b..74ee22c59d13 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -691,22 +691,6 @@ static inline u64 calc_delta_fair(u64 delta, struct
> sched_entity *se)
> return delta;
> }
>
> -/*
> - * The idea is to set a period in which each task runs once.
> - *
> - * When there are too many tasks (sched_nr_latency) we have to stretch
> - * this period because otherwise the slices get too small.
> - *
> - * p = (nr <= nl) ? l : l*nr/nl
> - */
> -static u64 __sched_period(unsigned long nr_running)
> -{
> - if (unlikely(nr_running > sched_nr_latency))
> - return nr_running * sysctl_sched_min_granularity;
> - else
> - return sysctl_sched_latency;
> -}
> -
> /*
> * We calculate the wall-time slice from the period by taking a part
> * proportional to the weight.
> @@ -715,7 +699,7 @@ static u64 __sched_period(unsigned long nr_running)
> */
> static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> - u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
> + u64 slice = sysctl_sched_latency;
>
> for_each_sched_entity(se) {
> struct load_weight *load;
> --
> 2.20.1
>
