Re: [PATCH 2/2] sched/fair: Increase wakeup_gran if current task has not executed the minimum granularity

[Vincent Guittot]

On Thu, 28 Oct 2021 at 11:49, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Commit 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
> moved the kernel.sched_wakeup_granularity_ns sysctl under debugfs.
> One of the reasons why this sysctl may be used may be for "optimising
> for throughput", particularly when overloaded. The tool TuneD sometimes
> alters this for two profiles e.g. "mssql" and "throughput-performance". At
> least version 2.9 does but it changed in master where it also will poke
> at debugfs instead. This patch aims to reduce the motivation to tweak
> sysctl_sched_wakeup_granularity by increasing sched_wakeup_granularity
> if the running task runtime has not exceeded sysctl_sched_min_granularity.
>
> During task migration or wakeup, a decision is made on whether
> to preempt the current task or not. To limit over-scheduled,
> sysctl_sched_wakeup_granularity delays the preemption to allow at least 1ms
> of runtime before preempting. However, when a domain is heavily overloaded
> (e.g. hackbench), the degree of over-scheduling is still severe. This is
> problematic as time is wasted rescheduling tasks that could instead be
> used by userspace tasks.
>
> However, care must be taken. Even if a system is overloaded, there may
> be high priority threads that must still be able to run. Mike Galbraith
> explained the constraints as follows;
>
>         CFS came about because the O1 scheduler was unfair to the
>         point it had starvation problems. People pretty much across the
>         board agreed that a fair scheduler was a much way better way
>         to go, and CFS was born.  It didn't originally have the sleep
>         credit business, but had to grow it to become _short term_ fair.
>         Ingo cut the sleep credit in half because of overscheduling, and
>         that has worked out pretty well all told.. but now you're pushing
>         it more in the unfair direction, all the way to extremely unfair
>         for anything and everything very light.
>
>         Fairness isn't the holy grail mind you, and at some point, giving
>         up on short term fairness certainly isn't crazy, as proven by your
>         hackbench numbers and other numbers we've seen over the years,
>         but taking bites out of the 'CF' in the CFS that was born to be a
>         corner-case killer is.. worrisome.  The other shoe will drop.. it
>         always does :)
>
> This patch increases the wakeup granularity if the current task has not
> reached its minimum preemption granularity. The current task may still
> be preempted but the difference in runtime must be higher.
>
> hackbench-process-pipes
>                           5.15.0-rc3             5.15.0-rc3
>                sched-wakeeflips-v1r1sched-scalewakegran-v3r2
> Amean     1        0.3890 (   0.00%)      0.3823 (   1.71%)
> Amean     4        0.5217 (   0.00%)      0.4867 (   6.71%)
> Amean     7        0.5387 (   0.00%)      0.5053 (   6.19%)
> Amean     12       0.5443 (   0.00%)      0.5450 (  -0.12%)
> Amean     21       0.6487 (   0.00%)      0.6807 (  -4.93%)
> Amean     30       0.8033 (   0.00%)      0.7107 *  11.54%*
> Amean     48       1.2400 (   0.00%)      1.0447 *  15.75%*
> Amean     79       1.8200 (   0.00%)      1.6033 *  11.90%*
> Amean     110      2.5820 (   0.00%)      2.0763 *  19.58%*
> Amean     141      3.2203 (   0.00%)      2.5313 *  21.40%*
> Amean     172      3.8200 (   0.00%)      3.1163 *  18.42%*
> Amean     203      4.3357 (   0.00%)      3.5560 *  17.98%*
> Amean     234      4.8047 (   0.00%)      3.8913 *  19.01%*
> Amean     265      5.1243 (   0.00%)      4.2293 *  17.47%*
> Amean     296      5.5940 (   0.00%)      4.5357 *  18.92%*
>
>                   5.15.0-rc3  5.15.0-rc3
>          sched-wakeeflips-v1r1 sched-scalewakegran-v3r2
> Duration User        2567.27     2034.17
> Duration System     21098.79    17137.08
> Duration Elapsed      136.49      120.2
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c     | 17 +++++++++++++++--
>  kernel/sched/features.h |  2 ++
>  2 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d00af3b97d8f..dee108470297 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7052,10 +7052,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  }
>  #endif /* CONFIG_SMP */
>
> -static unsigned long wakeup_gran(struct sched_entity *se)
> +static unsigned long
> +wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
>  {
>         unsigned long gran = sysctl_sched_wakeup_granularity;
>
> +       if (sched_feat(SCALE_WAKEUP_GRAN)) {
> +               unsigned long delta_exec;
> +
> +               /*
> +                * Increase the wakeup granularity if curr's runtime
> +                * is less than the minimum preemption granularity.
> +                */
> +               delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> +               if (delta_exec < sysctl_sched_min_granularity)
> +                       gran += sysctl_sched_min_granularity;

I need to think a bit more about corner cases but this change looks
much better than the previous one.

> +       }
> +
>         /*
>          * Since its curr running now, convert the gran from real-time
>          * to virtual-time in his units.
> @@ -7094,7 +7107,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>         if (vdiff <= 0)
>                 return -1;
>
> -       gran = wakeup_gran(se);
> +       gran = wakeup_gran(curr, se);
>         if (vdiff > gran)
>                 return 1;
>
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 7f8dace0964c..611591355ffd 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -95,3 +95,5 @@ SCHED_FEAT(LATENCY_WARN, false)
>
>  SCHED_FEAT(ALT_PERIOD, true)
>  SCHED_FEAT(BASE_SLICE, true)
> +
> +SCHED_FEAT(SCALE_WAKEUP_GRAN, true)
> --
> 2.31.1
>