Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Vincent Guittot]

On Wed, 3 Jun 2020 at 18:52, Qais Yousef <qais.yousef@xxxxxxx> wrote:
>
> On 06/03/20 16:59, Vincent Guittot wrote:
> > When I want to stress the fast path i usually use "perf bench sched pipe -T "
> > The tip/sched/core on my arm octo core gives the following results for
> > 20 iterations of perf bench sched pipe -T -l 50000
> >
> > all uclamp config disabled  50035.4(+/- 0.334%)
> > all uclamp config enabled  48749.8(+/- 0.339%)   -2.64%
> >
> > It's quite easy to reproduce and probably easier to study the impact
>
> Thanks Vincent. This is very useful!
>
> I could reproduce that on my Juno.
>
> One of the codepath I was suspecting seems to affect it.
>
>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0464569f26a7..9f48090eb926 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1063,10 +1063,12 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
>          * e.g. due to future modification, warn and fixup the expected value.
>          */
>         SCHED_WARN_ON(bucket->value > rq_clamp);
> +#if 0
>         if (bucket->value >= rq_clamp) {
>                 bkt_clamp = uclamp_rq_max_value(rq, clamp_id, uc_se->value);
>                 WRITE_ONCE(uc_rq->value, bkt_clamp);
>         }
> +#endif
>  }
>
>  static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>
>
>
> uclamp_rq_max_value() could be expensive as it loops over all buckets.
> Commenting this whole path out strangely doesn't just 'fix' it, but produces
> better results to no-uclamp kernel :-/
>
>
>
> # ./perf bench -r 20 sched pipe -T -l 50000
> Without uclamp:         5039
> With uclamp:            4832
> With uclamp+patch:      5729
>
>
>
> It might be because schedutil gets biased differently by uclamp..? If I move to
> performance governor these numbers almost double.
>
> I don't know. But this promoted me to look closer and I think I spotted a bug
> where in the if condition we check for '>=' instead of '>', causing us to take
> the supposedly impossible fail safe path.
>
> Mind trying with the below patch please?

I have tried your patch and I don't see any difference compared to
previous tests. Let me give you more details of my setup:
I create 3 levels of cgroups and usually run the tests in the 4 levels
(which includes root). The result above are for the root level

But I see a difference at other levels:

                           root           level 1       level 2       level 3

/w patch uclamp disable     50097         46615         43806         41078
tip uclamp enable           48706(-2.78%) 45583(-2.21%) 42851(-2.18%)
40313(-1.86%)
/w patch uclamp enable      48882(-2.43%) 45774(-1.80%) 43108(-1.59%)
40667(-1.00%)

Whereas tip with uclamp stays around 2% behind tip without uclamp, the
diff of uclamp with your patch tends to decrease when we increase the
number of level

Beside this, that's also interesting to notice the ~6% of perf impact
between each level for the same image

>
>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0464569f26a7..50d66d4016ff 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1063,7 +1063,7 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
>          * e.g. due to future modification, warn and fixup the expected value.
>          */
>         SCHED_WARN_ON(bucket->value > rq_clamp);
> -       if (bucket->value >= rq_clamp) {
> +       if (bucket->value > rq_clamp) {
>                 bkt_clamp = uclamp_rq_max_value(rq, clamp_id, uc_se->value);
>                 WRITE_ONCE(uc_rq->value, bkt_clamp);
>         }
>
>
>
> Thanks
>
> --
> Qais Yousef