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

[Qais Yousef <qais.yousef@arm.com>]

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?



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