Messages in this thread Patches in this message | | | Date | Wed, 3 Jun 2020 17:52:00 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value |
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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
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