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From: Vincent Guittot <vincent.guittot@linaro.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Mel Gorman <mgorman@suse.de>,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Quentin Perret <qperret@google.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	linux-doc@vger.kernel.org,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-fs <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value
Date: Thu, 4 Jun 2020 14:14:40 +0200
Message-ID: <CAKfTPtC6TvUL83VdWuGfbKm0CkXB85YQ5qkagK9aiDB8Hqrn_Q@mail.gmail.com> (raw)
In-Reply-To: <20200603165200.v2ypeagziht7kxdw@e107158-lin.cambridge.arm.com>

On Wed, 3 Jun 2020 at 18:52, Qais Yousef <qais.yousef@arm.com> 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

  reply index

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-11 15:40 Qais Yousef
2020-05-11 15:40 ` [PATCH 2/2] Documentation/sysctl: Document uclamp sysctl knobs Qais Yousef
2020-05-11 17:18 ` [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Qais Yousef
2020-05-12  2:10 ` Pavan Kondeti
2020-05-12 11:46   ` Qais Yousef
2020-05-15 11:08 ` Patrick Bellasi
2020-05-18  8:31 ` Dietmar Eggemann
2020-05-18 16:49   ` Qais Yousef
2020-05-28 13:23 ` Peter Zijlstra
2020-05-28 15:58   ` Qais Yousef
2020-05-28 16:11     ` Peter Zijlstra
2020-05-28 16:51       ` Qais Yousef
2020-05-28 18:29         ` Peter Zijlstra
2020-05-28 19:08           ` Patrick Bellasi
2020-05-28 19:20           ` Dietmar Eggemann
2020-05-29  9:11           ` Qais Yousef
2020-05-29 10:21         ` Mel Gorman
2020-05-29 15:11           ` Qais Yousef
2020-05-29 16:02             ` Mel Gorman
2020-05-29 16:05               ` Qais Yousef
2020-05-29 10:08       ` Mel Gorman
2020-05-29 16:04         ` Qais Yousef
2020-05-29 16:57           ` Mel Gorman
2020-06-02 16:46         ` Dietmar Eggemann
2020-06-03  8:29           ` Patrick Bellasi
2020-06-03 10:10             ` Mel Gorman
2020-06-03 14:59               ` Vincent Guittot
2020-06-03 16:52                 ` Qais Yousef
2020-06-04 12:14                   ` Vincent Guittot [this message]
2020-06-05 10:45                     ` Qais Yousef
2020-06-09 15:29                       ` Vincent Guittot
2020-06-08 12:31                     ` Qais Yousef
2020-06-08 13:06                       ` Valentin Schneider
2020-06-08 14:44                       ` Steven Rostedt
2020-06-11 10:13                         ` Qais Yousef
2020-06-09 17:10                       ` Vincent Guittot
2020-06-11 10:24                         ` Qais Yousef
2020-06-11 12:01                           ` Vincent Guittot
2020-06-23 15:44                             ` Qais Yousef
2020-06-24  8:45                               ` Vincent Guittot
2020-06-05  7:55                   ` Patrick Bellasi
2020-06-05 11:32                     ` Qais Yousef
2020-06-05 13:27                       ` Patrick Bellasi
2020-06-03  9:40           ` Mel Gorman
2020-06-03 12:41             ` Qais Yousef
2020-06-04 13:40               ` Mel Gorman
2020-06-05 10:58                 ` Qais Yousef
2020-06-11 10:58                 ` Qais Yousef
2020-06-16 11:08                   ` Qais Yousef
2020-06-16 13:56                     ` Lukasz Luba
  -- strict thread matches above, loose matches on Subject: below --
2020-04-03 12:30 Qais Yousef
2020-04-14 18:21 ` Patrick Bellasi
2020-04-15  7:46   ` Patrick Bellasi
2020-04-20 15:04     ` Qais Yousef
2020-04-20  8:24   ` Dietmar Eggemann
2020-04-20 15:19     ` Qais Yousef
2020-04-21  0:52       ` Steven Rostedt
2020-04-21 11:16         ` Dietmar Eggemann
2020-04-21 11:23           ` Qais Yousef
2020-04-20 14:50   ` Qais Yousef
2020-04-15 10:11 ` Quentin Perret
2020-04-20 15:08   ` Qais Yousef
2020-04-20  8:29 ` Dietmar Eggemann
2020-04-20 15:13   ` Qais Yousef
2020-04-21 11:18     ` Dietmar Eggemann
2020-04-21 11:27       ` Qais Yousef
2020-04-22 10:59         ` Dietmar Eggemann
2020-04-22 13:13           ` Qais Yousef

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