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From: Qais Yousef <qais.yousef@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
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: Wed, 3 Jun 2020 17:52:00 +0100
Message-ID: <20200603165200.v2ypeagziht7kxdw@e107158-lin.cambridge.arm.com> (raw)
In-Reply-To: <CAKfTPtAvMvPk5Ea2kaxXE8GzQ+Nc_PS+EKB1jAa03iJwQORSqA@mail.gmail.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

  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 [this message]
2020-06-04 12:14                   ` Vincent Guittot
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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