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Date: Thu, 4 Jun 2020 14:40:43 +0100
From: Mel Gorman <mgorman@...e.de>
To: Qais Yousef <qais.yousef@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>,
Jonathan Corbet <corbet@....net>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Luis Chamberlain <mcgrof@...nel.org>,
Kees Cook <keescook@...omium.org>,
Iurii Zaikin <yzaikin@...gle.com>,
Quentin Perret <qperret@...gle.com>,
Valentin Schneider <valentin.schneider@....com>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Pavan Kondeti <pkondeti@...eaurora.org>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
boost value
On Wed, Jun 03, 2020 at 01:41:13PM +0100, Qais Yousef wrote:
> > > netperf-udp
> > > ./5.7.0-rc7 ./5.7.0-rc7 ./5.7.0-rc7
> > > without-clamp with-clamp with-clamp-tskgrp
> > >
> > > Hmean send-64 153.62 ( 0.00%) 151.80 * -1.19%* 155.60 * 1.28%*
> > > Hmean send-128 306.77 ( 0.00%) 306.27 * -0.16%* 309.39 * 0.85%*
> > > Hmean send-256 608.54 ( 0.00%) 604.28 * -0.70%* 613.42 * 0.80%*
> > > Hmean send-1024 2395.80 ( 0.00%) 2365.67 * -1.26%* 2409.50 * 0.57%*
> > > Hmean send-2048 4608.70 ( 0.00%) 4544.02 * -1.40%* 4665.96 * 1.24%*
> > > Hmean send-3312 7223.97 ( 0.00%) 7158.88 * -0.90%* 7331.23 * 1.48%*
> > > Hmean send-4096 8729.53 ( 0.00%) 8598.78 * -1.50%* 8860.47 * 1.50%*
> > > Hmean send-8192 14961.77 ( 0.00%) 14418.92 * -3.63%* 14908.36 * -0.36%*
> > > Hmean send-16384 25799.50 ( 0.00%) 25025.64 * -3.00%* 25831.20 * 0.12%*
> > > Hmean recv-64 153.62 ( 0.00%) 151.80 * -1.19%* 155.60 * 1.28%*
> > > Hmean recv-128 306.77 ( 0.00%) 306.27 * -0.16%* 309.39 * 0.85%*
> > > Hmean recv-256 608.54 ( 0.00%) 604.28 * -0.70%* 613.42 * 0.80%*
> > > Hmean recv-1024 2395.80 ( 0.00%) 2365.67 * -1.26%* 2409.50 * 0.57%*
> > > Hmean recv-2048 4608.70 ( 0.00%) 4544.02 * -1.40%* 4665.95 * 1.24%*
> > > Hmean recv-3312 7223.97 ( 0.00%) 7158.88 * -0.90%* 7331.23 * 1.48%*
> > > Hmean recv-4096 8729.53 ( 0.00%) 8598.78 * -1.50%* 8860.47 * 1.50%*
> > > Hmean recv-8192 14961.61 ( 0.00%) 14418.88 * -3.63%* 14908.30 * -0.36%*
> > > Hmean recv-16384 25799.39 ( 0.00%) 25025.49 * -3.00%* 25831.00 * 0.12%*
> > >
> > > netperf-tcp
> > >
> > > Hmean 64 818.65 ( 0.00%) 812.98 * -0.69%* 826.17 * 0.92%*
> > > Hmean 128 1569.55 ( 0.00%) 1555.79 * -0.88%* 1586.94 * 1.11%*
> > > Hmean 256 2952.86 ( 0.00%) 2915.07 * -1.28%* 2968.15 * 0.52%*
> > > Hmean 1024 10425.91 ( 0.00%) 10296.68 * -1.24%* 10418.38 * -0.07%*
> > > Hmean 2048 17454.51 ( 0.00%) 17369.57 * -0.49%* 17419.24 * -0.20%*
> > > Hmean 3312 22509.95 ( 0.00%) 22229.69 * -1.25%* 22373.32 * -0.61%*
> > > Hmean 4096 25033.23 ( 0.00%) 24859.59 * -0.69%* 24912.50 * -0.48%*
> > > Hmean 8192 32080.51 ( 0.00%) 31744.51 * -1.05%* 31800.45 * -0.87%*
> > > Hmean 16384 36531.86 ( 0.00%) 37064.68 * 1.46%* 37397.71 * 2.37%*
> > >
> > > The diffs are smaller than on openSUSE Leap 15.1 and some of the
> > > uclamp taskgroup results are better?
> > >
> >
> > I don't see the stddev and coeff but these look close to borderline.
> > Sure, they are marked with a * so it passed a significant test but it's
> > still a very marginal difference for netperf. It's possible that the
> > systemd configurations differ in some way that is significant for uclamp
> > but I don't know what that is.
>
> Hmm so what you're saying is that Dietmar didn't reproduce the same problem
> you're observing? I was hoping to use that to dig more into it.
>
Not as such, I'm saying that for whatever reason the problem is not as
visible with Dietmar's setup. It may be machine-specific or distribution
specific. There are alternative suggestions for testing just the fast
paths with a pipe test that may be clearer.
> >
> > > With this test setup we now can play with the uclamp code in
> > > enqueue_task() and dequeue_task().
> > >
> >
> > That is still true. An annotated perf profile should tell you if the
> > uclamp code is being heavily used or if it's bailing early but it's also
> > possible that uclamp overhead is not a big deal on your particular
> > machine.
> >
> > The possibility that either the distribution, the machine or both are
> > critical for detecting a problem with uclamp may explain why any overhead
> > was missed. Even if it is marginal, it still makes sense to minimise the
> > amount of uclamp code that is executed if no limit is specified for tasks.
>
> So one speculation I have that might be causing the problem is that the
> accesses of struct uclamp_rq are causing bad cache behavior in your case. Your
> mmtest description of the netperf says that it is sensitive to cacheline
> bouncing.
>
> Looking at struct rq, the uclamp_rq is spanning 2 cachelines
>
> 29954 /* --- cacheline 1 boundary (64 bytes) --- */
> 29955 struct uclamp_rq uclamp[2]; /* 64 96 */
> 29956 /* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
> 29957 unsigned int uclamp_flags; /* 160 4 */
> 29958
> 29959 /* XXX 28 bytes hole, try to pack */
> 29960
>
> Reducing sturct uclamp_bucket to use unsigned int instead of unsigned long
> helps putting it all in a single cacheline
>
I tried this and while it did not make much of a difference to the
headline metric, the workload was less variable so if it's proven that
cache line bouncing is reduced (I didn't measure it), it may have merit
on its own even if it does not fully address the problem.
--
Mel Gorman
SUSE Labs
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