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From: Peter Zijlstra <peterz@infradead.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	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>,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value
Date: Thu, 28 May 2020 20:29:14 +0200
Message-ID: <20200528182913.GQ2483@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20200528165130.m5unoewcncuvxynn@e107158-lin.cambridge.arm.com>

On Thu, May 28, 2020 at 05:51:31PM +0100, Qais Yousef wrote:

> In my head, the simpler version of
> 
> 	if (rt_task(p) && !uc->user_defined)
> 		// update_uclamp_min
> 
> Is a single branch and write to cache, so should be fast. I'm failing to see
> how this could generate an overhead tbh, but will not argue about it :-)

Mostly true; but you also had a load of that sysctl in there, which is
likely to be a miss, and those are expensive.

Also; if we're going to have to optimize this, less logic is in there,
the less we need to take out. Esp. for stuff that 'never' changes, like
this.

> > It's more code, but it is all outside of the normal paths where we care
> > about performance.
> 
> I am happy to take that direction if you think it's worth it. I'm thinking
> task_woken_rt() is good. But again, maybe I am missing something.

Basic rule, if the state 'never' changes, don't touch fast paths.

Such little things can be very difficult to measure, but at some point
they cause death-by-a-thousnd-cuts.

> > Indeed, that one. The fact that regular distros cannot enable this
> > feature due to performance overhead is unfortunate. It means there is a
> > lot less potential for this stuff.
> 
> I had a humble try to catch the overhead but wasn't successful. The observation
> wasn't missed by us too then.

Right, I remember us doing benchmarks when we introduced all this and
clearly we missed something. I would be good if Mel can share which
benchmark hurt most so we can go have a look.

  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 [this message]
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
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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