linux-kernel - Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

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Message-ID: <20200529102125.GB3070@suse.de>
Date:   Fri, 29 May 2020 11:21:25 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Qais Yousef <qais.yousef@....com>
Cc:     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>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        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 Thu, May 28, 2020 at 05:51:31PM +0100, Qais Yousef wrote:
> > 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.
> 

As with all things, it's perfectly possible I was looking at a workload
where the cost is more obvious but given that the functions are inlined,
it's not trivial to spot. I just happened to spot it because I was paying
close attention to try_to_wake_up() at the time.

> On my Ubuntu 18.04 machine uclamp is enabled by default by the way. 5.3 kernel
> though, so uclamp task group stuff not there yet. Should check how their server
> distro looks like.
> 

Elsewhere in the thread, I showed some results based on 5.7 so uclamp
task group existed but I had it disabled. The uclamp related parts of
the kconfig were

# zgrep UCLAMP kconfig-5.7.0-rc7-with-clamp.txt.gz
CONFIG_UCLAMP_TASK=y
CONFIG_UCLAMP_BUCKETS_COUNT=5
# CONFIG_UCLAMP_TASK_GROUP is not set

-- 
Mel Gorman
SUSE Labs