linux-kernel - Re: Fwd: Nice processes prevent frequency increases - possible scheduler regression (known good in 2.6.35)

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Message-ID: <1351942405.16850.23.camel@maggy.simpson.net>
Date:	Sat, 03 Nov 2012 04:33:25 -0700
From:	Mike Galbraith <efault@....de>
To:	Michal Zatloukal <myxal.mxl@...il.com>
Cc:	linux-kernel@...r.kernel.org, cpufreq@...r.kernel.org
Subject: Re: Fwd: Nice processes prevent frequency increases - possible
 scheduler regression (known good in 2.6.35)

On Fri, 2012-11-02 at 21:09 +0100, Michal Zatloukal wrote:

> On the new kernel, the nice processes are never starved - even when
> starting a tab-laden chromium session, the processes for BOINC keep
> about 20% CPU each (that is normalized to all CPUs, ie 40% nice load
> on each core). The problem is, the governor now seems to consider the
> non-nice task unable to saturate the CPU, and the cores' frequencies
> are hovering between 1.0 and 1.8 GHz. The scheduler keeps scheduling
> the nice tasks, and the non-nice tasks are progressing much slower,
> caused by the lower CPU speed as well as less processing time
> allocated to them. HD video stutters often, and Chromium takes at
> least 2-3 times longer to fully load.

Your nice 19 tasks receiving 'too much' CPU when there are other
runnable tasks around sounds like you have SCHED_AUTOGROUP enabled.

With this enabled (or if tasks are placed in cgroups by another means),
group A and group B will each receive equal CPU if group shares/weight
are equal, regardless of group content.  Task nice level will affect CPU
distribution within a group, but group A containing a gaggle of nice 19
tasks will receive the same amount of CPU as group B containing your
nice 0 browser with zillion tabs.  Add nice 0 hogs to group A, the nice
19 tasks will receive much less CPU, but the total for group A will
remain unchanged relative to group B, unless group shares/weights are
twiddled.

(no idea what's going on with ondemand)

-Mike

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