linux-kernel - Re: [BUG] sched: fair group's bug

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Date:	Fri, 09 Jan 2009 10:45:06 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] sched: fair group's bug

on 2009-1-8 9:30 Miao Xie wrote:
> I tested fair group scheduler on my hyper-threading x86_64 box(2 CPU * 2 HT)
> and found the deviation of the groups' CPU usage was larger than 2.6.26
> when *offline* a CPU or do hotplug frequently. It is less than 1% On 2.6.26，but
> On current kernel, it is often greater than 4%, even than 10% by accident.
> 

This patch fixed the following problems. But the regression above still exists.

commit 0a582440ff546e2c6610d1acec325e91b4efd313
Author: Mike Galbraith <efault@....de>
Date:   Fri Jan 2 12:16:42 2009 +0100

    sched: fix sched_slice()

    Impact: fix bad-interactivity buglet

    Fix sched_slice() to emit a sane result whether a task is currently
    enqueued or not.

    Signed-off-by: Mike Galbraith <efault@....de>
    Tested-by: Jayson King <dev@...sonking.com>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>

> 
> Besides that, We found other problems by the attached program.
> 1. some tasks are hungry in the fair group.
> 	Steps to reproduce:
> 	# mkdir /dev/cpuctl
> 	# mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
> 	# ./cpuctl -g 1 -v
> 	--------------------
> 	1th Check Result:
> 	Group	Shares	Actual(%)	Expect(%)
> 	0	1024	100.00		100.00
> 	Each task's usage:
> 		Task in Group 0:
> 			Task	Usage(%)
> 			5395	0.000000
> 			5396	0.000000
> 		 	5397	0.000000
> 			5398	16.677785
> 			5399	16.677785
> 			5400	16.744496
> 			5401	16.611074
> 			5402	33.288859
> 
> 2. some groups broke the limit of the fair group and get more CPU time When
>    the groups is hiberarchy. Such as:
> 		top group
> 		    |
> 		 group 1
> 		/	\
> 	      task1	group 2
> 			   |
> 			task 2
> 	Steps to reproduce:
> 	# mkdir /dev/cpuctl
> 	# mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
> 	# ./cpuctl -H
> 	-------------------------
> 	Group	Shares	Actual(%)	Expect(%)
> 	0	1024	60.17		88.89
> 	1	1024    39.83		11.11
> 


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