linux-kernel - CFS group scheduler fairness broken starting from 2.6.29-rc1

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Message-ID: <20090723075735.GA18878@in.ibm.com>
Date:	Thu, 23 Jul 2009 13:27:35 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Ken Chen <kenchen@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: CFS group scheduler fairness broken starting from 2.6.29-rc1

Hi,

Group scheduler fainess is broken since 2.6.29-rc1. git bisect led me
to this commit:

commit ec4e0e2fe018992d980910db901637c814575914
Author: Ken Chen <kenchen@...gle.com>
Date:   Tue Nov 18 22:41:57 2008 -0800

    sched: fix inconsistency when redistribute per-cpu tg->cfs_rq shares
    
    Impact: make load-balancing more consistent
    
    In the update_shares() path leading to tg_shares_up(), the calculation of
    per-cpu cfs_rq shares is rather erratic even under moderate task wake up
    rate.  The problem is that the per-cpu tg->cfs_rq load weight used in the
    sd_rq_weight aggregation and actual redistribution of the cfs_rq->shares
    are collected at different time.  Under moderate system load, we've seen
    quite a bit of variation on the cfs_rq->shares and ultimately wildly
    affects sched_entity's load weight.
    
    This patch caches the result of initial per-cpu load weight when doing the
    sum calculation, and then pass it down to update_group_shares_cpu() for
    redistributing per-cpu cfs_rq shares.  This allows consistent total cfs_rq
    shares across all CPUs. It also simplifies the rounding and zero load
    weight check.
    
    Signed-off-by: Ken Chen <kenchen@...gle.com>
    Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>

======================================================================
			% CPU time division b/n groups
Group		2.6.29-rc1		2.6.29-rc1 w/o the above patch
======================================================================
a with 8 tasks	44			31
b with 5 tasks  32			34
c with 3 tasks  22			34
======================================================================
All groups had equal shares.

Regards,
Bharata.
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