linux-kernel - Re: RCU related performance regression in 3.3

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Date:	Thu, 5 Apr 2012 08:37:59 -0400
From:	Josh Boyer <jwboyer@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, kernel-team@...oraproject.org,
	pascal.chapperon@...adoo.fr
Subject: Re: RCU related performance regression in 3.3

On Wed, Apr 04, 2012 at 02:36:33PM -0700, Paul E. McKenney wrote:
> On Wed, Apr 04, 2012 at 11:27:27AM -0400, Josh Boyer wrote:
> > Hi Paul,
> > 
> > We've had a few reports of some boot slowdowns with the 3.3 rebase we
> > did in Fedora 16.  One of our users volunteered to bisect a vanilla
> > kernel and wound up at "rcu: Permit dyntick-idle with callbacks pending
> > (commit 7cb924990)".  You can find more details in the bug [1], but it's
> > been reported on both physical hardware and in virtual machines.
> > 
> > Have you seen anything like this in your testing?  Given the user used a
> > vanilla 3.3 kernel, I'm wondering if there is a targetted fix that might
> > be backported.  If so, it would be good to get that headed to the 3.3.y
> > stable tree.
> > 
> > josh
> > 
> > [1] https://bugzilla.redhat.com/show_bug.cgi?id=806548
> 
> I will look into this.  In the meantime, does setting
> CONFIG_RCU_FAST_NO_HZ=n get rid of the slowdowns?

Seems so.  Pascal tried 3.4-rc1 with and without that set and found that
disabling it lead to consistent timings.  He put his findings in the
bug, and I've also now CC'd him.

josh
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