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

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Date:	Wed, 4 Apr 2012 14:36:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Boyer <jwboyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kernel-team@...oraproject.org
Subject: Re: RCU related performance regression in 3.3

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?

							Thanx, Paul

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