linux-kernel - Re: Large slowdown with 'x86: Avoid invoking RCU when CPU is idle'

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Message-ID: <20120222013252.GZ2375@linux.vnet.ibm.com>
Date:	Tue, 21 Feb 2012 17:32:52 -0800
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: Large slowdown with 'x86: Avoid invoking RCU when CPU is idle'

On Tue, Feb 21, 2012 at 08:16:53PM -0500, Josh Boyer wrote:
> Hi Paul,
> 
> Over in Fedora land, I applied your patch from this thread:
> 
> https://lkml.org/lkml/2012/1/24/441
> 
> to our 3.3-rc3/rc4 based rawhide kernels.  The intention was to solve an
> RCU issue that was very similar to what Eric originally reported, and
> the RCU splat did indeed go away[1].
> 
> However, we then got a few reports of kernels containing that patch
> being extremely slow.  When the patch was dropped, the slowness goes
> away according to one reporter.  The details can be found in this bug:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=795050
> 
> The slowness doesn't seem to hit everyone, and in my local testing
> things seem to be working just fine.  The reporters have widely varying
> hardware as well, so it doesn't seem machine specific.
> 
> Perhaps I misdiagnosed the original issue, or perhaps I missed something
> else that needs to be applied prior to this but I thought I would point
> this out in case you had any ideas.

This patch has been obsoleted by patches #45-47 in this series:

	https://lkml.org/lkml/2012/2/3/459

And patch #47 in that series has been obsoleted by another series
from Steven Rostedt:

	https://lkml.org/lkml/2012/2/7/231

Hopefully these fix both splats and slowness.

							Thanx, Paul

> josh
> 
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=789641
> 

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