Message-ID: <20120224165130.GB2399@linux.vnet.ibm.com>
Date:	Fri, 24 Feb 2012 08:51:30 -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,
	rostedt@...dmis.org
Subject: Re: Large slowdown with 'x86: Avoid invoking RCU when CPU is idle'

On Fri, Feb 24, 2012 at 11:27:46AM -0500, Josh Boyer wrote:
> On Fri, Feb 24, 2012 at 08:17:43AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 24, 2012 at 09:40:32AM -0500, Josh Boyer wrote:
> > > On Tue, Feb 21, 2012 at 08:42:43PM -0500, Josh Boyer wrote:
> > > > > And patch #47 in that series has been obsoleted by another series
> > > > > from Steven Rostedt:
> > > > > 
> > > > > 	https://lkml.org/lkml/2012/2/7/231
> > > > 
> > > > Ok.
> > > > 
> > > > > Hopefully these fix both splats and slowness.
> > > > 
> > > > So again, I'm slightly confused on how RCU patches flow.  Eric
> > > > originally reported the bug for which you created the patch I applied
> > > > against 3.3.  The giant patch series above seems queued for 3.4.
> > > > 
> > > > I don't see stable CC'd on 45-47, nor any of Steven's patches.  I doubt
> > > > I'd want to go applying the 47-patch series on 3.3 at the moment, and
> > > > given you have these marked for 3.4 I don't think you do either.
> > > > However, is there some kind of fix for the original bug report against
> > > > 3.3?
> > > 
> > > I was being sincere when I asked the above questions.  Could you
> > > describe how you handle RCU patches across releases and if there is a
> > > fix for the 3.3-rcX issue Eric reported that is going into 3.3?
> > > 
> > > I know you're quite busy, but I'd like to understand your thinking so I
> > > know what to expect going forward.
> > 
> > Apologies for being slow, but could you please point me at the original
> > bug report that the old patch was designed to fix?  My email filing
> > seems to have failed me in this case.
> 
> Same thread I linked in my original email:
> 
> https://lkml.org/lkml/2012/1/24/203

Thank you!

> > My guess is that the best short-term fix for Fedora is to disable the
> > warning, but I do need to see the original bug to work out if that really
> > is a prudent course of action.
> 
> Honestly, I don't care from a Fedora perspective.  I can do what I need
> to do there without too much trouble.  I'm asking because afaik, upstream
> still has this problem.  The thread gets a bit curvy but from what I can
> tell it resulted in the patch I highlighted as having issues.  Maybe I
> overlooked something else that fixed Eric's problem?

"A bit curvy" is right -- which is why the fixes ended up at the end of
my large patch series for 3.4.

But after looking this over, Steven Rostedt's three-patch set should
suffice:

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

The reason that mine are not needed is that the problematic code is
called -only- from idle, not from process context, and also that the
problematic code is tracing.  My patch #45 is required for code that is
called from both process context and from idle.  My patch #46 is required
for non-tracing uses of RCU from within the idle loop -- along with TBD
patches to wrap those uses of RCU in the RCU_NONIDLE() macro.

So again, in your particular case of x86's power-tracing features,
Steven Rostedt's three-patch series called out above should be all
that you need.

I have CCed Steven in case there is some prerequisite to his patch set.

							Thanx, Paul

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