linux-kernel - Re: Unusually high system CPU usage with recent kernels

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Date:	Tue, 1 Oct 2013 13:27:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tibor Billes <tbilles@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Unusually high system CPU usage with recent kernels

On Sat, Sep 14, 2013 at 03:59:51PM +0200, Tibor Billes wrote:
> > From: Paul E. McKenney Sent: 09/13/13 02:19 AM
> > On Wed, Sep 11, 2013 at 08:46:04AM +0200, Tibor Billes wrote:
> > > > From: Paul E. McKenney Sent: 09/09/13 10:44 PM

[ . . . ]

> > > Sure. The attached tar file contains traces of good kernels. The first is with
> > > version 3.9.7 (no patch applied) which was the last stable kernel I tried and
> > > didn't have this issue. The second is version 3.11 with your fix applied.
> > > Judging by the size of the traces, 3.11.0+ is still doing more work than
> > > 3.9.7.
> > 
> > Indeed, though quite a bit less than the problematic traces.
> > 
> > Did you have all three patches applied to 3.11.0, or just the last one?
> > If the latter, could you please try it with all three?
> 
> Only the last one was applied to 3.11.0. The attachement now contains the
> RCU trace with all thee applied. It seems to be smaller in size, but still
> not close to 3.9.7.
> 
> > > > > I'm not sure about LKML policies about attaching not-so-small files to
> > > > > emails, so I've dropped LKML from the CC list. Please CC the mailing
> > > > > list in your reply.
> > > > 
> > > > Done!
> > > > 
> > > > Another approach is to post the traces on the web and send the URL to
> > > > LKML. But whatever works for you is fine by me.
> > > 
> > > Sending directly to you won again :) Could you please CC the list in your
> > > reply?
> > 
> > Done! ;-)
> 
> Could you please CC the list in your reply again? :)

This trace is quite strange -- there are a number of processes that sleep
and then wake up almost immediately, as in within a microsecond or so.
I don't know why they would be doing that, and I also cannot think of
what RCU might be doing to make them do that.

I am unfortunately out of ideas on this one.

							Thanx, Paul

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