linux-kernel - Re: [nohz] 2a16fc93d2c: kernel lockup on idle injection

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Message-ID: <20141217124724.GB20739@lerouge>
Date:	Wed, 17 Dec 2014 13:47:30 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	"Pan, Jacob jun" <jacob.jun.pan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [nohz] 2a16fc93d2c: kernel lockup on idle injection

On Wed, Dec 17, 2014 at 10:11:58AM +0100, Thomas Gleixner wrote:
> On Wed, 17 Dec 2014, Frederic Weisbecker wrote:
> > On Tue, Dec 16, 2014 at 10:21:27PM +0100, Thomas Gleixner wrote:
> > > So instead of evaluating the whole nonsense a gazillion times in a row
> > > and firing pointless self ipis why are you not looking at the obvious
> > > solution of sane state change tracking?
> > 
> > Because I had to do it wrong first before somebody like you with a fresh mind
> > comes, look at the whole picture and propose a rethink :-)
> 
> Point taken.
>  
> > >     I told all of you from the very beginning that remote accounting
> > >     is a key mechanism for this, but you keep insisting on hacking
> > >     around it.
> > 
> > I don't, and I think everybody has understood we are not going to accept
> > hacks to solve the 1hz issue.
> 
> You wish.
> 
> > I'm pretty sure you know what this is about to maintain a project with a
> > (variably) cruel lack of manpower ;-)
> 
> Oh yes! :(
>  
> > Moreover I'm not the kind of kernel developer who ignores reviews (whether pre or
> > post merge).
> 
> I know that.
>  
> > So, no point in threatening a dependency to BROKEN or ripping out except perhaps to
> > push me out to other projects with more thankful managers.
> 
> This was not directed to you, really.
> 
> This was directed at the folks who randomly "fix" that code by voodoo
> bandaids instead of analyzing the root causes in the first
> place. You're the least one to blame for that.

Well I was about to propose a bad fix for powerclamp and I sometimes take
overengineering directions. I'm still to blame for several things 8-)

Sorry I got too touchy :-)

Anyway, I'll give a try to your proposition. Somehow I have always felt that
something was wrong with this tick_nohz_can_stop_tick() called all over the place,
that plus the IPI that's often near the scheduler IPI.

I'm pretty sure this is going to help fixing some overhead issues I got reported.

Thanks a lot!
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