linux-kernel - Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs

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Date:	Tue, 30 Aug 2011 22:58:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to
 idle enter/exit APIs

On Tue, 2011-08-30 at 17:42 +0200, Peter Zijlstra wrote:
> On Tue, 2011-08-30 at 17:33 +0200, Frederic Weisbecker wrote:
> > > See all that is still kernelspace ;-) I think I know what you mean to
> > > say though, but seeing as you note there is even now a known shortcoming
> > > I'm not very confident its a solid construction. What will help us find
> > > such holes?
> > 
> > This: https://lkml.org/lkml/2011/6/23/744
> > 
> > It's in one of Paul's branches and should make it for the next merge window.
> > This should detect any of such holes. I made that on purpose for the nohz cpusets
> > when I saw how much error prone that can be with rcu :)
> 
> OK, good ;-)
> 
> > > I would much rather we not rely on such fragile things too much.. this
> > > RCU stuff wants way more thought, as it stands your patch-set doesn't do
> > > anything useful IMO.
> > 
> > Not sure what you mean. Well that Rcu thing for sure is fragile but we have
> > the tools ready to find the problems. 
> 
> Right that thing you linked above does catch abuse, still your current
> proposal means that due to RCU it will basically never disable the tick.

So how about something like:

Assuming we are in rcu_nohz state; on kernel enter we leave rcu_nohz but
don't start the tick, instead we assign another cpu to run our state
machine.

On kernel exit we 'donate' all our rcu state to a willing victim (the
same that earlier was kind enough to drive our state) and undo our
entire GP accounting and re-enter rcu_nohz state.

If between that time we did restart the tick, we take back our rcu state
and skip the donate and rcu_nohz enter on kernel exit.

I really should go read all those docs Paul send me to see how insane
the above is.
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