linux-kernel - Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle tasks

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Message-ID: <20140813141217.GU4752@linux.vnet.ibm.com>
Date:	Wed, 13 Aug 2014 07:12:17 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle
 tasks

On Wed, Aug 13, 2014 at 03:40:25PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 13, 2014 at 05:48:18AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 13, 2014 at 10:12:15AM +0200, Peter Zijlstra wrote:
> > > On Mon, Aug 11, 2014 at 03:49:04PM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > 
> > > > Because idle-task code may need to be patched, RCU-tasks need to wait
> > > > for idle tasks to schedule.  This commit therefore detects this case
> > > > via context switch.  Block CPU hotplug during this time to avoid sending
> > > > IPIs to offline CPUs.
> > > > 
> > > > Note that checking for changes in the dyntick-idle counters is tempting,
> > > > but wrong.  The reason that it is wrong is that a interrupt or NMI can
> > > > increment these counters without necessarily allowing the idle tasks to
> > > > make any forward progress.
> > > 
> > > I'm going to NAK this.. with that rcu_idle patch I send there's
> > > typically only a single idle function thats out of bounds and if its
> > > more it can be made that with a bit of tlc to the cpuidle driver in
> > > question.
> > > 
> > > This needs _FAR_ more justification than a maybe and a want.
> > 
> > Peter, your patch might be a good start, but I didn't see any reaction
> > from Steven or Masami and it did only x86.
> 
> That's not an excuse for doing horrible things. And inventing new infra
> that needs to wake all CPUs is horrible.

Does your patch even work?  Looks like it should, and yes, the idle loop
seems quite a bit simpler than it was a few years ago, but we really
don't need some strange thing that leaves a CPU idle but not visible as
such to RCU.

I have already said that I will be happy to rip out the wakeup code
when it is no longer needed, and I agree that it would be way better if
not needed.  But I won't base a patch on hypotheticals.  You have already
drawn way too much water from -that- well over the past years!  ;-)

							Thanx, Paul

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