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

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxx>
> > > 
> > > 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.

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