Re: [GIT PULL rcu/next] rcu commits for 2.6.40

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 16, 2011 at 09:08:08AM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > > Would it have been possible to split it in two, one for the movement of the 
> > > notifiers, the other for the barrier changes?
> > > 
> > > That way the bisection would have fingered the movement commit. Or so.
> > 
> > In hindsight, that certainly would have been better.
> 
> This is the Linux kernel and we *can* turn back the clock!

Yay for source-code control systems in general and git in particular!  ;-)

> > I was afraid of that...
> > 
> > On the off-chance that moving the memory barriers was at fault, the following 
> > patch restores all of them that don't have in situ replacements.  Grasping at 
> > straws, admittedly.
> 
> Well, the nice thing is that we really do not have to grasp at straws, and even 
> while we have no good ideas we can debug this *much* better.
> 
> Could you please do a simple test-tree that does has 3 commits:
> 
>  first one reverts the offending commit
>  second one applies the barrier part of it
>  this one applies the need_resched part of it
> 
> ( You can do even more finegrained steps, if you find harmless-looking bits of 
>   it that can be applied separately! )
> 
> Note, the important thing is that the tree should be a 'null pull' - i.e. the 
> revert plus the patches applied will not change anything in core/rcu.
> 
> Obviously it would be nice if each step built fine - no need to boot test each 
> step as long as you are reasonably sure it will boot fine.
> 
> Then i could take my reproducer and come up with a very precise bisection 
> result for you, with just a couple of minutes time spent on testing. One of the 
> commits after the revert will trigger the hang/slowdown.
> 
> My prediction is that we will be much wiser after that! :-)

I will put this together!

In the meantime, would you be willing to try out the patch at
https://lkml.org/lkml/2011/5/14/89?  This patch helped out Yinghai
in several configurations.

						Thanx, Paul