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

Peter Zijlstra
2011-08-30T20:59:25Z

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.