Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used

Paul E. McKenney
2014-06-14T11:26:47Z

On Fri, Jun 13, 2014 at 10:06:06PM -0700, Paul E. McKenney wrote:
> On Sat, Jun 14, 2014 at 01:39:36AM +0200, Frederic Weisbecker wrote:
> > On Fri, Jun 13, 2014 at 04:27:15PM -0700, Paul E. McKenney wrote:

[ . . . ]

> > > If I was to extend rcu_needs_cpu(), I would add a flag and another counter
> > > to the rcu_data structure. If rcu_needs_cpu() saw the flag set and the
> > > counter equal to the current ->completed value, it would return true.
> > >
> > > I already have the rcu_kick_nohz_cpu() in rcu_implicit_dynticks_qs(),
> > > so it is just a matter of also setting the flag and copying ->completed
> > > to the new counter at that point. I currently get to this point if the
> > > CPU has managed to run for more than one jiffy without hitting either
> > > idle or userspace execution. Fair enough?
> >
> > Perfect for me!
>
> One complication... So if the grace period has gone on for a long time,
> and you are returning to kernel mode, RCU will need the scheduling-clock
> tick. However, in that very same situation, if you are returning to
> idle or to NO_HZ_FULL userspace execution, RCU does -not- need the
> scheduling-clock tick set.
>
> One way I could do this is to have rcu_needs_cpu() return three values:
> Zero for RCU doesn't need a scheduling-clock tick for any reason,
> one if RCU needs a scheduling-clock tick only if returning to kernel
> mode, and two if RCU unconditionally needs the scheduling-clock tick.
> Would that work, or is there a better approach?

You know, it just feels like RCU -should- be able to solve this
internally. So if determining that you are returning to kernel mode is
at all inconvenient, give me a couple days to think this through.

Thanx, Paul