Messages in this thread | | | Date | Sat, 14 Jun 2014 04:26:39 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used |
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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
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