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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jun 13, 2014 at 03:49:26PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 13, 2014 at 02:10:35PM -0700, Josh Triplett wrote:
> > On Fri, Jun 13, 2014 at 01:48:22PM -0700, Paul E. McKenney wrote:
> > > On Fri, Jun 13, 2014 at 09:44:41AM -0700, Josh Triplett wrote:
> > > > On Fri, Jun 13, 2014 at 06:21:32PM +0200, Frederic Weisbecker wrote:
> > > > > On Fri, Jun 13, 2014 at 09:16:30AM -0700, Paul E. McKenney wrote:
> > > > > > > Is it because we have dynticks CPUs staying too long in the kernel without
> > > > > > > taking any quiescent states? Are we perhaps missing some rcu_user_enter() or
> > > > > > > things?
> > > > > > 
> > > > > > Sort of the former, but combined with the fact that in-kernel CPUs still
> > > > > > need scheduling-clock interrupts for RCU to make progress.  I could
> > > > > > move this to RCU's context-switch hook, but that could be very bad for
> > > > > > workloads that do lots of context switching.
> > > > > 
> > > > > Or I can restart the tick if the CPU stays in the kernel for too long without
> > > > > a tick. I think that's what we were doing before but we removed that because
> > > > > we never implemented it correctly (we sent scheduler IPI that did nothing...)
> > > > 
> > > > I wonder if timer slack would make sense here: when you have at least
> > > > one RCU callback pending, set a timer with a huge amount of timer slack,
> > > > and cancel it if you end up handling the callback via a trip through the
> > > > scheduler.
> > > 
> > > But in this case, we need the tick even if the current CPU has no callbacks
> > > because it might be in an RCU read-side critical section.
> > 
> > Don't we handle that case via the slowpath of rcu_read_unlock, and a
> > flag set via IPI?  ("Oh, that CPU has taken too long to note a quiescent
> > state; send it an IPI to set the special flag that makes unlock do the
> > work.")
> 
> There was once such logic on the force-quiescent-state path, and making
> that handle this new case was my first proposal.  As Frederic pointed
> out, that change requires rcu_needs_cpu()'s cooperation, because otherwise
> the CPU will take the IPI, see that it still has but one runnable task,
> and then keep its scheduling-clock interrupt off.

Exactly. So that's what happens currently, we call rcu_kick_nohz_cpu()
on extended grace periods but the IPI doesn't reconsider the tick.

In fact it doesn't do anything at all because the scheduler IPI,
when invoked without a reason, doesn't even call irq_enter()/irq_exit(),
so rcu_needs_cpu() isn't quite called from there.

Now that's going to change with https://lwn.net/Articles/601836/ if
we convert rcu_kick_nohz_cpu() to tick_nohz_full_kick_cpu().

Then we have the choice between two options:

* We can add a check in tick_nohz_full_check() and restart the tick if
necessary.

* Extend rcu_needs_cpu() to restore a similar periodic mode until the
grace periods get some progress.