Re: [PATCH] a local-timer-free version of RCU

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Nov 15, 2010 at 05:28:46PM -0800, Paul E. McKenney wrote:
> My concern is not the tick -- it is really easy to work around lack of a
> tick from an RCU viewpoint.  In fact, this happens automatically given the
> current implementations!  If there is a callback anywhere in the system,
> then RCU will prevent the corresponding CPU from entering dyntick-idle
> mode, and that CPU's clock will drive the rest of RCU as needed via
> force_quiescent_state().



Now, I'm confused, I thought a CPU entering idle nohz had nothing to do
if it has no local callbacks, and rcu_enter_nohz already deals with
everything.

There is certainly tons of subtle things in RCU anyway :)



> The force_quiescent_state() workings would
> want to be slightly different for dyntick-hpc, but not significantly so
> (especially once I get TREE_RCU moved to kthreads).
> 
> My concern is rather all the implicit RCU-sched read-side critical
> sections, particularly those that arch-specific code is creating.
> And it recently occurred to me that there are necessarily more implicit
> irq/preempt disables than there are exception entries.



Doh! You're right, I don't know why I thought that adaptive tick would
solve the implicit rcu sched/bh cases, my vision took a shortcut.



> So would you be OK with telling RCU about kernel entries/exits, but
> simply not enabling the tick?



Let's try that.



> The irq and NMI kernel entries/exits are
> already covered, of course.


Yep.


> This seems to me to work out as follows:
> 
> 1.	If there are no RCU callbacks anywhere in the system, RCU
> 	is quiescent and does not cause any IPIs or interrupts of
> 	any kind.  For HPC workloads, this should be the common case.


Right.



> 2.	If there is an RCU callback, then one CPU keeps a tick going
> 	and drives RCU core processing on all CPUs.  (This probably
> 	works with RCU as is, but somewhat painfully.)  This results
> 	in some IPIs, but only to those CPUs that remain running in
> 	the kernel for extended time periods.  Appropriate adjustment
> 	of RCU_JIFFIES_TILL_FORCE_QS, possibly promoted to be a
> 	kernel configuration parameter, should make such IPIs
> 	-extremely- rare.  After all, how many kernel code paths
> 	are going to consume (say) 10 jiffies of CPU time?  (Keep
> 	in mind that if the system call blocks, the CPU will enter
> 	dyntick-idle mode, and RCU will still recognize it as an
> 	innocent bystander without needing to IPI it.)



Makes all sense. Also there may be periods when these "isolated" CPUs
will restart the tick, like when there is more than one task running
on that CPU, in which case we can of course fall back to usual
grace periods processing.



> 
> 3.	The implicit RCU-sched read-side critical sections just work
> 	as they do today.
> 
> Or am I missing some other problems with this approach?


No, looks good, now I'm going to implement/test a draft of these ideas.

Thanks a lot!