Re: [PATCH] An RCU for SMP with a single CPU garbage collector

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Mar 08, 2011 at 05:53:55PM -0500, Joe Korty wrote:
> On Tue, Mar 08, 2011 at 10:57:10AM -0500, Joe Korty wrote:
> > On Tue, Mar 08, 2011 at 04:07:42AM -0500, Paul E. McKenney wrote:
> >>> Thinking about it some more, the tap-into-syscall approach might
> >>> work in my implementation, in which case the tap-into-preempt-enable
> >>> code could go away.
> > > 
> >> OK, please let me know how that goes!
> >> 
> >>> Nice thing about RCU, the algorithms are infinitely mallable :)
> >> 
> >> Just trying to keep the code size finite.  ;-)
> > 
> > I hope to get to it this afternoon!  I especially like
> > the lockless nature of JRCU, and that the dedicated cpus
> > are not loaded down with callback inovcations either.
> > Not sure how to support the PREEMPT_RCU mode though; so
> > if Fredrick is planning to support that, that alone would
> > make his approach the very best.
> 
> 
> 
> Hi Paul,
> I had a brainstorm. It _seems_ that JRCU might work fine if
> all I did was remove the expensive preempt_enable() tap.
> No new taps on system calls or anywhere else.  That would
> leave only the context switch tap plus the batch start/end
> sampling that is remotely performed on each cpu by the
> garbage collector.  Not even rcu_read_unlock has a tap --
> it is just a plain-jane preempt_enable() now.
> 
> And indeed it works!  I am able to turn off the local
> timer interrupt on one (of 15) cpus and the batches
> keep flowing on.  I have two user 100%  use test apps
> (one of them does no system calls), when I run that
> on the timer-disabled cpu the batches still advance.
> Admittedly the batches do not advance as fast as before
> .. they used to advance at the max rate of 50 msecs/batch.
> Now I regularly see batch lengths approaching 400 msecs.
> 
> I plan to put some taps into some other low overhead places
> -- at all the voluntary preemption points, at might_sleep,
> at rcu_read_unlock, for safety purposes.  But it is nice
> to see a zero overhead approach that works fine without
> any of that.

If you had a user-level process that never did system calls and never
entered the scheduler, what do you do to force forward progress of the RCU
grace periods?  (This is force_quiescent_state()'s job in TREE_RCU, FYI.)

							Thanx, Paul