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

[Joe Korty <joe.korty@ccur.com>]

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.

Regards,
Joe