On Fri, 20 Mar 2009 07:31:04 -0700
"Paul E. McKenney" [off-list ref] wrote:

quoted

that'd be throwing out the baby with the bathwater... I'm trying to
use the other cpus to do some of the boot work (so that the total
goes faster); not using the other cpus would be counter productive
to that. (As is just sitting in synchronize_rcu() when the other
cpu is working.. hence this discussion ;-)

OK, so you are definitely running multiple CPUs when the offending
synchronize_rcu() executes, then?

absolutely. 
(and I'm using bootgraph.pl in scripts to track who's stalling etc)

If so, here are some follow-on questions:

1.	How many synchronize_rcu() calls are you seeing on the
	critical boot path

I've seen only this (input) one to take a long time

 and what value of HZ are you running?

1000

	If each synchronize_rcu() is taking (say) tens of jiffies,
then, as Peter Zijlstra notes earlier in this thread, we need to focus
	on what is taking too long to get through its RCU read-side
	critical sections

I know that "the other guy" is not optimal and takes waaay too long.

 Otherwise, if each synchronize_rcu() is
	in the 3-5 jiffy range, I may finally be forced to create an
	expedited version of the synchronize_rcu() API.

I think a simplified API for the "add to a list" case might make sense.
Because the request isn't for a full sync for sure...

(independent of that .. the open question is if this specific case is
even needed; I think the code confused "send to others" with "wait
until everyone sees"; afaik synchronize_rcu() has no pushing behavior
at all, nor should it)

2.	If expediting is required, then the code calling
synchronize_rcu() might or might not have any idea whether or not
expediting is appropriate.  If it does not, then we would need some
sort of way to tell synchronize_rcu() that it should act more
aggressively, perhaps /proc flag or kernel global variable indicating
that boot is in progress.

	No, we do not want to make synchronize_rcu() aggressive all
the time, as this would harm performance and energy efficiency in
	the normal runtime situation.

	So, if it turns out that synchronize_rcu()'s caller does not
	know whether or not expediting is appropriate, can the boot
path manipulate such a flag or variable?

3.	Which RCU implementation are you using?  CONFIG_CLASSIC_RCU,
	CONFIG_TREE_RCU, or CONFIG_PREEMPT_RCU?

CLASSIC

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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