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Date:	Wed, 06 Aug 2008 07:30:13 +0200
From:	Manfred Spraul <>
Subject: Re: [PATCH tip/core/rcu] classic RCU locking and memory-barrier cleanups

Hi Paul,

Paul E. McKenney wrote:
> This patch is in preparation for moving to a hierarchical
> algorithm to allow the very large SMP machines -- requested by some
> people at OLS, and there seem to have been a few recent patches in the
> 4096-CPU direction as well.

I thought about hierarchical RCU, but I never found the time to 
implement it.
Do you have a concept in mind?

Right now, I try to understand the current code first - and some of it 
doesn't make much sense.

There are three per-cpu lists:

Obviously, there must be a quiescent state between cur and done.
But why does the code require a quiescent state between nxt and cur?
I think that's superflous. The only thing that is required is that all 
cpus have moved their callbacks from nxt to cur. That doesn't need a 
quiescent state, this operation could be done in hard interrupt as well.

Thus I think this should work:

1) A callback is inserted into ->nxt.
2) As soon as too many objects are sitting in the ->nxt lists, a new rcu 
cycle is started.
3) As soon as a cpu sees that a new rcu cycle is started, it moves it's 
callbacks from ->nxt to ->cur. No checks for hard_irq_count & friends 
necessary. Especially: same rule for _bh and normal.
4) As soon as all cpus have moved their lists from ->nxt to ->cur, the 
real grace period is started.
5) As soon as all cpus passed a quiescent state (i.e.: now with tests 
for hard_irq_count, different rules for _bh and normal), the list is 
moved from ->cur to ->completed. Once in completed, they can be 
destroyed by performing the callbacks.

What do you think? would that work? It doesn't make much sense that step 
3) tests for a quiescent state.

Step 2) could depend memory pressure.
Step 3) and 4) could be accelerated by force_quiescent_state(), if the 
memory pressure is too high.

-> nxt

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