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Message-ID: <48A93D49.2000601@colorfullife.com>
Date: Mon, 18 Aug 2008 11:13:45 +0200
From: Manfred Spraul <manfred@...orfullife.com>
To: paulmck@...ux.vnet.ibm.com
CC: linux-kernel@...r.kernel.org, mingo@...e.hu,
akpm@...ux-foundation.org, oleg@...sign.ru, dipankar@...ibm.com,
rostedt@...dmis.org, dvhltc@...ibm.com, niv@...ibm.com
Subject: Re: [PATCH tip/core/rcu] classic RCU locking and memory-barrier cleanups
Paul E. McKenney wrote:
>
>> 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:
>> ->nxt
>> ->cur
>> ->done.
>>
>> 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.
>>
>
> The deal is that we have to put incoming callbacks somewhere while
> the batch in ->cur waits for an RCU grace period. That somewhere is
> ->nxt. So to be painfully pedantic, the callbacks in ->nxt are not
> waiting for an RCU grace period. Instead, they are waiting for the
> callbacks in ->cur to get out of the way.
>
>
Ok, thanks.
If I understand the new code in tip/rcu correctly, you have rewritten
that block anyway.
I'll try to implement my proposal - on paper, it looks far simpler than
the current code.
On the one hand, a state machine that keeps track of a global state:
- collect the callbacks in a nxt list.
- wait for quiecent
- destroy the callbacks in the nxt list.
(actually, there will be 5 states, 2 additional for "start the next rcu
cycle immediately")
On the other hand a cpu bitmap that keeps track of the cpus that have
completed the work that must be done after a state change.
The last cpu advances the global state.
The state machine could be seq_lock protected, the cpu bitmap could be
either hierarchical or flat or for uniprocessor just a nop.
Do you have any statistics about rcu_check_callbacks? On my single-cpu
system, around 2/3 of the calls are from "normal" context, i.e.
rcu_qsctr_inc() is called.
--
Manfred
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