linux-kernel - Re: [PATCH, RFC] v7 scalable classic RCU implementation

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Message-ID: <48F38D73.8040804@colorfullife.com>
Date:	Mon, 13 Oct 2008 20:03:31 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org, penberg@...helsinki.fi,
	andi@...stfloor.org, tglx@...utronix.de
Subject: Re: [PATCH, RFC] v7 scalable classic RCU implementation

Paul E. McKenney wrote:
> Do you send a reschedule IPI to CPUs that are not in dyntick idle mode,
> but who have failed to pass through a quiescent state?
>   
Actually, I never send reschedule IPIs.

- For "usual" cpus, rcu_check_callbacks() checks for quiescent states.
There should be a set_need_resched() if a cpu holds up a grace period 
for too long. I just haven't implemented it yet.
IMHO it doesn't make sense to perform a "for_each_cpu() 
smd_send_reschedule()". rcu has a hook in each cpu, thus a 
set_need_resched() by the per-cpu hook is faster/simpler.

- For nohz cpus, a poller function [schedule_work(), enabled interrupts] 
peeks into the per-cpu data of the nohz cpu and checks if it is quiet or 
if it passed through a quiescent state.
If it didn't, then it sets a cpu_data->kick_poller flag and 
rcu_irq_exit() reports the grace period.
No need for an IPI either - rcu has a hook in the irq exit path.

Right now, I cheat if a nohz cpu is in a long-running nmi 
[while(other_cpu_is_in_nmi()) cpu_relax()], but I think I can fix that 
with an set_need_resched() in the rcu_nmi_exit().

--
    Manfred
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