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

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Date:	Wed, 15 Oct 2008 10:13:44 +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:
>> - 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.
>>     
>
> I considered adding a cpu_quiet() on the irq exit path, but eventually
> decided that I should instead place the added overhead in the infrequently
> invoked force_quiescent_state() function.  Could be argued either way,
> of course.
>   
rcu_irq_exit() is only called on idle cpus.
You are trading time spent by the idle cpu in 'hlt' with "real" cpu time.

>> 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().
>>     
>
> Hmmm...  I don't see where the NMI exit path checks the TIF_NEED_RESCHED
> flag, but I could easily be missing something.
>   
Good point.
I haven't looked at the issue yet.
Perhaps a smd_send_reschedule(smp_processor_id()) is necessary.

Btw, I found a bug in my state machine: Right now, the state machine 
will lock up if all cpus are in nohz mode.
I'm not sure if it applies to your code as well.

--
    Manfred
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