linux-kernel - Re: [patch 08/23] dynticks: prepare the RCU code

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Date:	Sat, 30 Sep 2006 15:09:58 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dipankar Sarma <dipankar@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Jim Gettys <jg@...top.org>,
	John Stultz <johnstul@...ibm.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [patch 08/23] dynticks: prepare the RCU code

* Dipankar Sarma <dipankar@...ibm.com> wrote:

> It is duplicating code. That can be easily fixed, but we need to 
> figure out what we really want from RCU when we are about to switch 
> off the ticks. It is hard if you want to finish off all the pending 
> RCUs and go to nohz state. Can you live with backing out if there are 
> pending RCUs ?

the thing is that when we go idle we /want/ to process whatever delayed 
work there might be - rate limited or not. Do you agree with that 
approach? I consider this a performance feature as well: this way we can 
utilize otherwise lost idle time. It is not a problem that we dont 
'batch' this processing: we are really idle and we've got free cycles to 
burn. We could even do an RCU processing loop that immediately breaks 
out if need_resched() gets set [by an IRQ or by another CPU].

secondly, i think i saw functionality problems when RCU was not 
completed before going idle - for example synchronize_rcu() on another 
CPU would hang.

what approach would you suggest to achieve these goals?

	Ingo
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