linux-kernel - Re: [PATCH, RFC] v4 scalable classic RCU implementation

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Date:	Tue, 16 Sep 2008 19:48:00 +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
Subject: Re: [PATCH, RFC] v4 scalable classic RCU implementation

Paul E. McKenney wrote:
>   
>> That means an O(NR_CPUS) loop with disabled local interrupts :-(
>> Is that correct?
>>     
>
> With the definition of "O()" being the worst-case execution time, yes.
> But this worst case could only happen when the system was mostly idle,
> in which case the added overhead should not be too horribly bad.
No: "was mostly running cpu_idle()". A cpu_idle() cpu could execute lots 
of irqs and softirqs.
So the worst case would be a system with 1 cpu/node for reserved for irq 
handling.
The "idle" cpu would be always in no_hz mode, even though it might be 
100% busy handling irqs.
The remaning cpus might be 100% busy handling user space.

And every quiescent state will end up in that O(NR_CPUS) loop.

--
    Manfred
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