netdev - Re: [PATCH] lglock: add read-preference local-global rwlock

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Message-ID: <51361540.3060603@cn.fujitsu.com>
Date:	Tue, 05 Mar 2013 23:54:40 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Michel Lespinasse <walken@...gle.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Lai Jiangshan <eag0628@...il.com>, linux-doc@...r.kernel.org,
	peterz@...radead.org, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, namhyung@...nel.org,
	mingo@...nel.org, linux-arch@...r.kernel.org,
	linux@....linux.org.uk, xiaoguangrong@...ux.vnet.ibm.com,
	wangyun@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	rusty@...tcorp.com.au, rostedt@...dmis.org, rjw@...k.pl,
	vincent.guittot@...aro.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	sbw@....edu, tj@...nel.org, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] lglock: add read-preference local-global rwlock

On 03/03/13 01:06, Oleg Nesterov wrote:
> On 03/02, Michel Lespinasse wrote:
>>
>> My version would be slower if it needs to take the
>> slow path in a reentrant way, but I'm not sure it matters either :)
> 
> I'd say, this doesn't matter at all, simply because this can only happen
> if we race with the active writer.
> 

It can also happen when interrupted. (still very rarely)

arch_spin_trylock()
	------->interrupted,
		__this_cpu_read() returns 0.
		arch_spin_trylock() fails
		slowpath, any nested will be slowpath too.
		...
		..._read_unlock()
	<-------interrupt
__this_cpu_inc()
....


I saw get_online_cpu_atomic() is called very frequent.
And the above thing happens in one CPU rarely, but how often it
happens in the whole system if we have 4096 CPUs?
(I worries to much. I tend to remove FALLBACK_BASE now, we should
add it only after we proved we needed it, this part is not proved)

Thanks,
Lai


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