linux-kernel - Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter

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Message-ID: <20100412181655.GA11872@redhat.com>
Date:	Mon, 12 Apr 2010 20:16:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nathan Fontenot <nfont@...tin.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sachin Sant <sachinp@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Shane Wang <shane.wang@...el.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by
	using percpu counter

On 04/12, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> >
> > But, I must admit, I'd like to avoid adding the new member to task_struct.
> >
> > What do you think about the code below?
> >
> > I didn't even try to compile it, just to explain what I mean.
> >
> > In short: we have the per-cpu fast counters, plus the slow counter
> > which is only used when cpu_hotplug_begin() is in progress.
>
> get_online_cpus() in your code is still read-preference.

Yes,

> I wish we quit this ability of get_online_cpus().

Heh. Since I never read the changelogs, I didn't even notice this was
one of the goals of your patch. I thought this is just the side-effect.

Yes, if we want to block the new readers, I don't see how it is possible
to avoid the counter in task_struct.

I can't judge whether this new member worth the trouble. Once again, I am
not arguing, just I don't know. And I think your patch is correct (apart
from pure theoretical race with cpu_hotplug_done afaics).

Oleg.

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