linux-kernel - Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Mon, 10 Dec 2012 18:28:59 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context

On 12/10, Srivatsa S. Bhat wrote:
>
> On 12/10/2012 02:43 AM, Oleg Nesterov wrote:
> > Damn, sorry for noise. I missed this part...
> >
> > On 12/10, Srivatsa S. Bhat wrote:
> >>
> >> On 12/10/2012 12:44 AM, Oleg Nesterov wrote:
> >>> the latency. And I guess something like kick_all_cpus_sync() is "too heavy".
> >>
> >> I hadn't considered that. Thinking of it, I don't think it would help us..
> >> It won't get rid of the currently running preempt_disable() sections no?
> >
> > Sure. But (again, this is only my feeling so far) given that get_online_cpus_atomic()
> > does cli/sti,
>
> Ah, that one! Actually, the only reason I do that cli/sti is because, potentially
> interrupt handlers can be hotplug readers too. So we need to protect the portion
> of the code of get_online_cpus_atomic() which is not re-entrant.

Yes, I understand.

> > this can help to implement ensure-the-readers-must-see-the-pending-writer.
> > IOW this might help to implement sync-with-readers.
> >
>
> 2 problems:
>
> 1. It won't help with cases like this:
>
>    preempt_disable()
> 	...
> 	preempt_disable()
> 		...
> 			<------- Here
> 		...
> 	preempt_enable()
> 	...
>    preempt_enable()

No, I meant that kick_all_cpus_sync() can be used to synchronize with
cli/sti in get_online_cpus_atomic(), just like synchronize_sched() does
in the code I posted a minute ago.

> 2. Part of the reason we want to get rid of stop_machine() is to avoid the
> latency it induces on _all_ CPUs just to take *one* CPU offline. If we use
> kick_all_cpus_sync(), we get into that territory again : we unfairly interrupt
> every CPU, _even when_ that CPU's existing preempt_disabled() sections might
> not actually be hotplug readers! (ie., not bothered about CPU Hotplug).

I agree, that is why I said it is "too heavy".

Oleg.

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