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

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Message-ID: <50C564ED.9090803@linux.vnet.ibm.com>
Date:	Mon, 10 Dec 2012 09:58:29 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 12/10/2012 01:52 AM, Oleg Nesterov wrote:
> On 12/10, Srivatsa S. Bhat wrote:
>>
>> On 12/10/2012 12:44 AM, Oleg Nesterov wrote:
>>
>>> But yes, it is easy to blame somebody else's code ;) And I can't suggest
>>> something better at least right now. If I understand correctly, we can not
>>> use, say, synchronize_sched() in _cpu_down() path
>>
>> We can't sleep in that code.. so that's a no-go.
> 
> But we can?
> 
> Note that I meant _cpu_down(), not get_online_cpus_atomic() or take_cpu_down().
> 

Maybe I'm missing something, but how would it help if we did a
synchronize_sched() so early (in _cpu_down())? Another bunch of preempt_disable()
sections could start immediately after our call to synchronize_sched() no?
How would we deal with that?

What we need to ensure here is that all existing preempt_disable() sections
are over and that *we* (meaning, the cpu offline writer) get to proceed immediately
after that, making all the new readers wait for us. And that is possible only if
we do our 'wait-for-readers' thing very close to our actual cpu offline operation
(which is take_cpu_down). Moreover, the writer needs to remain stable
(non-preemptible) while doing the wait-for-readers. Else (if the writer himself
keeps getting scheduled in and out of the CPU) I can't see how he can possibly
do justice to the wait.

Also, synchronize_sched() only helps us do the 'wait-for-existing-readers' logic.
What would take care of the 'prevent-new-readers-from-going-ahead' logic?

To add to it all, synchronize_sched() waits for _all_ preempt_disable() sections
to complete, whereas only a handful of them might actually care about CPU hotplug.
Which is an unnecessary burden for the writer (ie., waiting for unrelated readers
to complete).

I bet you meant something else. Sorry if I misunderstood it.

Regards,
Srivatsa S. Bhat

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