On 12/10/2012 10:58 PM, Oleg Nesterov wrote:

On 12/10, Srivatsa S. Bhat wrote:

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On 12/10/2012 02:43 AM, Oleg Nesterov wrote:

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Damn, sorry for noise. I missed this part...

On 12/10, Srivatsa S. Bhat wrote:

quoted

On 12/10/2012 12:44 AM, Oleg Nesterov wrote:

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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.

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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.

Ah, OK.

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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".

Got it :)

Regards,
Srivatsa S. Bhat