linux-kernel - Re: [PATCH] Avoid mask based num_possible_cpus and num_online

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Date:	Mon, 13 Feb 2012 11:54:26 -0800
From:	Tony Luck <tony.luck@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus -v5

On Thu, Feb 2, 2012 at 12:03 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> IIRC playing with 3 archs boot code seemed like a recipe for disaster.
> Feel free to try to fix this in -next though, and see what breaks...

ia64 is what breaks ... well not actually broken ... but some very
weird delays that
show up in different places depending on whether this patch is present.

First linux-next kernel to be blessed with this patch was
next-20120210. Booting it
I see:
[    7.164233] Switching to clocksource itc
[  146.077315] pnp: PnP ACPI init

An ugly 138.913 second delay.  Digging in the code showed that the bad bits
happened inside stop_machine()

Reverting just this patch makes this big delay disappear:

[   32.780232] Switching to clocksource itc
[   32.832100] pnp: PnP ACPI init

but notice that it takes 25 extra seconds to get to this point in the
boot (and while
we expect to save some time by not re-computing num_online_cpus each time we
need it ... this looks to be a lot more than I'd expect!)

-Tony
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