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

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Message-ID: <4F3971A1.5010305@linux.vnet.ibm.com>
Date:	Tue, 14 Feb 2012 01:55:05 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tony Luck <tony.luck@...il.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	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>,
	"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 02/14/2012 01:24 AM, Tony Luck wrote:

> 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!)
> 


Oh no!! ia64 directly uses cpu_set() and cpu_clear() on cpu_online_map!!
Grr.. It means num_online_cpus can be different from the actual number of
online cpus because it doesn't go through the set_cpu_online() path.. I haven't
yet pin-pointed the exact problem, but this definitely doesn't look good...
 
Regards,
Srivatsa S. Bhat

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