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

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Date:	Mon, 13 Feb 2012 12:43:00 -0800
From:	Venki Pallipadi <venki@...gle.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Tony Luck <tony.luck@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Mon, Feb 13, 2012 at 12:25 PM, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> 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...
>

This feels like a minefield in general. ia64, mips and um seems to
have cpu_set and cpu_clear of cpu_online_map and/or cpu_possible_map
in there.
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