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

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Date:	Mon, 27 Feb 2012 14:16:55 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>
CC:	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 02/27/2012 02:07 PM, Andrew Morton wrote:
> On Mon, 27 Feb 2012 13:55:55 -0800
> David Daney<ddaney.cavm@...il.com>  wrote:
>
>> On 01/31/2012 04:17 PM, Venkatesh Pallipadi wrote:
>>> Kernel's notion of possible cpus (from include/linux/cpumask.h)
>>>    *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
>>>
>>>    *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
>>>    *  that it is possible might ever be plugged in at anytime during the
>>>    *  life of that system boot.
>>>
>>>    #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
>>>
>>> and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
>>> and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
>>>
>>> i.e, We needlessly go through this mask based calculation everytime
>>> num_possible_cpus() is called.
>>>
>>> The problem is there with cpu_online_mask() as well, which is fixed value at
>>> boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
>>> in HOTPLUG case.
>>>
>>> Though most of the callers of these two routines are init time (with few
>>> exceptions of runtime calls), it is cleaner to use variables
>>> and not go through this repeated mask based calculation.
>>>
>>> Signed-off-by: Venkatesh Pallipadi<venki@...gle.com>
>>> Acked-by: Srivatsa S. Bhat<srivatsa.bhat@...ux.vnet.ibm.com>
>>> Acked-by: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>
>>
>> How is it that this patch got merged to linux-next before all the
>> cleanup patches for nr_online_cpus?
>
> <spends five minutes searching mailing list archives>
>
> I for one do not have a clue what patches the term "cleanup patches for
> nr_online_cpus" refers to.  Patches have names - please use them!
>

Sorry about that.  I was a little hasty:

   From:	Venkatesh Pallipadi <venki@...gle.com>
   Subject: [PATCH 0/3] Cleanup raw handling of online/possible map
   Date:	Tue, 14 Feb 2012 14:49:41 -0800
   Message-Id: <1329259784-20592-1-git-send-email-venki@...gle.com>

I don't really know a better way to refer to them.

David Daney



>>    From the looks of your follow-on patches it would seem that all MIPS,
>> hexagon, and um are now broken.
>>
>> I know for a fact that MIPS doesn't boot because of this.
>
> I shall drop
> cpumask-avoid-mask-based-num_possible_cpus-and-num_online_cpus.patch.
>
> That patch was sent as a single standalone patch and the changelog had
> no mention of any needed preparatory patches.  If resending, please
> send *all* patches in a single sequence-numbered series.  We know how
> to do this.

It is possible that Venkatesh did not know about the breakage when the 
original patch was sent
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