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

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Date:	Mon, 27 Feb 2012 14:07:46 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	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 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!

>  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.
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